Time & Frequency

© Brooke Clarke 2000 - 2009


Precision Clock Hardware Products in Development

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I suspect that it is Karma or whatever that causes me to have a fascination with Time and Frequency.
This has been with me forever.
Note about 3 Clocks

Three Clcoks

Russian Marine Chronograph

Russian Marine Chronograph
Russian Marine Chronograph

My Related T&F web pages
    Crystal Frequencies (factors)
    Precision Clock
    Simple PIC based Clock , 6 digit 7-segment LED clock, PC4 PIC based LCD clock data, time, DOW all leap year rules
    Patents relating to stable oscillators
    Time & Frequency Test Equipment
    Stellar Timekeeping  - an attempt to measure the Earth's rotational period.
Time -vs- Frequency
Time is the parameter that can be measured with the highest precision
Eliminate Leap Seconds?
Time Transfer
    Station Listings
    WWVB - 60 kHz
        HP 117A WWVB Receiver & Phase Comparator
        WWVB Time Receivers
    WWV -  WWVH - WWVS
    LORAN-C  - Loran Patents - LORAN-C & Daylight Savings Time
    Chirp Sounders
        PIC Microntroller Based Motorola Oncore VP+ Binary Time & Date Display
        Brooks Shera's GPS-Controlled Frequency Standard
       James Miller Simple GPS Stabilised 10 MHz Oscillator
       Murray Greenman - The GPS Disciplined GPS Clock 
        Motorola 1 PPS Sawtooth Corrector
    Network Time Synchronization
    CDMA Cell Phone 
Frequency Standards
        Heathkit Crystal Calibrator
        Stanford Research
Allan Variance
Astronomical Determination of the Time
    Sundials - seperate web page
    Photographic Zenith Tube (PZT) - pool of Mercury reflects stars that are directly overhead, 4 exposures on glass plate
    Geodetic Transit - observes the transit of a celestial body over the meridian
    Surveyors Transit with Solar Attachment - Solar or Polaris observation
    Very Long Baseline Interferometer (VLBI) - radio telescope
    Stellar Timekeeping - seperate web page
    Navigation, Orientation - seperate web page
Test Equipment  New page for more Time & Frequency Test Equipment   
Plotting Test Results
Official Links
Vendor Links 
    On Line Clocks
    Hardware Clocks
    Four Letter Word + GPS Time

Time -vs- Frequency

The oscillator used to drive a clock should be running at a precise frequency.  The clock counts the ticks coming from the oscillator.  Since the indicated time is the sum of all previous ticks any error in the frequency of the oscillator will be accumulated by the clock.  In addition the clock needs to be set.  It is not a trivial thing to set a precise clock.

Time is the parameter that can be measured with the highest precision

Time is the thing that can be measured with more precision than any other quantity.  If you can convert the parameter that you are trying to measure so that it shows up as a time change then you can measure it with high precision.  If on the other hand you end up measuring something like length or weight you can only get very limited precision.

Note that when frequency is measured most counters use a 1 second gate so that when measuring a 10 MHz frequency standard you only get 8 digits.  Extending the gate time or averaging takes 10x longer for each extra digit.  So to see 12 digits takes 10,000 seconds (2h 46m 40 seconds).

But by making a time interval measurement starting with the rising edge of a GPS timing receiver 1 PPS output and stopping on the rising edge of a 1 PPS signal derived from the oscillator under test gets you to 12 digit acuracy much faster.  For example assume the TI counter has a one shot resolution of 0.1 ns.  Now in 100 seconds you can see 1 part in 10^12.

It should be intuitive that a counter with the ability to make the second measurement is better than the counter that can make the first measurement by a factor of 100.  This is part of the Allan variance concept (see below)

Eliminate Leap Seconds?

There is a proposal by Dennis D. McCarthy to remove the current 0.9 second limit on the difference between UTC1 (Earth rotation based time) and UTC (atomic clock based time) and thereby remove the need for using leap seconds.  An article appears in the Nov. 1999 issue of GPS World.

I personally don't like the idea because then sundials would not tell the correct time.  A well made sundial today can be accurate to a few seconds since UTC is kept within a fraction of a second of Solar time (i.e. the Earth's rotation).  If we allowed the difference to get as big as 10 seconds, then sundials would have this amount of difference to common time.

Another larger problem is that if making the correction is put off then when it needs to be made there will be many more problems.  This is what happened with the Y2K problem.  There was a lot of scurrying for a few years prior to Y2K to fix the problems and a number of items have since been scrapped because they were not compliant.  If the leap seconds are left as they are then equipment designers will allow for their insertion and most designers will see a number of leap seconds during their carrier.  But if the correction is put off for much more than 20 years, then it will be a case of "I don't need to worry during my career so let's leave it to the next guy".

Time Transfer

This is a rough ranking of various methods of getting the time transferred to a local clock.  They are for a short observation.  By working with any of them for longer time periods the accuracy may get better.  These are transfer methods of setting a clock.  Another method involves using a standard clock.  The standard clock may be a flying atomic clock or someone's wrist watch.  When a standard clock is used there's a bunch of stuff that needs to be done to try and back out it's errors.
GPS carrier phase time transfer
GPS 1 PPS e-8
Loran-C 1
WWWB WVBH LF time stations 1  e-7
VLF radio carriers (military RTTY stations) 1 e-6
carrier of local AM or FM station 1 e-6
Chirp Receiver from GPS controlled Tx 1 e-6
WWV WWVH HF time stations 1 e-5
NTP e-5
CDMA e-5
Photographic Zenith Tube 2
Sun Meridian crossing w/current ephemeris 2  e-2
Telephone audio time recording e-1
TV color burst e-0
Internet web page clocks e+1
Sundial w/ EOT correction 2  e+2
Sundial w/o EOT correction 2 e+3
Astrolabe 2 e+3
Note 1 - depends on you location relative the the transmitter.  A weak signal can degrade the results by many orders of magnitude.
Note 2 - Any visual observation of an object in the sky is limited by the "seeing".  Seeing is caused by changes in the atmosphere so is minimized when looking straight up.  It is not a constant but rather is like a noise signal.  If a small telescope (under 12") is used to record 1,000 images with fast exposure times maybe one of them will be free of seeing distortion.
More on my Stellar Timekeeping web page.  Limited to about 30 milli seconds uncertainty in time.

Radio Station Listings

Time Broadcast Stations
The stations purpose is to disseminate time.  These time broadcasts occur in different frequency bands.  The most common are in the Low Frequency (around 40 to 80 kHz) and High Frequency bands (around 2.5 to 20 MHz).
Navigation Stations for Time
Some types of navigation radio stations have very precise frequency and or timing characteristics that can be used for time transfer.  LORAN-C and GPS both are extreamly good at time transfer.  GPS being the best method for most applications. 
Stations for Frequency
There are (were ?) special built receivers to Phase Track the military VLF radio teletype stations thus prividing a stable output frequency.

The television color burst signal used to be live from the network studio and could be used as a standard, but with the advent of digital time base correctors is no longer suitable.

L.F. Time Stations

China - BPC 68.5 kHz proprietary protocol
France -TDF -  162 kHz -
Germany - PTB - DCF77
Japan -  JJY - 40 kHz
Switzerland - HBG - 77 kHz - Clock makers -
UK - NPL - MSF -60 kHz -  BBC4 - 198 kHz -
US - WWVB - 60 kHz - manufacturers of time and frequency receivers .


