bottom & rear
Bottom & Front of 2709A10766
Back in the days of tube type equipment there was an HP 415 SWR (Standing Wave Ratio) meter and there have been a number of versions ending with the latest "E" model. This instrument is a very narrow band 1 kHz attenuator and amplifier and a precision rectifier. The relative amplitude of the 1 kHz signal can be read to scale ticks of 0.05 dB and interpolated to 0.01 dB. There is also a scale for the original purpose of reading SWR. The SWR scale was THE feature that separated the HP instrument from the GR 1232A Tuned Amplifier and Null Detector which took a lot of work when used to measure SWR. It makes use of what some call the Hall Network (pdf) "The only known commercial use of the Hall network is as a tunable band-pass filter for the 1961 General Radio product, Type 1232-A Tuned Amplifier and Null Detector (References 5 and 6). An examination of the manual for this instrument reveals that all capacitors are the same value. This product is still sold today by IET Labs, Inc. (http://www.ietlabs.com/) as part of the GenRad 1620 Capacitance Bridge." Maybe Ken did not know about the HP products that licensed the GR patent 2173427?
Note that if Full Scale is set on the "zero" range then when reading down 10 dB on the 60 range you are reading a signal that's down 70 dB.
The 415E is 6" high and 7 3/4" wide. This does not appear to be a standard Rack Mount instrument size.
June 2022 noticed an HP 200C (HP 1943 Catalog pg 12) on eBay (TS-312/FSM-1) and the label on the back referenced the same two patents as are on the HP 415A: 2173427 & 2268872.
It makes sense for the Hewlett patent 2268872, to appear on the oscillator that makes use of it, but why on the 415 SWR meter where only the GR patent (2173427) is being used? Maybe that was part of a cross licensing agreement between HP and GR? That would explain why the GR patent also appears on the HP audio oscillator where it is not used.
Let me know about the FSM-1.
The HP 3550A Portable Test Set was in the 1965 Catalog and consisted of the 204B oscillator, 353A Patch Panel, 403B Voltmeter all mounted in the 11046A portable case. Intended for telephone related testing. Maybe the FSM-1 was a similar test set?
I started this web page in 2003 because I like the HP 415E. The main reason is that I think of it as a AC voltmeter with a very narrow pass band filter centered at 1 kHz. Because of kTBR noise (Wiki) the narrower the instrument bandwidth the better the signal/noise ratio (Wiki). This also applies to radio receivers and spectrum analyzers.
Another reason is that I spent many hours with an HP 415E and a slotted line (Wiki) plotting the impedance of the circulators used to make Tunnel Diode Amplifiers (Wiki). About a year later HP came out with the HP 8410 Vector Network Analyzer that could show a real time plot of S11, i.e. the impedance of the circulator so the SWR meter and slotted line were relegated to the store room.
I searched for a patent on the HP 415 and for 19 years had no luck. Notice that the 415 Model History section has no mention of any patents because none of the HP 415 literature mentions any patents. But then found it was patented by Herman Hosmer Scott (Wiki) (who was working for General Radio). He later started a company that made High Fidelity stereo amplifiers under the trade name H.H. Scott (Wiki). That prompted me to get the HP 415A shown on this web page.
The HP 415A has a label that says: "Licensed by Western Electric Company, Inc. Under U.S. patents of American Telephone and Telegraph Co. and Western Electric Company, Inc. and by Radio Corporation of America.
(See Label Inside)
If this set contains a Resistance Tuned Oscillator, it is licensed under U. S. patent No. 2,173,427 and manufactured under U.S. patent No. 2,268,872."
I'm guessing that during W.W.II and for some years afterword the first sentence was boiler plate credit to AT&T, WE and RCA. But the second sentence is very specific to the HP 415 SWR meter. Since the HP 415 does not contain a resistance tuned oscillator the 2173427 Hewlett patent that was the basis of founding the company Hewlett-Packard (and probably why Hewlett is the first mentioned name) that patent has no relevance to the HP 415 SWR meters (Wiki: Wien bridge oscillator, HP HIstory, HP 200A).
