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Using a Spectrum Analyser

In this article I hope to show you some of the uses and advantages of computer-based spectrum analysis to discover the modulation of an unknown intruder (I would mention the disadvantages too, but I don't know of any). These computer programs (there are several and here are brief descriptions of them) combine a mathematical technique known as the Fast Fourier Transform (FFT) with the processing power of modern personal computers to perform a kind of analysis that was totally unavailable to ordinary people only a decade ago.

Just as you listened to a signal while tuning your receiver slowly through it using a narrow bandwidth in order to discover the spectral components of the signal, a spectrum analyser does this same sort of thing automatically and presents a picture of the signal spectrum, as "heard" in your receiver's audio output, so you can see right away how many channels there are, what is the frequency shift, is there any power supply "hum" on the carrier, and with a little practice you can guess at whether FSK or PSK is being used.

There are two common methods of display used by these analysers: one is an instantaneous picture of the spectrum which changes from moment to moment, in the same way as an oscilloscope displays a waveform; the other presents a history of the signal's spectrum over a short or long period of time by displaying succesive spectra in raster or "waterfall" fashion using different shades of gray or colour to show the amplitude of the signal at any one frequency and time. This second method provides a lot of information in one picture and is preferred for most purposes. All of the spectrograms shown here use the waterfall display method (the images here are also in reverse colour for easier viewing and printing).

On-Off-Keyed Carrier

Let's start with a simple case of on-off-keyed carrier. Sending continuous Morse dots or continuous dashes is a good example of a simple on-off-keyed (OOK) carrier. Some military stations use on-off-keyed carrier as a channel marker or as a synchronizing signal for their cipher equipment. In practice, the on and off periods are usually of equal duration, sometimes referred to "1:1 reversals". (A bit of useful jargon: the term "reversals" is often used to describe keying of the transmitter alternately between on and off, or between Mark and Space, or between 0 and 1.)

Spectrogram of on-off-keyed carrier. At right is the spectrogram of an OOK carrier being keyed at 38 Hz or 76 bauds. Frequency is displayed horizontally and time flows down the screen, like a waterfall. Notice how the carrier is accompanied by two spectral lines, one on each side, 38 Hz above and below the carrier frequency.  There are also many odd-numbered harmonics (x3, x5, etc) of these lines on each side of the carrier which are due to the fact that the carrier is 100% modulated and with fairly fast turn-on and turn-off times.

Well, this is an almost trivial example, but two related points should be noted. If the signal were in fact sending frequency-shift-keyed reversals, that is, alternating RTTY Mark and Space elements, the spectrum would look like two OOK carriers, one on the Mark frequency and one on the Space frequency. This is because FSK reversals are the same as on-off keying of the Mark frequency and off-on keying of the Space frequency. Such signals can often be seen; in fact they are more common than the simple OOK carrier that we started out with.

The second point is that an OOK carrier can often look like a carrier that is carrying no intelligence but which is amplitude modulated by "hum" in the power supply. Such hum might be caused by poor regulation of the power supply voltages to the transmitter so that they vary at the same frequency (usually 50 or 60 Hz) as the alternating current mains supply and/or harmonics of it. Routinely looking for power supply hum on any intruding signal can provide an important clue about the location of the transmitter, as few countries use both 50 and 60 Hz mains frequency.

Two FSK Printer Signals

Amateurs in Region 3 are often bothered by an RTTY signal which occupies 14212 kHz at odd times during the day. To my knowledge, no schedule has ever been discovered for these printer broadcasts and, although the broadcasts are often as short as a minute or two, the signal is strong.

Spectrogram of two printers on 14212 kHz Shown at right is a spectrum analyser display showing the transmission of two successive messages on 28 May 1999. The display spans a time of just over eight minutes. The first message began at 1147 Z and lasted about a minute and a half; the second began just after 1150 Z and lasted about two minutes. At any one time during these periods, you can see two broad vertical bands in the display representing the received Mark and Space frequencies of the RTTY signal. These spectral components are broadened in frequency by the keying transients of the transmitter and the shortness of individual Mark and Space elements. Notice how the transmitter operator tested the transmitter briefly on Mark and Space frequencies before beginning the message, resulting in a much more narrow spectral line for these periods when unmodulated carrier was being sent.  You can also see faint (weak) vertical bands on each side of the Mark and Space frequency; these are keying sidebands arising from the keying speed of the RTTY transmission. After the second message, the transmitter rested on the Space frequency (again sending unmodulated carrier) for about 20 seconds before going off the air.  Nearly horizontal smears across the display were the result of interfering signals present in the bandpass of the receiver.