Brooke's Frequency Assignment Table
Gude analog & Digital - Time receivers for: DCF, HBG, MFS & GPS (no WWVB)
HKW-Electronics - UE6010/11/15/20 LF time receiver ICs, antennas, modules, etc.
Radio Österreich International - list of time stations -
Standard Frequency and Time Signal Stations On Longwave and Shortwave
Time Stations by AC6V

WWVB - 60 kHz

WWVB can be used for two different purposes:
(1) as a phase reference to determine the frequency stability of an oscillator
(2) as a source of amplitude modulated digital time code data to set a clock that includes leap years and leap second data.
The leap second data is done with 2 bits in such a way that the clock can make the jump at the proper time.
Starting in the 1990s there are a number of low cost clocks and watches that set themselves called "atomic clocks" based on this service.
Note that GPS is a world wide system that does NOT have any leap year or second data.  It does have a UTC to GPS time differential that will allow a clock to display the correct UTC time to within a second after about 12.5 minutes of the correct time to make the jump.  GPS does not have any bits for DST.

HP 117A WWVB Receiver & Phase Comparator

I bought the HP 117A VLF Comparator from EIP Microwave since they had replaced it with a Loran-C based system.  It uses Nuvistors in the loop antenna and the 60 kHz amplifier block that are hard to find.  I have replaced them and now it works well.  I would like to use a PIC microcontroller to decode the time signal.  The stock unit only looks at the phase difference between the external signal and the phase of WWVB.  You can see the hourly 45 degree phase shifts on the WWVB carrier as well as the dinural variation in phase of the signal from WWVB. 

Found a source of contact type strip chart paper (see Amprobe below) : width = 2 1/2", plot width = 1 15/16", sprocket holes on both sides of paper = 0.1 ID on one side and slots 0.1 x 0.15" with a 1/4" c-c pitch.  along the edge is says: HEWLETT-PACKARD RECORDING CHART NO. 9281-0081
Example plot showing adjustments being made to an HP 107 oscillator.  You can see the hourly phase shifts on the WWVB carrier.
The strip chart drive in the 117 may be the same as an Amprobe model DB81 that uses Amprobe model 850D chart paper.
10509A antenna for the HP 117A

WWVB Time Receivers

NIST has increased the power of their time station WWVB on 60 kHz so that the East coast of the U.S. will have a stronger signal all year. 

For the month of March 2005 - NIST does experiment by using a 20 dB modulation depth instead of the standard 10 dB modulation depth on WWVB & WWVBH  to help time code type clocks.  But this may cause phase lock type receivers, like the HP 117, to have trouble maintaining lock.

Temic (2007 now C-Max) has a series of Time Code Receivers that have selectable 40, 60 or 77.5 kHz input frequency and built in knowledge of the time codes used in diffeent countries.  C-Max is in Germany, where Temic was and there is some continuity.  Also make time transmitter simulators to test LF time receivers (needed since the LF signal strength to the receivers has a very wide range.

Historical Info on Temic
They have changed their web page and I can no longer find information on the U 4223 B-CFS.  It is packaged in an SSO20 surface mount package, the only other version is a raw chip.  The SSO20 package has 0.025" pitch connections which is 4 times closer than a DIP with 0.1" pitch.  So far this has been a problem in building a prototype.   A loop that should work with C-Max IC.
PSN network note about Temic receivers.
Searching the Temic-semi web site for WWVB returns 5 documents:
  • Time-Code Receiver with TC Output - U4226B
  • Time-Code Receiver with Digitized Serial Output - U4224B
  • Low-Cost Time-Code Receiver - T4225B
  • Time-Code Receiver with A/D Converter - U4223B
  • Time-Code Reception -
  • Agilent 4395A -  If an active external antenna like the Dymec DA-100, that can pick up the 60 kHz signal from WWVB is connected and the center frequency is set to 60 kHz, span to 100 Hz, RBW to 100 Hz, then the sweep time is 45.71 ms.  The display jumps up and down 10 dB in step with the time code modulation on WWVB.  The number of points is set automatically to 5.  4395A Spectrum Plot 50 to 150 kHz.  The triangular peak centered on 100 kHz is LORAN-C.
    Arctime  - sells a number of Ziet clocks that work by receiving LF time signals
    A Radio-controlled Digital Clock - PIC based with LCD display
    B Roehrig, "Most Accurate Frequency Standard, Parts 1,2,3", 73 magazine, Jan/Feb/Mar 1994.
    Circuit boards available from FAR Circuits  This is a WWVB frequency standard and needs additional circuitry to demodulate and decode the time signals.
    Elmer's Guide to Atomic Clocks for the Home and Office -
    Gunter - AK2124 SOP 24IC - AK2125 SOP 14 IC -  AK2127 raw chip or SOP 16 IC -
    Klockit - sells a movement to drive conventional hands. 
    This ships with a pin to lock the hands than must not be removed until it is installed and the hands attached.  I used one to convert a Wal-Mart clock to WWVB controlled. (You might look for an ALL plastic clock for better reception.) Klockit now has a complete kit in addition to the movement.   When I built mine I needed to order the second hand separately, be sure to get all three hands with the raw movement.
    Precitel Co. - Time Products for VLF and GPS including public clock movements
    Quartzlock Instruments - WWVB receiver and precision time standards
    Radio Clock - a PC sound card based software package that decodes the audio WWVB signal from your receiver and sets the PC clock.  Made to support BeaconSee for ham radio propagation monitoring.  They also have a free applet for GPS PC clock synch.
    Radio Clock Control, the Netherlands - offers slave clock pulsers that are mains or LF time signal driven.
    Radio Shack's Radio Controlled Clock to SDR Interface - info on Blob cable
    Ultralink, Inc - OEM TTL version, RS-232 and one with analog signal strength meter
    Ultralink 333A
                WWVB Receiver with special diaplay
    Ultralink WWVB
    Ultralink 333A WWVB receiver with special firmware
    Ultralink WWVB antenna (marked 301)
                WWVB Receiver
    U1 (on right) is a U4226B Temic WWVB receiver and U2 (at the left) is a TI 2771C Op Amp
    The RJ-11 Pins are: 1 = nc, 2=Vcc (2 V), 3=TimeCodeOut, 4=AGC (Signal Strength), 5=Gnd, 6=nc

    UL333 Antenna & RF decoder , Data Decoder & Display  it's now a UTC only time display, but might have be upgradable to local time
    The US Time zones are selectable as well as UTC.  With UTC there is also an option for continious RS-232 output (9600-8N1).
    Universal Time Company - custom clocks as art
    WWVB Receiving Antenna (bike wheel & soup can) - WWVB signal processor -

    HF Time Stations WWV -  WWVH - WWVS

    The WWV (Colorado) and WWVH (Hawaii) stations can be used in a number of ways:
    (1) audible voice announcement of the time every minute with audible second ticks
    (2) audible voice announcements about a number of topics at specified minutes past the hour
    (3) AM modulated digital data on a 100 Hz subcarrier with time code data similar to WWVB.  The Heathkit GC-1000 used this signal.

    It had a bug in that the switch to/from standard time/DST happened when Colorado changed, not in your time zone although there were DIP switches for time zone as well as propagation delay in ms.  In California it was wrong by an hour or two twice a year if set to display local time.  There may be a modern CPU chip replacement for the GC-1000.
    Power supply reliability upgrade information by Jeff Thomas - replaces the linear 5 volt regulator with a PT5101A-ND switching mode power supply on a daughter board.
    More Accurate Time from the Heathkit Most Accurate Clock by USAF Academy - from +/- 29 ms to +/- 3 ms by adding a 1 PPS output on the DB-25 pin 7.
    Another problem was that when the power failed the GC-1000 used the backup battery to keep the LED display on thus quickley draining a small battery.  It would have been better to shut down the LEDs when the AC line failed and keep the oscillator going.  Heath April 15, 1986 patent on the GC-1000 is 4582434.
    An improved WWV-WWVH receiver is the Precision Standard Time Model 1020, it locks quicker and has less problem with false locks.

    In my T&F rack  there is an HP 1148BR Time Comparator and HP 120 oscilloscope and an HP 113AR Frequency Divider and Clock.
    The 114 takes in the audio from a HF receiver tuned to one of the WWV stations.  It feeds that audio to the 120 scope Vert. and a 1 PPS to the Hor.
    The 1 pps can be moved in 1 ms steps using the 3 thumb wheel switches on the 114.

