Crystal Locked FM Bug

A number of simple FM devices, (without the use of a crystal) and showed how the power and frequency depends on a number of factors including the voltage of the supply and the design of the stages. The broad term given to the oscillator stage of these transmitters is `voltage dependent` as the frequency of the output is dependent on the voltage of the supply
Crystal Locked FM Bug - schematic

. We designed transmitters capable of transmitting 100 metres (the Ant), 400 metres (the Amoeba), 800 metres (the Voyager), 1km (the Ultima) and others with ranges between 100 metres and 1km. It was simply a matter of adjusting the spacing of the coil or the value of the capacitor in the oscillator circuit to shift the frequency anywhere on the band. As the battery voltage fell at the end of its life, the frequency did move slightly. Although this shift was very small, the change was noticeable in some applications, such as long-term monitoring of alarms etc. The only way to achieve this is to have an extremely stable oscillator - one that is independent of the supply voltage. Designing such a circuit is not an easy task and it has taken us quite a number of attempts to get it to work properly. At last we have come up with a suitable design and we have called it the Crystal Locked Bug - or Xtal Bug for short. During our designing we had two major problems to overcome. One was getting a low-cost crystal that would produce a frequency on the 88 -108 band and the other was getting good quality audio. When audio is fed into a crystal locked oscillator, it must "pull and push" the frequency of the oscillator - after all, that is why it is called Frequency Modulation. In other words it must pull and push the oscillator against the rigidity of the crystal and in doing so, the audio gets distorted. This is one of the major problems...

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