One frequently hears FM radios on the VHF bands that high or low in modulation level which exacerbates the problem of copying stations whilst mobile.
The defence often given is that it is so hard to measure frequency modulation, that it take an expensive deviation meter and they are scarce.
This article explains how to make accurate measurements using equipment often found around ham shacks, and could certainly be cobbled together from the resources of a few ham shacks. The figures and example given apply to nominal 25kHz channeled radios, adjustments are need for narrow channel radios.
There are three steps where calibration is progressively transferred through a measurement chain:
- calibrate a modulator (an ordinary FM transmitter);
- calibrate a demodulator (an ordinary FM receiver) using the calibrated modulator;
- measure the unknown transmitter using the calibrated modulator.
1. Calibrate a modulator
The usual method of calibrating a modulator is to use the spectral properties of an FM signal.
One could use a spectrum analyser to find the calibration point, adjusting the modulation level and detecting the null of the carrier or sidebands according to the Bessel function.
Since the instrumentation is used to detect the null of a carrier or sideband component, and the null is very sensitive, a narrow band receiver can be used for the calibration procedure.
A practical approach
This is a procedure to calibrate a frequency modulator at a single modulating frequency using an SSB receiver to detect the first carrier zero.
- Prepare to modulate the carrier source (the transmitter) with a 1kHz (exactly) sine wave modulation source, adjust to zero modulation level and key the transmitter up.
- Couple a small amount of the carrier to an SSB receiver and tune in the carrier to a beat note of about 800 Hz.
- Slowly increase the modulation until you hear the carrier beat disappear. Carefully find this null position of the carrier beat note. Note that you will also hear one or more sidebands when the modulation is applied, ignore these and just listen for the null of the carrier.
The modulation index is now 2.4, and therefore the deviation is 2.4kHz.
The technique is very sensitive and very accurate, and error will mostly be attributed to the accuracy of the modulating frequency.
You have read about it, click to listen to a demonstration. This demonstration uses an SSB receiver with a 3.5kHz IF bandwidth, but I have used the technique with receivers with a 10kHz IF bandwidth, you just hear more of the sidebands, but concentrate on the carrier beat and null it out. The test receiver could be a high quality communications receiver or a scanner with a BFO. You could sample the modulated signal at the carrier frequency, or by sniffing some signal from the IF of a super-heterodyne receiver.
2. Calibrating a demodulator
Having calibrated a modulator, we can set a receiver up to demodulate that signal and calibrate its output voltage against the known deviation of the source.
Above, an oscilloscope is connected to the receiver output and the volume control is adjusted until the peak voltage is 2.4 divisions, corresponding to peak deviation of 2.4kHz.
3. Measure the unknown transmitter
Having calibrated the receiver and display, we can go on to measure the unknown transmitter(s).
Above is a capture of speed input to the transmitter under test. It can be see that most of the time, peaks are reading the equivalent of 3kHz, very occasionally reaching 4kHz.
But isn’t it actually PM… or what about pre-empahsis and de-emphasis?
Well, although everyone refers to it as FM, it is FM with 6dB/octave pre-mphasis over the entire speed spectrum which gives the characteristic as PM. The demodulation process approximately equalises the pre-emphasis so the audio channel is nearly flat from end to end.
To a certain extent then, it is a nonsense to talk of peak deviation of frequency in kHz when in fact it is PM and the peak deviation is phase in radians. Nevertheless, that is the convention.
The process above adjusts the transmitter for speech drive equivalent to 3kHz peak deviation at 1kHz. Many test sets used in the Land Mobile Radio field measure deviation in a non de-emphasised demodulator, some offer a choice of with or without de-emphasis. In practice, it makes little difference on voice tests of male speakers.
The procedure above was compared with that obtained using a Motorola R2009D Communications Monitor, and the results are consistent.
It is important to understand that FM Land Mobile Radios often (usually) include a peak limiter in the tx audio path, and there is likely to be two adjustments, one prior to the limiter often labelled “mic gain” and another between limiter and modulator often labelled “deviation”. The deviation control is usually adjusted so the peak deviation does not exceed 3-4kHz under the loudest speech peaks, and the mic gain is adjusted so that normal speaking levels cause 3kHz peak deviation (equivalent).
If stations report your deviation is high or low, firstly make sure you address the microphone properly and speak with a consistent and strong voice. If it turns out your voice is unusually soft or load, it is probably the mic gain control that requires adjustment rather than the deviation control, yet people tend to thing “low deviation – wind the deviation control up”.
Not the only way
This is not the only way to achieve the outcome, but an example to show that practical tests of good quality can be designed to exploit available equipment. This is after all, the role of the measurement technician.