    ESE - ES-180A WWV master clock -
    Austrailian Standards Leaflets - Radio VNG 2.5, 5.0, 8.638, 12.984 and 16.0 MHz.


    The LOng RAnge Navigation system version "C" was a follow on to the A version that worked around 1.9 MHz which is near the peak  absorption frequency (1.6 MHz) of the ionosphere's "D" layer.  It has maximum density when the Sun is at local noon and goes away at night.  This has a strong effect on AM broadcast stations and I'm sure on the old LORAN system.  LORAN-C operates at 100 kHz where the ground wave is very stable.  Since it's a pulsed system a receiver can seperate ground wave from sky wave unlike WWVB where they get mixed up.  This makes LORAN-C able to transfer time more accurately than WWVB at 60 kHz.

    Data Channel

    The current LORAN-C system does not now broadcast data packets.  A part of the eLORAN modernization program may be to add a Data Channel.  This would be similar to what's now done as part of GPS and would include the time and date, station ID and location, etc.  Like the GPS system the LORAN system will not have leap seconds, but will have a couple of bits to allow determining exactly when a new leap second will happen as well as the current offset between LORAN and UTC so that UTC can be accurately determined.  What's missing is domestic Daylight Savings (Summer) time bits, like are on WWV and WWVB.

    Why the LORAN-C Data Channel should include Daylight Savings Bits

    It would enable precision clocks that keep local time.

    Currently it's straight forward to make a clock that tells UTC and these are used for scientific purposes throughout the world.  But it's very difficult to make an equally accurate clock that tells the local time.  For example Heathkit used to offer the GC-1000 digital clock that was synchronized by the WWV and/or WWVH radio stations but it has a couple of problems, 1) the switch to/from DST happened when the WWV(H) transmitter switched, not at the proper local time and 2) H.F. radio is about the poorest quality time synchronization method available today, many orders of magnitude from GPS or LORAN-C. 

    WWVB is much better and is now used to synchronize "atomic" clocks.  It too has a few problems: 1) there's a dinural variation caused by the interaction of the ground wave and the sky wave,  and 2) the signal is very weak in many U.S. locations so that most clocks can only get a strong enough signal around local midnight, and 3) even when it's working the precision is far from GPS or LORAN-C.

    Note that the GPS signal is very weak and does not penetrate into buildings and so can not be used for economical local clock control.  Even if an outside antenna is used GPS can not be used because it's not practical to have a couple of DST bits on GPS where each country in the world would have their own, i.e. it would take hundreds of bits.  No DST bits = no local time.

    That's not a problem with LORAN-C since the signal only carries for some hundreds of km.   If DST bits were in the LORAN-C Data Channel specification as a local option then each country could choose to use them for their local time or not and it would not burden the system or neighboring countries.

    LORAN-C stations are not only very powerful but are spread all over the world.  The LORAN-C signal is very strong when compared to a LF time signal and so could be used to synchronize local clocks.  LORAN-C blankets the entire US with a signal  stronger than   WWVB (1800 UTC part of east coast is off coverage map).

    Currently each country has it's own LF time system for local clocks.  There is no world wide standard so there's a bunch of independent companies working on a clock for just their country.   Since they are on different frequencies and use different protocols this work can not be shared.  By standardizing local time broadcasts everyone would be using the same clock leading to a greater range of products from lab quality instruments to dime store clocks more accurate then the existing WWVB clocks.

    Timing of Transmitted Pulses

    In a hyperbolic navigation system the thing a receiver measures is the time interval between the pulses from each station.  The original LORAN systems were thought of as a group of 3 to 5 stations acting alone (not relating to any other chains).  The master station of each chain waits for a time that's called the Group Repetition Interval (GRI) between sending it's pulses.  The slave stations each wait for their own unique amount of time after the master station then transmit.  All the stations in a chain have the GRI between their transmissions.  So if you trigger a scope at one of the published GRIs from a stable time base and feed the scope input from an antenna  you will see some pulses at fixed time delays from the members of that chain.  Also visable will be unsynchronized pulses from other chains.  When one of the unsynchronized pulses lands on top of one of the stable pulses it's called cross chain interference. 

    Another aspect of the modernization is controlling all the LORAN stations time of transmission based on UTC instead of the master then slave method now used.  When all the LORAN stations have their transmission times controled by UTC then instead of thinking of each chain seperatly the whole system can be thought of a one big chain.  Since each classical chain is transmitting using it's unique GRI the existing LORAN receivers will not see any difference.  But then it would be possible to build a LORAN-C receiver that's "All in View" instead of chain specific like is now done.

    Table of Loran-C Chain Stations -

    International Loran Association - Prior to GPS Loran-C was the best available broadcast time signal (except for an expensive satellite system)
    The U.S. Coast Guard controls LORAN since it's primarly a coastal navigation system.  So this is where you find the specs, status, etc.

    Chirp Sounders

    There are many H.F. radio transmitters that sweep from 2 to 30 MHz for the purpose of studying the ionosphere.  Many of them now use GPS to keep their start times very accurate and could be used as time transfer transmitters. RCS-5 based and DSP based receivers.


    WWVS (WWV Satellite) is a time system that is was replaced by GPS) on two GEOS sats, one one the East coast and one on the West coast.  The sat was nominally geosnychronous, but there were variations in the orbits that showed up as time variations.

    See my write up on how Sputnik led to the Transit satellite navigation system that lead to GPS.  GPS receivers do not need the precision clock that was required for Transit receivers.

    This is done by requiring 4 satellites, 3 for position (X, Y and Z) and a forth to remove the requirement for a precision clock in the receiver.  There are no bits in the GPS data for the U.S. DST to/from Standard Time.  There is an offset number that describes the number of seconds that GPS time differs from UTC.  This is needed because the GPS system does not have any leap seconds and UTC does so you can know UTC by listening to GPS.

    GPS is now the best time transfer method available for most applications.

    GPS All in View

    The GPS system is designed so that you can get a fix if 4 satellites are being tracked and early GPS receivers worked with 4 satellites.  Later GPS receivers are of the "All in View" type where as many satellites as can be tracked are used in the solution since this reduces the error in final position (or time).  To take advantage of "All in View" the total number of satellites that a GPS receiver can track has been raised from 6 or 8 for the early receivers to 12 which is practically the most that could be seen for a nominal constellation of 24 satellites.  (But there's more like 30 operational GPS satellites now so the receivers should have 15 or 16 channels.

    PIC Microntroller Based Motorola Oncore VP+ Binary Time & Date Display

    This is a PIC16F84 based circuit that reads the binary data stream from the VP+ and formats the data into DOW, DOM, Month as a 3 letter name, YYYY, hh:mm:ss and displays the data on any standard 44780 based alphanumeric LCD.  Since the display is only reading the data it will automatically show leap seconds.  It will show local or UTC time depending on how the VP+ was programmed by an external computer.  I plan to add dip switches so that you can select the number of columns and number of rows.

    GPS Disciplined Oscillators

    These combine the long term stability of GPS with the short term stability of a crystal or Rubidium oscillator to end up with an oscillator that does not to be calibrated every 6 months.  For more about this see the Allan section below.

    Brooks Shera's GPS-Controlled Frequency Standard

    I think this one has the best specs, but it was designed when the Motorola 6 & 8 channel timing receivers were common.  Now there are newer 12 channel timing grade GPS receivers that have more like 9 ns jitter instead of the 52 ns of the older receivers.  This means a new design is needed to take advantage.  There is a PIC microcontroller used to steer an oscillator based on the 1 PPS from a Motorola GPS receiver.  

    James Miller Simple GPS Stabilised 10 MHz Oscillator

    Based on the Rockwell/Conexant/Navman Jupiter GPS engines is that they have a 10 kHz output.
    Complete Unit
    Do-It-Yourself version

    Murray Greenman - The GPS Disciplined GPS Clock

    Uses a frequency lock loop to minimize the total cost, but at the expense of some performance.