It's the 2268872 patent by H.H. Scott working for General Radio that's the heart of the HP 415 SWR meters. It is for what I'd call an active filter (Wiki) circuit that has the ability to implement either a very narrow pass band filter (the HP 415 SWR meter, GR 760 Audio Analyzer, &Etc) or a very narrow stop band filter, or an audio oscillator.
It turns out that the HP200 series of audio oscillators make use of GR patent 2268872 in the positive feedback portion of the oscillator and the Hewlett 2173427 light bulb patent in the negative feedback portion of the oscillator. While this dependence on two patents is clear from the labels on the HP 200 series audio oscillators as well in the HP patent this is the first time I've been aware of it.
This example is marked:
|Licensed by Western Electric Company, Inc.
Under U.S. patents of American Telephone and
Telegraph Co. and Western Electric Company, Inc. and by Radio Corporation of America.
(See Label Inside)
If this set contains a Resistance Tuned Oscillator, it is licensed under U. S. patent No.
2,173,427 and manufactured under U.S. patent No. 2,268,872.
115 Volts 50 - 60 Cycles
Manufactured in the United States of America
Palo Alto, California (R)
Indicator, Standing Wave
Supply: 115V or 220V 1(phase) 60 (cycles)
A unit of indicator, standing wave AN/USM-37
Navy Department - Bureau of Ships
Palo Alto California
For the past 19 years (on and off) I've been trying to find the
HP patent for the 415 SWR meter. Today (2022 Apr 18) I
found a photo on an eBay ad for a 415A (it's on order) that
showed patent numbers 2268872 which is THE audio oscillator that
started HP. I don't understand why it's on the 415 SWR
meter? And another patent ending ....427. I could not find
any HP patents between 1941 and 1954 ending with 427. But
did find a photo of a USM-37 at NavyRadio.com
where, with a magnifying glass I made out patent 2173427
assigned to General Radio. The Inventor was H.H. Scott (Wiki).
PS I bought a new H.H. Scott stereo audio amplifier and it had so much noise out into my Voice of the Theater speakers that I traded it in for a McIntosh MC2505 (Wiki). The Scott amp probably would be fine when used with Acoustic Suspension loudspeakers (Wiki) but not with efficient speakers like the Altec Lansing (Wiki) Voice of the Theater speakers.GR made the 1232A Tuned Amplifier and Null Detector (IET: manual) based on thier patent 2268872. This requires tuning to the desired frequency unlike the 415 which has a single pre-tuned frequency. Also the 415 has a scale calibrated in SWR whereas with the GR 1232A you need to manually calculate the SWR (Wiki) which involves taking two square roots which in the 1950s was not trivial so the HP 415 was a great time saver.
|Fig 4 Note aluminum can marked 1 kc.
In theory it could be unplugged and replaced with a circuit for a different frequency.
|Fig 5 Terminal
415E Controls & Connectors
Front PanelThe analog meter movement is a mirrored type where you look for the reflection of your eye's pupil bisected by the needle and thus eliminate parallax in the measurement. There is also a mechanical meter zero pot. The correct way to use this is to turn the pot in one direction so that the needle moves up scale and and then comes back down scale and you stop turning when the needle is over "2" on the expanded scale. If you over shoot, do not turn the screw in the opposite direction, but rather, continue turning and approach the zero position from up scale.
The POWER switch has positions for OFF, ON, (and for instruments with the optional internal battery supply: Battery: ON, TEST and CHARGE).
The INPUT switch has positions for XTAL IMPED: HIGH, LOW or BIASED (+1.3 VDC on the center contact with 12.6 mA of current capability) and BOLOMETER: 4.5 mA or 8.7 mA. To match a wide variety of input sources. The HIGH position is also used for signal sources that are not from a detector or power sensor.
