Most audio listening devices, also called spy bugs, use simple oscillators with LC tank circuits as frequency determining elements. Their frequency is not even close to anything that could be considered stable. Despite the fact that many of those wannabe spy shops claim that they sell that sort of low-grade rubbish to authorities all over the world, those devices are virtually useless in a real life scenario.
More accurate listening devices derive their carrier frequency from a vibrating crystal of piezoelectric material. Crystals are suitable to create an electrical signal with a very precise frequency. With an integrated PLL clock multiplier like the ICS501 , we can easily build a crystal-controlled listening device ourselves.
The ICS501 is hardwired to a multiplication factor of 6.25 (S0 = Ground, S1 = Open). There’s a bunch of standard crystal frequencies available which will result in a practical VHF frequency if multiplied by 6.24. For instance, the standard frequency 23.16 MHz results in an output frequency of 144.75 MHz (2m amateur radio band). The following standard crystals are suitable for transmissions in the FM radio band (88 – 108 MHz): 16.128 MHz (100.8 MHz), 16.384 (102.4 MHz), 16.9344 (105.84)
The crystal’s frequency is being pulled slightly off target by the variable capacitance diode (CD1). The control voltage is biased by R3 and R5 to about 2.5 V. The audio entering the electric microphone (MIC) is being amplified by T1 and then applied to the variable capacitance diode (CD1) by C1. The change in voltage across CD1 will alter the capacity of CD1. As a result, the frequency of the oscillator is altered.
The circuit is pretty sensitive because the change in frequency of the crystal oscillator is also being amplified by factor 6.25. 160 Hz of change in frequency of the crystal oscillator and will therefore result in 1 kHz deviation of the output signal.
One thing that is somewhat problematic to get nowadays is a variable capacitance diode. I used a NTE618, which is available for $ 3.11 from Newark , and a 30 pF capacitor in series in my assembly.
Real audio surveillance circuits used by authorities nowadays rely on the cell phone network to bring the audio back to the corresponding agency. The times of a surveillance truck with a bunch of high ranking surveillance technicians in front of the target’s house is far from over. But just as much as the myth of a ‘clicking noise’ on a tapped phone line, this sort of outdated ‘knowledge’ will probably stick around for another few centuries.
Links and Sources:
 ICS501 Clock Multiplier, Jaunty Electronics: http://baltic-lab.com
 NTE618, Newark: http://www.newark.com/