    ??? new GPSDO  from Australia 2007

    Mot GPS 1 PPS saw
                tooth correctorMotorola 8 Channel Sawtooth Corrector
    I used this combination to try and use the saw tooth correction feature on the 8 channel Motorola VP GPS receiver, but there was a glitch in the Motorola saw tooth.  Some times the corrected 1 PPS would be low, but never high.  So you were better off averaging the saw tooth than correcting it since the saw tooth error was symmetrical.  There's a rumor that HP figured out how to get around the bug based on the timing between the glitches being related to the clock speed of the GPS receiver and how it related to the granularity of the saw tooth.

    The circuit uses a PIC16C63, ECS-300C programmable clock, 74HCT14, Data Delay Devices3D7408 8-bit programmable time delay, Opterex  DMC20481 LCD and 78M05.

    An adapter was made from 2x7 ribbon cable to 0.3 inch DIP plug so that the LCD could be plugged into the white protoboard.  A ZIF socket holds the windowed PIC.  RS-232 wiring to the Synergy GPS development black box not shown in photo.

    PIC based GPS Mot
              binear readerMotorola 8 Channel Binary Packet Reader
    This is the PIC circuit that reads Motorola 8-channel binary data an displays some packets in plain text on an LCD.  Uses 1 PIC16F84, 10 MHz unit oscillator, MAX232N and 78M05A.  It's powered by a 9 volt battery.  Also an adapter from 2x7 ribbon cable to 0.3" DIP for the 2x40 LCD that has no markings.  I expect that the GPS interface cable shown here was used on the above saw tooth corrector.

    The HP-48 calculator has a serial interface and can be programmed to read the packets.

    About this time I had written a Lab VIEW program to compare the SC 10 to GPS over a few hours and could fit a parabolic curve to the difference and solve for the aging of the SC 10.

    GPS related links

    CNS Systems - commercial TAC32 clock - GPS Time - Low Cost High Accuracy GPS Time paper from ION
    TAPR - TAC32 - for TAPR members for non-commercial and personal use
    Horita - GPS based Video Time Code generators
    HP GPS based timing products & app notes
    KEK GPS Clock System - TACGPS backup module -
    New Zealand and Australia Time Resourses - sub second timing for astronomical use
    Symmetricon - is taking over the HP (Agilent) GPS based timing products 58333A, 58540A - I think these used the discontinued Motorola Encore VP series OEM boards.
    W7CQ page on the Shera setup with onfo on the HP(Agilent) 10544A
    Zyfer - GPS diciplined 1PPS & 10 MHz -
    Luis Cupido - Board to phase lock an oscillator to either a 10 MHz input or a 1 PPS input

    Rockwell/Conexant/Navman Jupiter GPS engines that has a 10 kHz output

    Simple GPS Stabilised10 MHz Oscillator - would be better using the TVB PIC as the divider rather than the 74390 parts.

    Probably the Simplest GPS Disciplined Oscillator possible ! - by Andrew Talbot, G4JNT

    Network Time Synchronization

    Network Time Protocol (NTP) - The Electrical Engineering Computer Information Systems department of the Univ. of Delaware is the center of action for NTP.  The internet requires time synchronization in order to operate and this is handled by NTP.  You can also set your computer clock using NTP type software (but not the time server software on this server which is intended for the server rather than the client. 
    Galleon Systems - NTP Time synchronisation products including GPS and MSF (But not WWVB nor DCF)
    Symmetricom, Inc - (NASDAQ: SYMM) -multimode time sync including GPS
    Proposed new <time.h> for ISO C 200X -

    CDMA Cell Phone

    Each CDMA base station has a GPS receiver for system synchronization.  By receiving the CDMA signal it's possible to get a 1 PPS signal good to better than 10 micro seconds.  But this only works if you have a nearby CDMA cell site.  A big advantage of the CDMA time signal is that you do not need an outside antenna.
    EndRun Technologies - hand held and rack mount time standards based on GPS or CDMA.

    Frequency Standards


    There have been many mechanical time keepers.  Some used sand or water, later the pendulum clock was highly developed.
    Harrison spent most of his life developing a chronometer that would work at sea so that sailors would know their longitude.
    Knowing latitude is relatively simple by measuring the elevation angle of the Sun or stars, but knowing the longitude requires knowing what time it is.  Great Britain had a standing reward of 10,000 pounds sterling for someone to produce a workable way to know the longitude at sea.


    Quartz crystals are probably the most common devices used to control the frequency of electrical oscillators. They are used in wrist watches, clocks and most microcontroller based devices.  There are many ways to slice a quartz crystal from the piece that's found in nature or grown and these different "cuts" yield different properties.

    There are a number of environmental conditions that effect the frequency of a quartz based oscillator.  A crystal will change resonant frequency when the temperature changes depending on the cut of the crystal and the actual temperature.  HP used to offer a precision temperature meter that used a crystal optimized to vary with temperature.  Crystals change frequency with time, called aging.  It turns out that this has a lot to do with how the crystal is packaged.  If there are any molecules that can land on the crystal they will change it's frequency. The acceleration due to gravity will effect crystal frequency.  See the plot on my PRS-10 web page showing the effect of gravity on it's internal 10 MHz crystal. Atmospheric pressure and humidity also can have an effect on crystal frequency.

    The highest precision crystal oscillators place the crystal inside an oven (OXO) that keeps the temperature constant.  Another way to compensate for the temperature dependence of the crystal is to measure the temperature and use that information to change the frequency of the oscillator (TCXO).

    There is a ceramic resonator technology that has performance slightly poorer than a simple crystal and costs less that is used in high volume low cost microcontroller applications although my guess is that overall the crystal is still the most used frequency determining element.

    There are a number of companies that make test equipment to measure the electrical parameters of quartz crystals.  It turns out that the 32,768 Hz crystals used in watches are extremely difficult to measure because they have high Q which translates into measuring impedance in the 1 to 10 Meg Ohm range.  The HP (Agilent) 4194A can make this measurement.

    7 July 2007 - I now have a couple of clocks running side by side.  One is a Self Winding Clock Co. "Western Union" pendulum clock and the other is a Standard Electric Time Co. slave clock that gets it's 1 Pulse Per Minute from a quartz crystal based circuit.  As I'm writing this the pendulum clock is 3.5 minutes slow and the quartz based clock is 5 minutes fast.  They were both set at about the same time when they both were adjusted for rate.  This demonstrates that although a crystal has the potential to be a very good timekeeper a simple implementation may not be any better and can easily be worse than a pendulum clock.

    A Quartz Watch Time Base Monitor - from RF Design magazine, maybe 1989 - uses a piezo transducer to listen to the 32,768 sound and an op amp/filter then a PLL to get a square wave so the period can be measured.

    The Aging of Bulk Acoustic Wave Reasonators, Filters and Oscillators by John R. Vig and Thrygve R. Meeker 1991
    This paper discusses the possible aging mechanisms.  As of 1991 the best aging rates that can be found are about the same as were possible using 1960s technology.  Note that the best possible aging rate of a crystal oscillator depends on it's surface area to volume ratio.  Therefore lower frequency crystals have lower aging rates.  This is why the 32,768 Hz watch crystals have such good aging specs.

    Heathkit Crystal Calibrator

    Radios made prior to the introduction of digital synthesizers typically used an LC (inductance capacitance) controlled oscillator as the tuning element and did not have the ability to tune to a station directly, you needed to tune up and down the dial until you heard the station.  To help find stations a crystal calibrator could be used.  It generated harmonics spaced by the 100 kHz crystal frequency and when you heard the harmonic yo know you were tuned to some multiple of 100 kHz.

    Easy to adjust by listening to WWV and zero beating the calibrator.