The INPUT is a BNC(f) connector with the shell 50 Ohms above ground.
The FREQ adjust pot allows the center frequency of the 415 to be set to match the frequency of the source generator.
The BANDWIDTH control adjusts the width of the pass band and therefore also controls the signal/noise ratio for faint input signals. If the 415 is used with a sweeper this control should be set fully CCW to increase the BW to allow for signal variations as the sweep progresses. Also the amplifier output should be used to drive a scope or chart recorder instead of the recorder output since the latter has a low pass filter that limits it's bandwidth.
The RANGE-DB control has an outer knob with positions for 0, 10, 20, 30, 40, 50, and 60 dB a very large dynamic range, and an inner EXPAND knob with positions of: NORM, 0, 2, 4, 6 and 8 dB. By using the expanded (red) scale positions you can read amplitude to 0.01 dB over the full 60 dB range. On the 0-NORM range you can bring the needle to full scale with an input signal between 0.120 VAC and __?__.
Comparing the 34401 DVM with the 415E shows that the noise bandwidth of the 34401 limits its use:
FS 10 range
FS 20 range
FS 30 range
FS 40 range
FS 50 range
FS 60 range
can not get signal this low
* 34401 is reading noise and 415E has a Full Scale reading.
The Gain & Vernier controls will bring the needle to Full Scale with an input of more then 0.12 Volts up to more than 5 Volts.
Square LawNote that the 415 reads 10*LOG(voltage ratio), this is called "Square Law" and when the 415 is fed from a microwave detector that has a low level signal at it's input the 415 will read the same as a power meter because the detector has a "square law" response. If you were trying to measure the dB difference in two voltages you would normally use dB = 20 * LOG(voltage ratio), but that's not what the 415 was designed to do.
Rear PanelOn the rear panel there is a BNC(f) RECORDER output that has a 0 to 1 VDC signal to drive a chart recorder and a pair of banana jacks on the standard 3/4" centers that has the amplified 1 kHz signal. There is a covered hole that can be used to change the input connector from the front panel to the rear panel.
SWR MeterI used the 415E for making microwave Smith Chart plots in conjunction with a slotted line and for making attenuation and/or gain measurements using a crystal detector. The 415E is very good for both of these measurements and probably is the highest dynamic range scalar measuring instrument around. It would also work fine for making optical measurements with an optical to DC converter.
A key feature is the narrow (slightly adjustable) bandwidth. Note slotted screw pot at lower right of front panel. One article says the Q is 25 to 35 centered at 1 kHz, so in the range of 40 to 28 Hz.
Note that the 1 Hz Resolution Bandwidth of the HP 4395A spectrum analyzer goes a long way to seeing very weak signals.
Also see Microwave Test Equipment\Hot-Cold Noise Source.
I'm considering using it to measure the output of magnetic field sensors, where it's narrow noise bandwidth may greatly extend the sensitivity of the sensor.
When a 5.04 k Ohm resistor is connected to the input and the range set for Xtal Impedance High and both the gain and vernier knobs are turned fully clockwise the meter reads 6.5 dB below full scale. The manual addresses the theoretical KTBR noise (Wiki) of a 5 k Ohm resistor and how the 4 dB noise figure of the 415E will show up in the measurement. This shows how sensitive the meter is.
Typical uses for the 415E are: SWR measurements using a slotted line, Power measurements using a Bolometer (Wiki), Relative power measurements using a crystal detector. In all cases the source must have a 1 kHz square wave modulation.
When a short is installed on the unknown port and the slotted line (Wiki) carriage is moved back and forth there should be a couple of nulls a half wave apart. This means as the test frequency gets lower the slotted line needs to get longer.
Record the location of a null. Install the Device Under Test (DUT) and find the nearest null. The movement of the null and it's direction (either toward the DUT or away from the DUT) corresponds to be rotation on a Smith Chart (Wiki). The SWR will determine the radial distance on the Smith Chart. This is relative to the transmission line impedance of the slotted line, typically 50 Ohms.
