Digitally controlled FM transmitter with 2 line LCD display

Each step will result in a self functional unit. In the end of this journey, you will be able to link the steps together into a very powerful FM transmitter. This part will explain the main controlling unit for the FM transmitter. This part is very important since the transmitter frequency is digitally controlled and thereby very stable. The PIC16
Digitally controlled FM transmitter with 2 line LCD display - schematic

F870 will always remember the last frequency you used and when the power is connected, the first thing that happens is that the transmitter PLL and display will be uppdated. (If you never will change the frequency with the buttons and don`t want a LCD display, you can set the initial frequency and still use this controling circuit. ) The easiest way to make a RF oscillator stable is to implement some kind of frequency regulating system. Without any regulating system, the oscillator will start to slide in frequency due to temperature shift or other influences. A simple and common regulating system is called PLL. In this construction the VCO range is 88 to 108 MHz. As you can see from the blue arrows, some energy goes to an amplifier and some energy goes to the PLL unit. What the PLL do is that it compare the VCO frequency with the reference frequency (which is very stable) and then regulated the VCO voltage to lock the oscillator at desired frequency. The last part that will affect the VCO is the audio input. (Synthesizer and PLL can be broke down into complex regulating system with lot of math. I hope all PLL experts have indulgence with my simplyfied explanation below. I try to write so even fresh born homebrewers can follow me. ) The picture above shows you the phase detector. It has two inputs A, B and one output. The output of the phase detector is a current pump. The current pump has three states. One is to deliver a...

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