    This design shows up in the book  "Evolution of Naval Radio-Electronics and Contributions of the Naval Research Laboratory" (NRL Report 7600, January 1976) on page 289 is a photo showing the Sultzer labs unit at the top of the Omega nav receiver rack as a major improvement in crystal oscillator technology.  It has a cylindrical shape and uses a proportional oven control coupled with a large thermal mass.  This was a big improvement over the prior art which used bang-bang type temperature control and low mass.  The crystal frequency was 2.5 MHz and it was the first all transistor frequency standard.


    The Sultzer design was the basis of the HP 105 crystal oscillator and the cylindrical crystal oscillator used in many HP time and frequency products.  HP used a 5 MHz crystal for all their versions.  This is a fairly large and heavy unit.


    This unit used a double proportional oven surrounding a Bliley glass enclosed 10 MHz crystal.  The inner oven is set at the turnover temperature of the crystal to minimize the effect of any temperature change.  By plotting the oscillator frequency vs. temperature you can find the sweet spot.  The size and weight of the complete oscillator assembly (excluding the power supply) was much smaller and lighter than the Sultzer design.

    It was designed so that the lead acid batteries were in the same rack space as the double oven crystal oscillator.  The acid fumes etched the PC boards inside the oven and it died.  I removed the batteries, and rebuilt the boards.  It ran for a few more years and I learned a lot about precision crystal oscillators.  I have replaced the old double oven unit with a new Stanford ResearchSC-10 crystal oscillator.  It has both mechanical trim as well as external electrical frequency control.

    The power supply uses two 723 based voltage regulators in series.  The first converts normal 120 VAC mains power down to about 20 volts to charge 3 each 6 Volt gel cell batteries.  The battery voltage goes through another 723 based power supply that drives the heaters and electronics using separate circuits.  When the mains power fails there is no switch over since the batteries are always feeding the final supply.  When the mains power comes back on the batteries charge.  If the blackout lasts too long a relay will disconnect the batteries to prevent battery damage from over discharge.

    Stanford Research

    My Gibbs got to where it was not economical to repair and was replaced by the SC 10 crystal oscillator.
    Stanford Research was founded and still is based on physics rather on electrical engineering.  For me, their products offer very good value and performance for the money.  The SC 10 has both mechanical coarse frequency setting and a number of different options for how the electronic fine tuning will work.

    By adding a printed circuit board that Radio Shack used to sell that has a number of decades of division the 10 MHz output can be divided down to a 1 PPS signal.  This can be compared to the 1 PPS from a GPS receiver.  At this time SA was turned on and the 1 PPS had about 100 nS of random jitter (although there was always one sat with SA turned off). 

    Quartz Crystal Resonators and Oscillators for Frequency Control and Timing Applications by Rakon


    The Rb and Cs "standards" are not absolute, they are just like a crystal in that their frequency needs to be set.  What they offer is a lower aging rate i.e. they are more stable.
    Stanford Research Systems PRS10 Rb Source with built in time tag & GPS lock plus extensive RS-232 communications for only $1,495 is single qty!!!
    TrueTime Inc - has a broad line of timing products, they are in Santa Rosa, near my location


    HP 5060A Cesium Beam Frequency Standard Option H21
    FTS 4060 Cesium Time and Frequency Standard option S24
    Theory of Operation -

            Buffer AmpInterstate Electronics, Cesium Beam, Freq-Std Buffer, Part No. 7292000, Serial No. 10, Contract No. APL 601951-L
    This is a group of three independent amplifiers, two have 1 input and 1 output and the other has 1 input and 2 outputs.  Although they are labeled as 5 MHz, 1 MHz & 100 kHz, they work very well from below 100 kHz to 30 MHz.  The gain is near 0 dB and is good for input levels up to +15 dBm input.  The BNC-f input and output connectors have isolated shells and the amplifier circuit used Mini Circuits Labs transformers on the outputs.  Has built-in 110 VAC power supply.  Power connector is MS3106A-10SL-3S (same as the 24 Volt connector on the PRC-104 battery box).  Pin "C" is ground and A&B are line for this instrument.  An LH0002 IC & MCL  T2.5-6 is used for each output.  I think this unit was surplused because the labels indicate that it will not operate at 10 Mhz, but in fact all the channels will operate at 100 kHz through 30 MHz.

    Allan Variance

    The Allan variance is a statistical measure of the stability of an oscillator.  The slope of the Allan variance vs. sampling time plot will tell you about the kind of noise that is disturbing the oscillator.  The modified Allan variance adds another noise type and is preferred by some as a more inclusive characterization.  Typically Allan variance is measured on 1 PPS type of data using a time interval counter that has no dead time, i.e. it measures every tick.  The data is stored in a computer and then processed with different decimation intervals.

    A similar characterization can be done in the frequency domain.  There has been a lot of high powered math done demonstrating that the Allan variance information in the time domain can be converted back and forth into the frequency domain called phase noise.


    Lets look at two counters:

    1)    A classical 1 second gate time frequency counter can display "10,000,000" (8 digits or 1E-8 resolution) for a 10 MHz input.  If you want 12 digits of resolution then you can extend the gate time (assuming the time base is of adaquate stability) to 10,000 seconds (2h 46m 40 seconds, or 1E4) and can set the decimal point to display the frequency as "10,000,000.000,0" (1E-12 resolution).

    2)    If a Time Interval Counter is used that has 0.1 ns one shot resolution then making a measurement of the time interval between the unknown oscillator and a reference oscillator that's better you can see 1 part in E-10 in one second or to see 1E-12, like in the above 10,000 second test, it takes only 100 seconds.

    The first counter only needs to be able to operate at 10 MHz.  If the first counter was used in the time interval mode (i.e. counted an internal 10 MHz clock gated by the START and STOP inputs) it would have 0.1 us one shot resolution.  To get that down to 0.1 ns one shot resolution the internal clock singal would need to be 1,000 times faster or 10GHz.  Real counters don't use 10 GHz oscillators but make up the difference by being very cleaver.  In any case you can see that a counter that has faster one shot resolution is a better counter.

    On an Allan plot the first counter has a point at 1 second with a stability of 1E-8.  A line can be drawn through this point with a slope of -1.  The line will for example go through a point at 1E-12 at 10,000 seconds.  So you can see that although it can make a measurement with a resolution of 1E-12 the counter only has a stability of 1E-8.

    The second counter has a point at 1E-12 at 1 second.  Again a line with slope of -1 can be drawn.

    In both cases you can only measure at or above the line representing that counter.

    A measure of the stability of a counter is it's true one shot resolution when measuring the Time Interval between two pulse trains each of which is at 1 PPS and the counter reports all of them (i.e. the dead time is short enough that no pulses are missed).

    The stability of a counter is different than it's resolution.

    The Plot

    The plot is done on Log-Log paper.  The Y-axis is the stability in seconds (higher stability is a lower number and is at the bottom of the scale) and the X-axis is the time interval between two points in seconds.  When testing crystal and atomic standards it's common for the X-axis to run from 1 to 100,000 (1E5) seconds since the minimum interval is the time between measurements and for a 1 Pulse Per Second signal (1 PPS) that's 1 second.  If the Time Interval data comparing a house standard's 1 PPS output with a Device Under Test's 1 PPS output for 1 day the max X-axis value would be 86,400 seconds, rounded up is 100,000 seconds.

    For crystal oscillators the left edge of the z axis will be much shorter than 1 second.

    The Slope

    One of the nice features of the Allan plot is that the slope of the curve gives you some idea of what's causing that stability. 


    For example if the line is sloping down and to the right with a slope of -1:1 you are seeing the effect of the longer time between the start and stop pulses.  In the case of a frequency counter  when the gate open time is increased by a factor of 10 and the result is divided by 10 the resolution improves by a factor of 10.  One point on the plot would be the resolution on the y axis and the gate time on the x axis.  The next point would be to the right a decade and down a decade, i.e. the line has a -1 slope.  So averaging or using longer gate times does not really change the quality of the measurement but it does allow you to apply whatever quality of measurement you have at different x axis locations.

