Adjusting Foxtrak tx audio using an SSB/FM receiver exploiting Bessel zeros to calibrate the modulator

This article explains a method of setting the tx audio level of a Foxtrak using an SSB/FM receiver exploiting  Bessel zeros to calibrate the modulator. Whilst the Foxtrak is used in this example, the method can be used with any tracker or TNC that allows single modem test tones to be transmitted.

The concept behind the test lays in the spectral content of an FM signal which comprises a carrier and many sidebands. The characteristic that is exploited in this test is that the carrier amplitude is zero at modulation index 2.4 (modulation index m=Deviation/ModulatingFrequency) under sinusoidal frequency modulation.

Calibrating the modulator

The first step is to set the audio level for a known deviation level, the Foxtrak’s 1.2kHz tone will be used.

Using Bessel zeros, we know that the first carrier zero occurs with m=2.4, and if the modulating frequency is 1.2kHz (the low tone), then deviation will be 1.2*2.4=2.88kHz.

The low tone is activated from the serial port using the command ESC T 0.

So, using an SSB receiver to monitor the carrier beat note, and winding the audio level up from zero to find the first carrier zero, we have set the deviation to 2.88kHz.

This is quite an accurate measure (though subject to the frequency accuracy of the test tone, about 1% for Foxtrak).

Adjust to 1.64kHz deviation on the low tone

Now 2.88kHz is too much deviation for our needs, we only want 1.64kHz, or 57% (-5dB), so we switch the monitoring receiver to FM and measure the audio output due to 2.88kHz deviation.

We then reduce the tx audio level to reduce the measured receiver output to 57% and then we have 1.64kHz deviation.

Confirm deviation on the high tone

As a check, we can change test tones to 2.2kHz and we should get close to 3kHz deviation, subject to the accuracy of the pre-emphasis and de-emphasis curves of the tx and rx.

The high tone is activated from the serial port using the command ESC T 0.

Show me

So, here is the video…


Tx audio drive can be adjusted with excellent accuracy using an SSB receiver and exploiting Bessel zeros.

It is only one of many ways to achieve the outcome, and it may suit the equipment you have at your disposal.