Slotted line in patent 2673963 below.
The label says:
Palo Alto, California
Let me know what the FSM-1 is.
The 200CD Wide Range Oscillator with a military label: NSN: 6625-518-4659 AF Contract No. AF 36/600/12149 also has U.S. Patent 2852675 on the serial number 212-38626 label.
That is the Bernard Oliver - HP balanced oscillator patent.
Fig 1 Front
Fig 2 Back
Fig 3 Back Inside
range switch behind power transformer
Fig 4 Bottom either C4 or C10 missing under bracket.
Cut white wire near line cord. So not complete.
While the photo below is from an HP 330B Distortion Analyzer I found on eBay, I expect the HP 300A and 320 Distortion Analyzers also make use of the GR patent.
HP 330B Distortion Analyzer
Inside upper left corner of right side panel just above the power transformer.
U.S. Patent No. 2268872Licensed under U.S. Pat. No. 2173427, also licensed by Western Electric Company, Inc., under U.S. patents of American Telephone and Telegraph Company and Western Electric Company, Inc., for use only for testing or measuring apparatus or phenomena or for other scientific or technical investigation, development, or instruction, except for testing or measuring physiological phenomena and except as part of or in association with apparatus for the transmission or reception, by wire or radio, of intelligence of any kind, other than in association with, but not as part of, radio broadcast transmitting or receiving systems for testing and measuring the transmission or reception thereby of programs.
MODEL _________________ SERIAL _____________
Palo Also California
Also see the Boonton Q-meter for similar licensed by labeling.
HP Journal Aug 1951, Vol 2 No. 12 Distortion Measuring Equipment by Burnton Bauer: 300A, 320B, 330A
2418568 Frequency divider, Bauer L Brunton, HP, 1947-04-08, - Fout =[Fin* (n+1)]/n
There were as many of these HP slide rules as there were technicians and engineers or maybe even more. They were mostly used with reflectometer setups for measuring reflection coefficient and doing calculations related to that. During the early years at Aertech we mostly used HP 690A and 8690A BWO sweepers.
Slide rule set at 14 dB RL = 1.5 VSWR.
That point also appears on the Allis Associates coffee cup.
HP App Note 183 "High Frequency swept Measurements", Dec 1978 is mentioned on the slide rule. The 8620 Sweeper is featured along with the 11691D Directional Coupler and 423 Detector in the ALC loop, covering 0.1 to 18 GHz. Also mentioned is the 8755 Scalar Network Analyzer. The 415E, 432A and 435A are also mentioned.
The Slide Rule description starts on pdf page 24 (doc pg 18). Fig 19 shows the slide rule set for 1.5 SWR (14 dB) as in the photo above. Mismatch Error Limits starts on pdf page 46 (doc pg 34).
App A covers 2 and 3 resistor power splitters.
App B covers Errors in Reflection Measurements.
App C covers Errors in Transmission Measurements & mentions the 8750A Storage Normalizer that was supposed to replace grease pencils.
While B and C do not mention S-Parameters the diagrams have the look and feel of S-Parameters.
HP licensed the use of GR patent 217342 for the HP 200 series of audio oscillators and for the HP 415 SWR meter. Both the HP 200 audio oscillator and the HP 415 SWR meter make use of the GR patent. I think most people do not know that the HP 200 series audio oscillators also depend on the GR patent. Note H.H. Scott patents 2173426 and 2173427 were issued the same day and are seminal in the filed of active circuits.
2106785 Electric filter, Herbert W Augustadt, Bell Labs, 1938-02-01, 363/45; 330/202; 330/123 - balanced bridged-T?
2163403 Stabilized oscillator, Larned A Meacham, Bell Labs, 1939-06-20, 331/139; 331/140; 333/188; 331/66; 331/183; 333/17.1- Bell Labs patent 2319965 explains how there's a better way, such as 2173427.