Significantly, the spectrogram shows that the frequency shift used for each message was about 850-860 Hz but the center frequency of the FSK transmission was different for each message. Over a period of several months, it was found that there were two distinct frequencies being used for these broadcasts and on a few rare occasions transmissions were heard on both frequencies at the same time, proving that two separate transmitters were being used. In the spectrogram displayed here, the receiver tuning was not changed between messages so if you use the audio frequency scale at the bottom of the display, you can calculate for yourself the difference in center frequency between these two transmitters. If you find a difference of about 125 Hz, your eye is good.

But It Just Sounds Like Noise!

It can be difficult sometimes to tell the difference between some of the multiplexed digital signals, broadcast jammers, and other complex modulations now being heard daily on the air.  In fact the multichannel digital signals known as "voice frequency telegraphy" (VFT) which squeeze 8, or 12, 16 channels of data into one voice-grade channel (hence, the name) sound to the ear very similar to some of the jamming signals still used by a few countries.

Spectrogram of hum jammer from Iran The spectrogram at right shows a jammer which to the ear sounds just like some loud buzzing signal or indescribable noise. The base frequency of the display was 7058.5 kHz and the signal was recorded in USB mode, so the signal was on a center frequency of about 7060.1 kHz in our 40 m band. The jammer left this frequency at about 0150 UT. Using the frequency scale at the bottom of the display, you can see the many spectral components at intervals of 100 Hz with a smaller number at intervals of 50 Hz, and it is easy to see that the signal occupies a bandwidth of almost 2 kHz.
This signal is heard almost every evening (in the Americas) from Iran, using different frequencies in the 40 m band as it attempts to prevent reception there of a radio broadcast from Iraq. The Iraqi broadcast jumps around the band at odd times using frequencies at multiples of 10 kHz as it attempts to avoid the jammer.

Spectrogram of 8-channel VFT data signal. For comparison, the spectrogram at right shows a Russian (or Chinese?) VFT signal on 14154.3 kHz center frequency, recorded in May 1999. As the recording begins, the signal is sending traffic and you can see each of the 8 data channels, separated by 300 Hz (except 450 Hz between the two center channels). These 8 channels just fit into the 2.4 kHz bandwidth of the receiver and the maximum frequency span of the spectrum analyser. Soon after 1133 Z the traffic ended and the signal changed to an "idle" condition in which you can see two synchronizing lines separated by about 70 Hz in each channel. In this condition the signal will make a strong buzzing noise in your receiver.  Then, just before 1134 Z the signal went off the air. It is not uncommon for these VFT signals to remain in an idle condition for long periods of time, during which it is easy to mistake them for a broadcast jammer, unless of course you use a spectrum analyser to show the true nature of the signal.

If you could see this spectrogram in its original detail and resolution (the original image is 640 x 350 pixels), you would notice that each of the spectral lines appearing during idle condition is actually two lines separated by a very small frequency difference. I am told that this splitting of the spectral lines indicates the use of PSK modulation rather than FSK, but I do not know the explanation for this helpful observation.

Spectrogram of 12-channel VFT. There is another VFT data signal which is often heard coming from Russia or China. It is a 12-channel signal with 200 Hz spacing between channels and a pilot carrier 400 Hz above the highest frequency data channel. The spectrogram at right shows the highest 9 channels of the total 12, and the pilot carrier above them. The signal is believed to be PSK and, because traffic is being sent throughout the recording shown here (no idle periods), only a band of noise is observed in each data channel. In practice it is usual for the pilot carrier of these signals to be on a radio frequency ending in "x.3 kHz". It is believed that the pilot carrier might be a tuning aid and does not necessarily mean that single-sideband is being used.

I hope the examples above have given you some ideas on how a computer-based spectrum analyzer can be used to discover the structure of some of the weird intruding signals that invade our Amateur bands. With a litte practice you will become quite good at recognizing and identifying the common kinds of signal and with a little more effort you can "unwrap" the more complex signals too. Good luck!

In the next article we will describe the ITU emission codes and how to use them to describe the intruders that are often encountered in our Amateur bands.

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