for enhanced sound quality. While assembling the transmitter care must be taken to make sure that C1 is directly connected to L1 and C9 to L2. These caps eliminate the distortions form the DC supply and improve the sound quality greatly. 9V voltage supply is also very important because it provides the exact amount of current to Q1 to produce loud and clear sound quality. I hope that you'll have as much fun as I had while building this transmitter
If you want to use Phase Locked Loop (PLL) to digitally tune the frequency of TX200 then you will need to make small changes to the transmitter's oscillator. Theoretically, change original variable frequency oscillator (VFO) to voltage controlled oscillator (VCO). PLL uses voltage to change the capacitance of the varicap diodes / frequency of the oscillator so two BB105 varicap diodes or any 15-30pF varicaps will be perfect for this.
You vary the capacitance of the varicap diode(s) by applying voltage supply between 1 - 8V. If you apply 1V the capacitance will be around 34pF, and if you apply 8-30V the capacitance will be 12pF (minimum). Now, you may wonder why do we need to start from 1V? The reason is that if you apply voltages below 1V the capacitance may go as high as 200pF and we simply can't use that. 100K resistor will be used to lower the current of the voltage, because varicaps require only 0.2mA of current to operate!
With this knowledge we replace C6 - 15pF capacitor with two varicaps and one resistor. That much is enough to be able to use 1 - 8V voltage to change the oscillator's frequency. The second change that we will need to make is to use 2pF capacitor and connect it to the Q2 transistor's collector. PLL will use this to sample the frequency and compare it with digitally selected frequency. PLL will then compare those frequencies and if they are not the same PLL with vary the voltage that is applied to the varicaps, until those two frequencies are the same.
Before you even connect PLL circuit to the TX200 transmitter you will need to test if the oscillator is working properly. We can tune through entire FM band (88-108MHz) by applying the voltage to the varicaps. To do so, connect "PLL - Voltage Control" PIN to the 10K potentiometer as shown below. With this setup you will be able to use potentiometer to change the frequency of the transmitter. If you can't go through the entire FM band or if your frequency is out completely then you will need to change the inductance of the L1 coil.