The Bridge Stabilized Oscillator BSTJ (vol17-1938/articles/bstj17-4-574.pdf)
"The frequency controlling resonant element is used as one arm of a Wheatstone resistance bridge."
2173426 Electric system, Scott Hermon Hosmer, (Wiki HH Scott: Company, man), General Radio, 1939-09-19, 330/98; 330/85; 330/106; 330/109; 330/128; 333/171; 330/75; 330/96; 330/108; 330/127; 330/142; 333/172 -
"One of the objects of the present invention is to provide a selective electrical circuit, the response of which may be varied over wide frequency ranges without complicated tuning and switching mechanisms and without any serious shifts in the input and output impedances of the system or its sensitivity. One of the important features of the invention is that this result may be obtained without necessitating the use of any inductances whatsoever, using a frequency determining system which may consist of resistances and condensers only. The frequency to which such a system is selective can, accordingly, be varied by merely varying resistances or condensers, or both resistances and condensers."
This patent is on the label of the GR 760-A Sound Analyzer, photos at left.
2173427 Electric oscillator, Scott Hermon Hosmer, General Radio, 1939-09-19, 331/138; 315/163; 330/86; 330/100; 330/106; 331/140; 331/142; 333/170; 330/104; 330/109; 331/141; 331/175 - (on back of 415A) - Filters in feedback networks.
"A chief object of this invention is to provide a simple and satisfactory means of obtaining an oscillator which will provide a sinusoidal output voltage, but which does not contain any inductances as necessary circuit elements in order to obtain this sinusoidal wave-form."
This patent is at the heart of both the HP 200 series audio oscillators and the HP 415 SWR meter. That's why it is printed on their labels.
aka: HP 200C
Licensed by Western Electric Company Inc. under U.S. patents of American Telephone and Telegraph Co. and Western Electric Company Inc., and by Radio Corporation of America.
See Label Inside.
If this set contains a Resistance Tuned Oscillator, it is Licensed under U.S. patent No. 2173427 and manufactured under U.S. patent 2268872.
115 Volts Serial No. S2014 50-60 Cycles
2268872 Variable frequency oscillation generator, William R Hewlett, HP Inc, 1942-01-06 331/141; 331/183 - THE HP patent (Wiki) with light bulb stabilization.
Makes use of positive feedback (GR patent) for determining the frequency and negative feedback to reduce distortion using a light bulb. This clearly is making use of the GR patent 2173427 for frequency selection.
The level of oscillation is set to a low enough level, by means of negative feedback, so that there's very little distortion. It is kept at that low level even though different tubes may be installed, or at different temperatures, or over component drift over time &Etc. by means of the non linear properties of the lamp (R3) in the cathode circuit of tube (10).
See: A real gem: HP's audio oscillator patent turns 60 - Model 200A (Wiki)
Photos at left of the TS-312/FSM-1 which is an HP 200C audio oscillator.
2319965 Variable frequency bridge stabilized oscillator, Raymond O Wise, Bell Labs, 1943-05-25, 331/142; 331/176; 331/183; 66/172E -
2173427, H.H. Scott
Called by the Oliver/HP patent 2852675
2583649 Modified wien-bridge oscillator, William R Hewlett, HP, 1952-01-29, 331/141 - to extend upper and lower frequency limits. Calls the same patents as 2583943.
2583943 Modified wien-bridge oscillator, William R Hewlett, HP, 1952-01-29, 331/141 - to extend upper and lower frequency limits.
2072946 Harmonic analyzer, Paul O Farnham, RCA, 1937-03-09, 333/171 - this is an RCRCRC phase delay circuit, not an active filter.
2093665 Star and delta connection of impedances, Tellegen Bernardus Do Hubertus, RCA, 1937-09-21 333/171; 330/176; 333/170; 333/216 - various circuits including bridged-T circuits, but no active filters.