      This is an important concept for many reasons.

    Overlaying Two Plots

    Say one plot is made using an atomic standard vs. a specific GPS receivers 1 PPS output.  The plot looks like a -1:1 slope straight line starting out much worse than the crystal oscillator and  ends up better than atomic standards.

    The other plot is made comparing a crystal oscillator with an atomic standard.  The x-axis for an oscillator usually starts some decades faster than 1 second.  A bathtub shape is typical where the bottom is reached around the 1 second area and the slope is positive by 100 seconds.

    When the plots are overlaid you can see that they intersect at one point.  For intervals less than the intersection x value the crystal oscillator has superior performance and to the right of that point GPS has better performance.  By using GPS to control the long term frequency of the crystal oscillator the performance of the combination is better than either one by itself.  This is called a GPS Disciplined Oscillator (GPSDO) and is a product made be a number of companies and there are a number of hobbyist kits to roll your own.

    The Time Interval Counter

    When making the Time Interval (TI) measurements a counter is being used.  A classic way to do this is as follows.  The Start input enables a gate and each cycle of the reference oscillator is totaled until the Stop input closes the gate.  The frequency of the reference oscillator determines the One Shot Resolution (OSR) of the measurement.  If the reference oscillator is 10 MHz then the OSR is 100 ns, 100 MHz is 10 ns, 1 GHz is 1 ns.  A 10 MHz counter can be built easily using common logic ICs.  A 100 MHz counter requires selected parts and careful circuit layout and might be classified as beyond most hobbyists ability to design.  Yet TI counters are available that have a OSR in the tens of ps (0.020 ns).  Needless to say they do not use multi GHz reference oscillators, but instead they use a number of  clever methods.  One such method is the Time to Digital converter (Google it).

    If the OSR is 10 ns and a 1 PPS signal is being measured then that's a point on the Allan plot for the TI counter.  A line with a -1:1 slope can be drawn through this point to define the lower test limit of this TI counter. This counter can only measure things that are above it's line, i.e. have a worse stability.

    If you look at the Allan plot for something and superimpose the line for your counter on the plot if any part of the plot is below your line, then you need to use a better counter to make that measurement.

    The design of the TI counter that's used for a GPSDO has a big impact on the overall performance of the combined unit.  The first generation Motorola VP Oncore GPS timing receivers have a sawtooth error a little bigger than 50 ns so a TI reference frequency of 20 MHz would be in the ballpark.  But the current GPS timing receivers have a sawtooth error less than 10 ns and when corrected more like 3 ns so now a ballpark TI reference frequency of 350 MHz is needed if the overall design is going to be near optimum.

    I have the feeling that sometimes the measuring equipment is limiting the results of a test.  This can be seen by plotting the -1 slope line for the counter on the Allan plot for the result.  While surfing Allan Variance papers I came across a number related to Wavelets.  The language they are using and for example the example of modling the weather makes me think there is some Chaos Theory involved.  (Google: Chaos theory, Glick, fractal) (Wiki: Chaos Theory)
    Time Interval Metrology Enterprise - Allan's web page - The Allan Variance (brief overview) -
    Agilent (HP) app note 1289 The Science of Timekeeping (pub# 5965-7984E) <- very good coverage of Allan Variance
    USNO - Clock Performance and Performance Measures.
    Hamilton Technical Services - stability software and a number of on line papers
    Stable32 - Techniques for Frequency Stability Analysis - extensive linked bibliography
    NIST - An Introduction to Frequency Calibrations - good  overview of Offset, Stability, Allan Deviation
    NIST - Fundamentals of Time and Frequency - good overview
    NIST - general interest time and frequency publications -

    Extending the One Shot Resolution of a Time Interval Counter


    One of the first methods is based on analog interpolators.  Assuming that the input start and stop signals are asynchronous with the internal clock there will be a START to rising clock edge error and there will be a STOP to rising clock edge error.  By starting a ramp generator with START and stopping the ramp generator with the next rising clock edge that time interval can be measured by a voltmeter.  In a similar manner the STOP to rising clock edge error can be measured.  These two corrections can now be applied to the integer count.

    I&Q Sine & Cosine

    By using In phase and Quadrature phase versions of the internal clock to generate pure sine waves then using a sample and hold circuit to capture the value on both channels at START and again later at STOP you can figure out the angle between 0 and 360 degrees.  Where 360 degrees is the period of the internal clock  So, using the above 10 MHz internal counter as an example if the votage measurement is good to 14 bits then the one shot resolution is improved by about 16,000 times, i.e. from 0.1 us to 6E-12.

    Time to Digital Converter

    These are ICs that use gate delay times to measure time intervals into the ps range but have a max time interval limit that reauires a slower TI counter to make up the interval to get to 1 second.

    Meta Stability

    There is a specification for a flip-flop that says it will not work properly unless the time between signals exceeds some setup time.  But if the signals happen to arrive too close together the operation of the flip-flop can be wrong (it just hangs for a long time).  So in the above schemes the flip-flop used for the gate needs to be protected from this problem.  There are parts called Synchronizers that will do this.

    Test Equipment

    I have a collection of Hewlett Packard time and frequency instruments that goes back to the late fifty's.
    Office Instruments   Time and Frequency Rack
    I am using the Stanford Research  SR620 Time Interval Counter.  It seems optimized for working with 1 PPS signals.  It is an older instrument and some of the printing functions are written for Epson format, but the 25 pS one shot time resolution and display with 16 digits makes it ideal for 1PPS work.  The Stanford Research PRS10 Rubudium source supplies the house 10 MHz to the SR620 and is daisy chained to other instruments like the HP 4395A, HP 8648A, SR DG535, etc.
    HP 5216A 12.5 MHz  Electronic Counter - Nixie tube display, DIP ICs, 4 bits/digit output
    In the 1950s I had a cardboard box given to me as a gift from an HP employee that contained a bunch of AC-4A stuff, some complete, some parts and a bunch of NE-2 bulbs in strings of 10.
    eBay "Frequency Standard" - Army 1994 probably audio derived from a 1 MHz crystal, 11 relay controlled frequencies
    Austron 2100F LORAN-C Frequency Monitor -
    HP 5110A Synthesizer Driver & 5100A Frequency Synthesizer -

    Plotting Test Results

    A Time Interval vs. date & time plot for a Cesium standard will be a straight line.  The slope is an indication of the frequency offset.  For example if the slope is 10 ns per day (10E-9 / 86400 ) then the frequency if off by about 1E-13.  There are many things that will add bumps and dips

    A Time Interval vs. date & time plot for a non Cesium standard will be a parabolic curve.  That's because not only is there some initial frequency offset but there's also aging.  How long the test needs to be in order to see the aging depends on the quality of your house standard.

    You can get more information by making one of the Allan or related plots.  The gold standard for doing this is the Stable32 commercial software package.  Free software is available from Ulrich Bangert called Plotter.  It can be used with just a series of TI measurements in a text file or with two columns of data where the first is a time stamp.  In order to get the Allan plot the vertical scale needs to be in seconds either as the raw data or there's a way to scale it inside the Plotter program.  Ulrich is very helpful in getting you going.


    There are many styles of sundials.  Some of them are very accurate time keepers.

    Official Links

    USNO Logo
    The USNO is the keeper of time.  In the past they determined the time by means of the Photographic Zenith Tube (PZT) [Interesting Google Images].  This was a telescope that had a pool of mercury acting as an optical reflector so that it would look straight up.  A glass plate was exposed four times at precise times determined by the USNO house clock.  After developing the plate, errors could be corrected based on the geometry of the 4 star images and the exact time of exposure determined.  Now there are a number of ways of determining the time based on observing astronomical phenomena (including Earth satellites) either optically or using radio telescopes.  But those methods are really not as accurate as the time scales determined by ensembles of atomic clocks. In the paper "Astronomical Time" by David McCarthy, Proceedings of the IEEE, Vol 79, #7, July 1991 he mentiones that one  night's work with the PZT would yield time to withing +/- 5 ms.  The more modern methods are VLBI, Lunar Laser Ranging (to a retro reflector), Satellite Laser Ranging (to a retro reflector on some satellites.

