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. Continue reading Adjusting modulation level on FM mobiles etc.