Project of the Month

A deviation meter.

This project will turn your radio into a piece of test equipment by incorporating a deviation meter into it. As this will mean soldering a wire inside your radio, you should only attempt this project if you are comfortable with soldering inside your radio.

How it works

The audio output from the IF (intermediate frequency) chip in your radio is passed through filters and limiters before it is amplified for the speaker. If we sample the audio before the filters, the amplitude of the audio signal will tell us the deviation of the received signal. We will pass this signal to a 'peak-detect' circuit, and display it on a meter.

You will need:

A 1mA (milli-amp) linear panel meter. This time a meter out of an old tape deck won't do, because they are logarithmic.

A 7808 8-volt voltage regulator.

2 x LM358 op-amp IC.

A LM334 constant current source.

An enclosure for the meter.

Sundry resistors, capacitors, trimpots, and some veroboard.

Step 1

Identify your FM IF IC.

The IF IC can usually be found nearby the 455 KHz filter. This filter is usually a black or blue or yellow small plastic box with '455' stamped on it. Another way to identify the chip is to Google the part numbers printed on the top of the chips on your circuit-board until you find one identified as 'FM decoder' or similar phrase. Once you have identified the FM IF IC, download a datasheet for it, so that you can locate the 'demod out' pin.

Here are some IF IC's commonly used by UHF CB radio manufacturers, including GME, Uniden, Icom, etc,

and the pin number for the demod out.

Step 2

Build the meter circuit. This circuit is purely audio frequencies and DC voltages, so there are no special construction requirements or shielding needed.

Solder a wire from the 'demod out' pin of your FM IF IC to the following circuit. I built this circuit on a piece of veroboard.

How it works - Deviation Meter

The 'demod out' from your IF IC is buffered by one part of an LM358 op-amp IC. The 12K and 6K8 resistors set a reference voltage of 3 volts for the buffer and following amplifier stage.

Step 3

Calibration.

Set your radio to a channel with no traffic. If you have your radio and this circuit powered-up now, you should have a reading on the meter. This is because the output from the IF IC is usually un-muted. What you are seeing on the meter is the deviation level of the 'static' or 'hash' (the sound you get when you turn the squelch down). If you do not have any reading, it is possible that your radio does mute the RX audio. In this case, turn your squelch down till your mute opens. Adjust VR1 till you get 5.5 volts at TP1.

Use another radio to transmit a silent carrier to your radio. Adjust VR2 till the meter reads zero.

VR3 adjusts the deviation reading. For this part, you will need a radio that has a known deviation, and set VR3 till the meter reads the correct deviation.

Final Notes

Because the output from the IF IC is usually un-muted, the meter reads the deviation of the 'hash', or 'static' when no signal is being received. A handy add-on to this meter is a circuit to force the meter to zero when the mute is closed. I built my deviation meter to suit my Icom IC-400. This IF IC in this radio also has an RSSI output. I used this output to control a mute circuit, to make the meter read zero when not receiving a signal. If your IF IC has an RSSI output, you can build this circuit HERE. (coming soon)