     The USNO has their own ensemble of about 50 atomic standards which are combined mathematically to form the USNO time scale.  That scale is used to steer an
    Auxiliary Output Generator driven from a single atomic standard to that there's a physical realization of the USNO time scale called Master Clock 2. There is a 9 second discrepancy between the voice announcement of the time from the USNO and an accurate clock.  The USNO says that it is caused by the audio streaming of the internet, but who knows?
    The USNO ensemble is part of the world wide ensemble maintained by the IERS, see below.


    NIST Time & Frequency The Time & Frequency Division of NIST maintains atomic clocks that are set from USNO observations.  Has a page for radio station WWVB (60 kHz) and another one for WWV & WWVH (2.5, 5, 10, 15 & 20 MHz).  There are a number of other quality pages at this site.  Their primary focus is on stable Frequency whereas the USNO is concerned with Time. Atomic Web Clock -Java clock showing both computer & Nist time

    PTTI Precise Time -Time Interval The PTTI is an organization that has meetings and publishes papers related to time and time interval.

    Intl Earth Rotation Service The IEEE Ultrasonics, Ferroelectrics and Frequency Control (UFFC) Society has annual meetings, publishes a periodical and papers.  This page also has a  large number of links.

    The National Earth Orientation Service was organized to coordinate, collect, analyze, and distribute data from the various operational U. S. programs that monitor variations in the orientation of the Earth. It serves as the Sub-Bureau for Rapid Service and Predictions of the International Earth Rotation Service

    Brief History of the Development of Ultra-precise Oscillators for Ground and Space Applications by Norton, Cloeren & Sulzer

    IERS was created in 1988 by the International Union of Geodesy and Geophysics (IUGG) and the International Astronomical Union (IAU). It replaced the Earth rotation section of the Bureau International de l'Heure ( BIH ), and the International Polar Motion Service ( IPMS ). It is a member of the Federation of Astronomical and Geophysical Data Analysis Services (FAGS ).

    JPL Frequency Standards Lab - The Frequency Standards Laboratory houses the National Aeronautics and Space Administration's (NASA) Lead Center for Frequency and Time. The Laboratory supplies NASA's Deep Space Network (DSN) with the hardware, expertise and technology for state-of-the-art frequency standards, clocks, distribution networks, and time synchronization to enable deep space navigation and advanced radio science experiments.

    Vendor Links

    ACAM - Time to Digital Converters -
    Antique Watch Images -
    Antiquarian Horological Society
    Bode Research Group (BRG) - lots of different types of clocks including 60 kHz
    British Horological Institute - Links
    Casio - timepieces - Triple Sensor (Digital Compass) - Altimeter, Barometer, Compass, Thermometer & Watch
    Cesium, Songs of -
    Chinese Military & Professional Optics Corp - Astrometric Instruments
         Horizontal Meridian Circle
        Photoelectric Astrolabe Mark 2
    Photoelectric Astrolabe Mark 3
        Vacuum photographic zenith tube 
    Chronométrophilia - the Swiss association of collectors and amateurs of ancient and classic horology.
    Clock making books -
    C-Max - LF Time ICs and related parts (looks like Temic based or improved versions)
    Crystals - quartz as resonators for oscillators
    Data Delay Devices - programmable delay chips
    Datum - manufacture of time test instruments 2003 -> now part of Symmetricom
    Deutsche Optik - Clocks & Watches -
    Directory of Companies in Frequency Control, Timing, and Related Devices @ UFFC
    Distrubution Amplifier, home brew - work in progress
    Doug Hogarth's Niceties - Doug also has an interest in Time & Frequency
    Earth: An Oscillator and Frequency Standard - Lab test Report
    Electric Clocks - with a classification system & Great Macromedia animations - new web page
    ElectronicsUSA.com - LED clock kits & assembled - not NOT 1 PPS input compatible
    Elmer's Guide to Atomic Clocks for the Home and Office -
    Falcon Watch - 24 hour watches for about 50 years
    Fifteenth Century Navigation - Patricia Seed
    Frequency Standards and Metrology Research Group (FSM Group) - working with Sapphire instead of quartz oscillators.
    German society for chronometry - all in German
    Global Time Systems - digital clocks for public display
    GeoClock - software for PC computer showing sun on world map, Ham radio options also available
    Gordon Uber's web page - very comprehensive links
    Highland Technology - picosecond timing products
    HOROLOGY - The Index -Fortunat F. Mueller-Maerki maintains this page of links. Horology is the Science of Time , Timekeepers ( Clocks, Watches ) and Timekeeping .
    HP K34-59991A Phase Comparator patent 4282482 Method and apparatus for phase detection having a narrow dead band and a linear output
    Information Leaflet No. 75: `The longitude of Greenwich'
    Institute of Photonics and Electronics Czech Republic - Division of Electronics Systems and Signals - Department of Standard Time and Frequency - English - have T&F cal standards
    Introduction to Quartz Frequency Standards -
    Jackson Labs - GPS disciplined oscillator, Freq Syn (bench & Board versions)
    John Ackermann N8UR - Sultzer vs. WWVB using Fluke 207 -
    KIWI - Precision Timestamp Utility - freeware, runs in pure DOS with 1 ms accuracy based on GPS 1PPS
    LeapSecond.com by Tom Van Baak - Tools - Museum of HP Clocks -
    Leitch - time code driven clock systems
    Locus - All In View DSP based Loran-C timing receivers
    Meinberg Radio Clocks - DCF77 or GPS
    MicroSet - Instrument for measuring pendulum clocks and mechanical watches - many interesting white paperes
    Mumford Micro Systems - Micro Set - Watch, grandfather clock testing instrument with many examples - Horology products
    National Association of Watch and Clock Collectors - member web page links - Horological Science Chapter #161 -
    National Physical Laboratory - Time Metrology -
    Network Time Protocol (NTP) - The Electrical Engineering Computer Information Systems department of the Univ. of Delaware is the center of action for NTP.  The internet requires time synchronization in order to operate and this is handled by NTP.  You can also set your computer clock using NTP type software (but not the time server software on this server which is intended for the server rather than the client.  I am using a shareware program called
    NIST - Properties of Oscillator Signals and Measurement Methods
    Nixie-clock using neon lamps as logic elements -
    Nixie Tube Clocks -
    Novatech Instruments - DDS based T&F instruments
    Odetics - Zyfer - timing products
    Planetarium 200 - watch with planet positions shown
    Poseidon Scientific Instruments Pty. Ltd. (PSI) - sapphire instead of quartz oscillators
    High-precision timing references -
    Richard Karlquist - HP/Agilent Cs standards and related T&F stuff
    Royal Observatory, Greenwich
    Seiko Japan- US
    Sidereal Time -
    SocketWatch to set my computer's clock - now using TAC32 from TAPR & Motorola VP GPS rcvr
    Spectracom Corporation - 8182 NTP sync receiver & time of day, precise frequency, telecommunications products
    Spectrum Geophysical Instruments - time and frequency control
    Stocker & Yale - military wrist watches which conform to MIL-W-46374F
    Stable32 - 32 bit Windows time interval analysis software
    Standard & Daylight savings Time -
    Suunto Finland - Wrist top Computers - Altimeter, Barometer, Thermometer, Compass & Watch
    Symmetricom - telcom time standards
    Syntonics Corp - Precision quartz oscillators (Tom Clark is Senior Scientist here )
    Temex Neuchâtel Time - Synchronized Rubidium Oscillator and other precision sources
    Time and Frequency Solutions Ltd.Timing Technology - Time Transmission and Reception. -  National Time and Frequency Broadcasts -
    Time and Frequency Measurement by John Ackermann
    Time & Frequency Standards - all Japanese
    Time Conventions from JPL that includes outer space -
    Time-Nuts -- Precise Time and Frequency for Amateurs
    Time to Digital converters -
    Time Tools - GPS and MSF NTP receivers
    Time Zone - Watch Forums - Ulysse Nardin Marine Chronometer 1846 - a modern 28 jewel self winding work of art
    Tom Clark WB2TNL - ftp files - GPS contains timing stuff, counters.pdf schematic, SiRF stuff, VLBI at Haystack Observatory - VLBI allows very precise time-location determinations
    Timing Technologies - Cyberchron, Locus, Naviman, Starlink, Zyfer
    True Time - T&F product manufacturer 2003 -> now part of Symmetricom
    WWV/WWVH usabilities near Washington DC -
    Vigilante Electronics  - has used Rb oscillators
    Waltham Aircraft Clocks -
    Watches & Clocks -
    Wenzel Associates, Inc. - precision crystal oscillators with very low phase noise - Techlib.com - for amateur scientist and technical hobbyist
    World of Watches -
    Zyfer - GPS timing solutions

