Crystal Controlled FM Transmitter

Posted on Feb 5, 2014

The FM transmitter is relatively simple to build and with only one adjustment it is ideal for the absolute beginner. But before I continue, let me just make one thing clear. With Wide-Band deviation, the VCO is never locked in phase, only in frequency. It uses a Varicap diode to modulate an oscillator, then lock the oscillator in a Phase Locked Lo

Crystal Controlled FM Transmitter
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op, and compare it to a crystal oscillator. The circuit follows that of a simple textbook synthesiser comprising a Voltage Controlled Oscillator (VCO), a damped Loop Filter, a Reference Oscillator (Crystal) and a Frequency Comparator. The only difference is that I have added a frequency divider chip to divide the VCO frequency by 64. This means that if the VCO operates at 100MHz, the output from the divider will be 1. 5625MHz. If the crystal oscillator is also 1. 5625MHz then the loop will be "in-lock. " The control voltage from the filter to the VCO steers the frequency of the VCO so that the output of the divider is ALWAYS 1. 5625MHz. Any deviation from this will result in a change of the loop voltage to move the VCO back to 100MHz. I will not delve too deeply into the in`s and out`s of synthesisers. I have already written several pages of information about them and the stages that are needed to make a working synthesiser. The specific circuit is shown above. The "Prescaler" (divider) chip needs to have a supply voltage of only 5v (+/- 0. 25v) so the LM317 has been included. I used the LM317T due to it`s larger can size (and I have got a lot of them) so it will tolerate a supply voltage of greater than 13. 8vDC without burning. All has been somewhat over-engineered. IC1 (CD4001) is the crystal oscillator with an extra gate used as nothing more than a buffer stage. This feeds the frequency comparator of a CD4046 (IC2). The...

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