PIC870 Controller
This is 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 heart of this unit is a called PIC16F870, a 2 x 16 char display and four buttons. As you can see I have chosen a PIC16F870 because it is easy to find and cheap to buy.

FM PLL Controlled VCO

Voltage Controlled Oscillator(VCO) range is 88 to 108 MHz. You can see that the PLL can control the frequency of the VCO. 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>. The amplitude of the audio will make the VCO change in frequency FM (Frequency Modulation).

It is not good to load or “steal” to much energy from the oscillator because it will stop oscillating or give bad signals. Therefore I have added an amplifier.

The oscillator give about 15mW of energy and the following amplifier will bring up the power to 150mW.
The amplifier can be pressed a bit more (maybe 400mW-500mW) but that is not the best solution.

PIC16F870 Programs (INHX8M format)
The zip file contains hex file made for this project.
I have made two programs, each one are made for different crystal frequency of the PIC.

You should use the first hex file, when you drive the PIC with a VCTCXO of 16.8 MHz.
You should use the second hex file, when you drive the PIC with a crystal of 2-5MHz.
You should use the third hex file, when you drive the PIC with a crystal of 13MHz.

Download Component Lists

Source:

• Digitally controlled FM transmitter with 2 line LCD display
• FM PLL controlled VCO unit