    Online Clock Web Pages

    Italian erasing Date & Time -
    People Photographs Time -
    Nixie Tube Data & Time -
    next LORAN-C TOC for 9940 -
    Current UT1-UTC values -
    Timeline -
    Balls & Bars -
    Ying-Yan Clock -
    NIST Official U.S. Time -
    A collection of clocks that can be used as screen savers
    Javascript Clocks -
    Multi slider (like a slide rule) 7 sliding horizontal scales - showing your local time
    Time Flow Clock - http://home.tiscali.nl/annejan/swf/timeline.swf
    Matchstick Clock - Artist's page - animated view - uses over 150 hour hands.  They are in 12 groups and speed out numbers.  For example at the 4 o'clock position there are 15 hands and once per hour they are arranged as FOUR.
    Java based LST Clock Applet - but it's for the local time zone, not some exact location (like where a telescope is).
    Men walk stand or sit to form analog or digital clock (click on clock to toggle mode)

    Hands On Digital Clock - 4 x 12 meter 7-segment display where the segments are boards held in place by C-clamps.  One man can change one segment, but it takes 11 men and a couple of ladders to change two digits.  If fewer men were used the time to make a change would be longer than 1 minute and the clock not be good to a minute.  It looks like they started out using 4 bolts at each joint with power tools and later changed to multiple C-clamps.  You can buy a DVD movie  that can be played on a PC where the video is synchronized to the PC time.  This is not an online clock, not a hardware clock, hard to classify.

    Hardware Clock Web Pages

    There is a limit on how good you can make a pendulum clock that comes about because of the change in gravity caused by the Sun and Moon.  This was observed by Alfred Loomis in 1931 presented to the American IEE and the Royal Astronomical Society in 1932 papers.  A good book about Loomis is "Tuxedo Part".  He also has a chapter in
    Biographical Memoirs V.51 (1980) published by National Academy of Sciences.  For more see the paper Lunar/Solar Tides and Pendulum Clocks (part 1).
    Alfred Loomis Patents:
    1435073 Gun Mount
    1409304 Chronograph
    1376890 Chronograph
    2884628 Long Range Navigation System  (LORAN)
    He also patented other things, but these I find interesting.
    Resistor Color Code Clock -

    My Clocks

    Note about 3 Clocks photo

    Three Clcoks

    The Standard Electric Time Co Slave Clock is driven by the Piexx IMP2 at 1 pulse/minute.
    The Self Winding Clock  Co pendulum clock winds itself twice per hour using a couple of "D" batteries, and has no hourly sync pulse.  It's running about 2 minutes slow /day so in the photo below it's slow about 10 minutes.
    The FTS4060 Cesium standard is running about -3E-14 and is driving my PC4 clock not shown in this photo.
    The two books in the above photo are The writings of Thomas A Edison Vol 1 and Vol 2.  They cover his work on stock tickers that took him from pennyless to very rich and also his work with telegraphy.  My interest in wet batteries led to the telegraph which led to clocks that use electromagnets (need to make this page).

    2 Clocks
    The Self Winding Clock Co "Western Union clock on the left is running on it's pendulum.  The Standard Electric Time Co. slave clock is driven by the IMP2 slave clock pulser that's based on a watch crystal.  The photo taken at 08:10:30. They both were set a week or so ago.  The pendulum is slow about 2 minutes and the crystal is fast about 3 minutes.  So although a crystal based oscillator has the potential to keep very good time it does not always happen.

    Standard Electric Time Co. Slave Clock - now running from a Piexx pulser
    Self Winding Clock Co - "Naval Observatory Time, Western Union" -
    Second SWCC clock is a model 37SS with a Sweep Second hand (that means the hand turns from the center of the clock, i.e. it's not a small seperate hand).  This one needs a lot of work.
    Third SWCC clock is the same model as the first one.   Got for a good price because the prior owner burned out the motor by connecting 400% voltage.
    Forth SWCC is another model 37SS that has not arrived yet.

    Four Letter Word + GPS Time

    This clock displays a new four letter word once each 10 seconds and the time for 10 seconds starting at the top of each minute based on a GPS receiver.  The time is underlined (Fig 5).

    Fig 1 EATS
    Four Letter Word + GPS Time
    Fig 2 PERK
    Four Letter Word + GPS Time
    Fig 3 OVAL
    Four Letter Word + GPS Time
    Fig 4 SPIT
    Four Letter Word + GPS Time
    Fig 5 11:10
    Four Letter Word + GPS Time
    Fig 6 WOGS
    Four Letter Word + GPS Time
    Fig 7 WHOA
    Four Letter Word + GPS Time
    Fig 8 KEEL
    Four Letter Word + GPS Time
    Fig 9 TYPE
    Four Letter Word + GPS Time

    Other Hradware Clocks

    Pictorial Guides to Synchronome Movements, Cases and latches -
    Clemens Riefler Astronomical Clock -
    Electric Master Clocks -
    Pendulums - a web page about pendulums where the period is the key parameter.
    Abbey Clock Clinic - good info including a free eBook on Clock & Watch escapment design.
    Sync-netic - makes parts for master clocks

    Tower Clocks

    Tower Clock Collection 1 - 2 - 3 - 4 - 5 -

    Bank Vault Time Locks

     Bank Vault Time Lock Collection

    Electronic Hardware Clocks

    Cathode Corner - CRT & Nixie Clocks
    Clockvault - Riefler and Shortt Clocks -
    ESE - modern Master - Slave clock systems including IRIG Time Codes
    history of the evolution of electric clocks -
    NixiChron - Nixie Tube Clocks and the Nixie Wristwatch
    Nixie Clocks.de - Scope Clock
    Dutchtronix AVR Oscilloscope Clock  - $35 kit uses Philips RTC PCF8563 IC, ATmega168 uC and 32.768kHz crystal (low end DSO does not work)
    Piexx - Driver for Secondary Clocks -
    Precision Horology -
    Real Nerds - Binary Clock & Electronic Sand Timer
    Sapling - modern Master - Slave clock systems
    Scope Clock DG7 tube and PIC16F876
    Electronic Vector Clock -
    Zetalink - GeekKlok - Four Letter Word - various types of clocks
    PIC® Electronic Projects & Kits - most of the clocks include a C-Max WWVB receiver.

    Hardware Watchs

    ETA - movements used in many watches
    Otto Frei - watch makers supplies - ETA 2892-A2 watch kit - ETA 2824-2 Kit or Assembled watch -
    National Association of Watch and Clock Collectors -
    TimeZone - watch info
    Time Savers - Electric clock battery adapters and watch maker tools
    The Dial House - repainting clock dial faces

    Brooke's Home Page

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