Brooke's NRD-545 page

There is a report in the Passport to World Band Radio that says that the DSP chip in the 545 has digital noise crosstalk that causes the ultimate rejection to be tens of dB lower that it should be. This may be the case, but the reason that I got this receiver is that it has the narrowest IF filter bandwidth available at 10 Hz. This allows listening for the carrier of an AM station or the carrier of a TV station. It also allows decoding WWV's 100Hz digital data by reading the s-meter with a computer.

CW

This is one of the earliest modes and consists of turning the Continuous Wave carrier on and off using Morse Code. The bandwidth needed is the narrowest of the commonly used modes and most receivers have a 500 Hz wide IF filter for CW that also works for narrow shift RTTY.

A disadvantage of CW is that when the key is up no power is being transmitted. FSK has an advantage in that power is always being transmitted so it's easier to define the two conditions (zero or one).

Coherent CW

The idea was to make the on/off and off/on transitions at times known to the receiver allowing better demodulation that when they occur at random times. But it did not have any control of the phase of the transmitted signal, which would improve the s/n even more. Far better in terms of s/n are the Pseudo-random codes and spread spectrum for getting the most signal for the power used, although at the expense of modulation low bandwidth.

AM

Amplitude Modulation is used by medium wave and short wave broadcast stations as well as aircraft. It changes the carrier amplitude with the loudness of the modulation signal. This is probably the oldest method of broadcasting voice and when viewed on a spectrum analyzer there is a lower sideband with voice modulation, a fixed carrier and a upper sideband with voice modulation. It can be detected by a simple crystal radio or by using the LSB or USB mode on a modern receiver. Doing that has an advantage if there's interference on either the upper or lower sideband. It's an easy listening mode in that when the speaker's voice stops and there's silence the radio goes silent. This is because the carrier remains when there is no voice or music to modulate the sidebands.

AM has another advantage in that if there are two stations on the same frequency and they are at the same power level a receiver will hear both stations. This is not the case with FM modulation where the stronger station completely suppresses the lower power station. That's an unacceptable safety consideration for aircraft.

FM

In Frequency Modulation the frequency of the carrier is changed with the loudness of the modulation signal. Since the carrier is always on at full power the audio output will be quiet when there is no modulation signal. This is the mode used on the VHF broadcast band. The modulated signal has a bandwidth that on the order of 100 kHz, much wider than the bandwidth of a music modulation hence there is some process gain that works in a similar way to antenna gain.

Narrow band FM that's used for utility communications has a signal that's 5 kHz wide and a voice signal is about 3 kHz wide so it doesn't have much process gain, but you can get a lot of channels into a given slice of the spectrum. In this application the FM capture effect that pervents hearing two stations at the same time is a benefit because the other station is most likley one you don't want to hear.

SSB

In Single Side Band modulation either the lower sideband or the upper sideband that's in an AM transmission is sent to the antenna and the carrier is supressed completly. This makes SSB verry efficient in terms of getting the highest radiated signal for the power used. But it requires much higher frequency stability in the transmitter and the receiver.

If you were listening to a SSB radio and music went soft the noise level would come up, not at all an entertainment type of mode.

FSK

Frequency Shift Keying is a way of sending digital data. The transmitter is at one frequency for Mark and another frequency for Space. It's much easier to make the binary decision with FSK than with CW where one of the conditions is the transmitter is turned off. This mode was used to send Radio Teletype messages.

MSK

Minimum Shift Keying is a version of FSK where the shift is as narrow as possible. It's used on LF, VLF and ELF stations to keep the transmitter frequency within the bandwidth of the high-Q antenna matching network.

PSK

Phase Shift Keying is similar to FSK except it's the phase of the transmitter that's being modulated.

Spread Spectrum

By making the transmitter signal much wider than the modulation signal bandwidth there is a process gain that has the same effect as using more RF power or an antenna with gain. This is used in satellite TV systems where the RF bandwidth is on the order of 36 MHz compared to a signal bandwidth of 5 MHz. In the GPS system the signal level at the Earth's surface is below the ktb noise level, it's only after demodulation of the pseudo-random code that the s/n comes up. Note that since all the GPS satellites transmit on the same frequency they differ in what pseudo-random code each satellite uses so the codes are chosen to have very low cross correlation. This also means that the s/n radio will never be very high because there's always going to be interfering signals from other satellites.

Modern Digital Modes

Early telephone computer modems used simple FSK and could only achieve limited data speeds on the plain old telephone system (POTS). The next generation modems used a modulation that sounded like noise and could support much higher baud rates. The latest Digital Subscriber Line (DSL) modulation uses frequencies above hearing (allowing the POTS telephone to still work on the same line) and is adaptive in that it changes what frequencies are used depending on line condition. It's not uncommon when there's trouble on the line for the POTS phone to quit working while the DSL signal still gets through (although at a slower speed).

NRD-545	Cable	Computer
DB-25(m)		DB-9(f)
2 Tx	Red	2 Rx
3 Rx	Green	3 Tx
4 RTS	White	7 RTS
5 CTS	Black	8 CTS
7 Gnd	Shield	5 Gnd
		1, 4, 9*

JRC NRD-545 Receiver

Background

The Setup

Latitude: 39:11:24.692 N Longitude: 123:09:50.548 W Altitude: 249.7 meters 819' 3" (WGS-84)

Modulation Modes

CW

Coherent CW

AM

FM

SSB

FSK

MSK

PSK

Spread Spectrum

Modern Digital Modes

Modulation Test Equipment

Oscillioscope

Spectrum Analyzer

Modulation Domain Analyzer

Specalized Analyzers

Cable

JRC NRD 545 Controlling Software N545PRO.EXE

NRDWIN Spectrum Plots

Low Frequencies

AM Broadcast Band

FM Broadcast Band

Wide Band Scans

44 Meter Aero & Fixed

34 Meter Maritime Coastal

22 Meter SW BCB

19 Meter SW BCB

LabVIEW

Modification

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545

Decoders & Computer control

Multicoupler (Rx)

Antenna Coupler (Tx)

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