GB524314 Improvements in and relating to thermionic valve oscillators and amplifiers, RCRCRC
2268872 THE Hewlett patent
2298177 Electric filter circuit, Scott Hermon Hosmer, GR, 1942-10-06, 333/171 - active RCRCRC
2354141 Universal resistance capacitance filter, Ellison S Purington, RCA, 1944-07-18, 333/170 - bridged-T passive
2444084 Resistance-capacitance oscillator, Artzt Maurice, RCA, 1948-06-29, 331/136; 331/144; 331/141 -
2446821 Resistance-capacitance tuned oscillator, Gassel William, Gritz Henry, Philco Ford, 1948-08-10, 331/135 -
2673963 Ultrahigh-frequency measuring apparatus, David Packard, HP, 1954-03-30, 324/645 - 809 (USM-37D) Universal Carriage
This carriage can be equipped with waveguide or coax lines to cover different frequency ranges.
* 805A is a long slab line two parallel grounds with a rod center conductor.
500 - 4000 MHz
500 - 4000 MHz
3 - 12 GHz
2852675 Modified wien bridge oscillator, Bernard M Oliver (Wiki), HP, 1958-09-16, 331/141; 330/109; 330/119; 330/82; 330/110; 330/121 -
HP Journal Feb.1956 Vol 7 No. 6 - 201C, 200J & 202C
Balanced R-C Oscillator circuit B.M. Oliver.
2852743 Electronic ratio meter, Nicholas L Pappas, HP Inc, App: 1954-07-23, 324/140D; 324/76.83 - by measuring relative to a reference signal variations in the signal generator amplitude are automatically cancelled. This probably was the HP 416 Ratio Meter.
The only patent related to the HP 415 that I could find as of 2021.
---------------- Other old HP patents ------3294988 Transducers, Packard David, HP, 1966-12-27, 374/177; 257/288; 257/467; 310/319; 374/178; 600/488; 600/549; 327/564; 327/509; 601/4; 257/254; 257/417; 257/785; 310/338; 374/E7.035; 374/E7.002 - piezo pressure or temperature - can be used for medical probe
I suspect all of these products are based on either 2173426 or
GR 760-A Sound Analyzer is based on the 2173426 1937 patent. (GR Experimenter Feb 1939: An Analyzer for Noise Measurement, H.H. Scott. "The new analyzer, however, has a constant percentage band width at all Frequencies.") (GR 760-B manual)
GR 761-A Vibration Analyzer (GR Experimenter June 1941: The Vibration Meter - A new electronic tool for industry, H.H. Scott)
GR 1231-B Amplifier and Null Detector (GR Experimenter Feb 1948) (1231-B manual, May 1961)
GR 874-VI Voltage Indicator - for use with 874 slotted lines and crystal detectors (1959 Catalog P, page 64, GR 11231-B page75)
GR 1232-A Tuned Amplifier and Null Detector (1961 Catalog Q, page 72)
GR 1234 SWR Meter (GR 1968 Catalog T, pdf page 126) (1234 manual, May 1968) Note the HP 415 came out in 1949. HP introduced the 12 GHz 8410 Vector Network Analyzer in 1967. (HP Memory). It worked with the HP 690 series of Backward Wave Oscillator (BWO) microwave sweep generators. Note that the parameters used prior to the introduction of S-parameters along with the HP 8410 were things like "y", "z" and "h" parameters that required either an open or short at the test frequency. These tended to cause solid state devices, like microwave transistors to break into oscillation at some random frequency, maybe much higher in frequency than the test frequency, thus making the related measurements near meaningless. Note there was a similar problem testing Tunnel Diodes until stable test fixtures were developed.
While looking at the GR offerings related to the microwave engineering I was doing in the 1965 time frame the GR offerings seem antiquated when compared to HP.
Page created 28 March 2003.