Subcarrier Decoder for a FM Surveillance Bug |
Overview
This is a 67 kHz subcarrier decoder which should be compatible with the "Subcarrier Generator for a FM Surveillance Bug" project described in GBPPR 'Zine Issue #102.
Normally, a Signetics NE565 Phase Lock Loop chip is used to demodulate the 67 kHz subcarrier, but the NE565 can be difficult to locate. This project consists of a similar subcarrier decoder, but instead will be based around an Exar XR-2211 FSK Demodulator/Tone Decoder. The XR-2211 is still being manufactured and is quiet easy to find.
The decoder is based on the example Frequency-Shift Keyed (FSK) demodulator circuit in the XR-2211's datasheet. The datasheet lists the equations used to determine the center decoding frequency and loop filter components. A LM358 op-amp is added on the output of the XR-2211 to act as a buffer. The LM358's output signal is essentially still at a "line level," so an additional amplifier may be required if you want to directly drive a speaker or a pair of headphones.
High-quality 1% metal-film resistors and 5% or better tolerance film-based capacitors should be used in the audio chain, loop filter, and center frequency determing sections of the circuit. A multiturn 5 kohm potentiometer is used to tweak the decoder's oscillator final center frequency.
The input to the subcarrier decoder is the baseband/discriminator output from a wideband FM receiver, like the Radio Shack PRO-2004/5/6 or PRO-2035/2042 series of scanners. This input signal needs to be the raw discriminator output before any de-emphasis or low-pass filtering circuits.
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.
You'll need to slowly adjust the 5 kohm Center Frequency Tune potentiometer to trim the center demodulation frequency. You can do this by tuning the receiver to a properly subcarrier modulated signal and adjusting the pot "by ear" until the demodulated audio is clear. You can practice this by trying to decode a valid SCA subcarrier on a local FM broadcast station.
Scanner / Receiver 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

67 kHz subcarrier decoder circuit board overview.
The baseband 67 kHz signal comes in on the left. It passes through a simple resistor/capacitor 30 kHz high-pass filter to clean it up a bit.
The signal then feeds the Exar XR-2211 for FM demodulation. The output of the XR-2211 is buffered by a LM358 op-amp.
A 78L05 voltage regulator provides a stable +5 VDC for the XR-2211 and the LM358 from a 9 volt battery.

Alternate view, showing the components which make up the loop filter on the output of the XR-2211.
The XR-2211's datasheet provides the equations used to determine these values. Feel free to tweak them a bit to try and get better audio output. These components should be fairly high quality, with a 1% or 5% tolerance.
The 5 kohm multiturn potentiometer sets the XR-2211 center tuning frequency. There isn't a test point availble to tune the center frequency, so you'll need to "do it by ear" or with an oscilloscope by watching the demodulated audio output.

Mounting the 67 kHz subcarrier decoder circuit board in a metal case.
Two RCA phono jacks provide the baseband 67 kHz input and "line level" audio output.
A 9 volt battery is also mounted inside the case. A SPST controls the circuit's power.

Testing the decoder circuit with a 67 kHz signal at -40 dBm which is being modulated by a 1 kHz tone at a 5 kHz deviation.
Ugg! This is what the audio output looks like when XR-2211 isn't on frequency.
If you don't have an oscilloscope, use an audio amplifier and a speaker to monitor the output audio. If it's off frequency, it will sound like shit...

Slowing tweaking the 5 kohm Center Frequency Tune potentiometer should eventually result in something like this, a nice clean 1 kHz sine wave output.

Finished outside case overview.
The baseband 67 kHz signal input is via the left RCA jack.
The line level audio output is via the center RCA jack.
The switch on the right controls the circuit's power.
