Subcarrier Generator for a FM Surveillance Bug

Overview

A subcarrier generator is a method commonly used for "masking" the audio from your surveillance bug by hiding it within the main RF carrier.

Normally, if you are using a conventional FM audio bug, anyone with a receiver tuned to your transmitting frequency will also be able to monitor your audio.  This can easily give away your surveillance operation (or location), especially if you're using frequencies within the normal 87.5 MHz to 108 MHz FM broadcast band.

The idea of the subcarrier generator is to generate a "carrier on a carrier."  In this example, we'll be using a frequency modulated 67 kHz subcarrier.  A subcarrier of 67 kHz was choosen as this is often used for FCC-authorized Subsidiary Channel Authorization (SCA) transmissions within the FM broadcast band.  This is often how Muzak, audio books, stock quotes, and other low-bandwidth services are transmitted over the airwaves.  Decoders tuned for this specific 67 kHz subcarrier are quite easy to build, so we'll stick to using that frequency.

The subcarrier generator itself is based around a frequency-modulated oscillator using a common 555 timer IC.  The 555 timer will be configured to generate a 67 kHz square wave output which is then frequency modulated by a signal from a LM833-based microphone pre-amplifier circuit.

The output from the 555 timer will then be low-pass filtered and attenuated before being applied to the unfiltered audio input on the FM surveillance bug.  There must be no input low-pass filtering on the modulation input of your surveillance bug for this subcarrier method to work.

To receive the subcarrier-modulated audio, you need a receiver tuned to the proper carrier frequency and equipped with a subcarrier converter on the receiver's discriminator ("baseband") output.  The discriminator output on the receiver is the directed demodulated audio before reaching any low-pass filtering or deemphasis stages.

In the Radio Shack PRO-2005/6 line of scanners, this output can be taken from the Samsung KA2243N wideband FM demodulator test point #1 (TP1 on the silkscreen).  For other receivers, you may have to poke around a bit with an oscilloscope.  Look for the 38 kHz stereo pilot tone when the receiver is tuned to a strong FM broadcast station.

You can make your own 67 kHz subcarrier converter by viewing the example circuit in the Signetics NE565 Phase Lock Loop datasheet.  The NE565 can be difficult to track down, but they do appear on eBay from time-to-time.

Scanner                      Wideband FM Discriminator Tap

Radio Shack PRO-2004         IC-1, pin 6 (TP3)
Radio Shack PRO-2005/6       IC-1, pin 10 (TP1)
Radio Shack PRO-2035/2042    IC-1, pin 10 (TP1)
Uniden BC-9000XLT            IC-9, pin 11
Icom R-1                     IC-1, pin 9
AOR AR-8000                  U3, pin 8


Pictures & Construction Notes

Overview of the 67 kHz frequency modulated subcarrier generator circuit.

A LM833-based microphone preamplifier is on the left.

It provides around 40 dB of gain and audio is from a standard electret microphone.  The extra op-amp on the LM833 is used as an active bias to aid in circuit stability.

The 555 timer subcarrier generator is on the right.  Its output frequency is set by a few resistors and a single high-quality 1000 pF capacitor.  The audio output from the LM833 microphone preamplifier is fed to pin 5 on the 555 timer.  This then frequency modulates the 67 kHz carrier generated by the 555 timer.

The entire circuit should be run from a well regulated +5 VDC power supply.

Use 1% metal-film resistors throughout the circuit for maximum low-noise performance and frequency stability.

This was a just a test circuit so it was physically quite large to allow for easier experimentation.  Using SMT components will reduce the size of the circuit dramatically.

Alternate view of the subcarrier generator circuit.

On the output of the 555 timer (lower-left) is a 3-pole low-pass filter to remove any harmonics created by the subcarrier generator.


Tuning the frequency of the 555 timer subcarrier generator.

Monitor the output frequency on pin 3 of the 555 timer.

A multiturn 10 kohm potentiometer may be used to tweak the final output frequency.

The 555 timer generates a square wave, so a low-pass filter needs to be added on its output to remove any harmonics which may cause audio distortion in the transmitter.

Oscilloscope view of the final subcarrier output signal.

The low-pass filter turns the square wave output of the 555 timer into a sine wave.

The Audio Level adjustment potentiometer then further reduces the signal level before being applied to the modulation input on the transmitter.

Be sure not to over modulated the subcarrier generator, otherwise it will cause interference or splatter on nearby frequencies.

The subcarrier generator should be narrowband modulated (5 kHz deviation), while the actual RF transmitter may be wideband modulated (75 kHz deviation).