Download Parts List here.

Printed Circuit Board (PCB)

You may choose between single-sided PCB (only bottom layer) and double-sided PCB (+ ground layer on the top). The difference is in output power. The double-sided PCB gives higher output power (this fact is not so interesting in rf issue).

Control Software for U3
The software for U3 is provided for download here: plltsa.zip (version 1.3). Please read included license text first.Suitable free PIC programmer is for example here: http://www.members.aon.at/electronics/pic/picpgm. Actually Pira CZ does not provide the PIC programming or complete kit sale for this device. Software modifications included: pll64.hex – 6.4 MHz crystal, pll32.hex – 3.2 MHz crystal, pll.asm – for any changes. Fuses: WDT: Enabled, OSC: INTRC-I/O, MCLRE: I/O.

Use the buttons to set the frequency. After a few seconds of idle the frequency is tuned and stored to EEPROM (icon of diskette). Finally the buttons are locked (icon of key) to avoid unwanted frequency changes. Unlock by pressing any button for a longer time.

The display and LED indicate PLL in-lock state. This state is also provided at pin 1 of U3. It can be used for control of additional power amplifier. When in-lock state is indicated, the PLL is switched to low speed for maximally flat low audio frequencies response.

Source: Pira CZ 5W PLL FM Transmitter

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

This very stable FM transmitter based on TSA5511 synthesizer from Philips. Frequency is performed with three buttons through PIC16F84 microcontroller and displayed on single line 16×1 LCD.

With best of 2SC1971 final amplifier circuit design was able to reach about 9 mil distance. But such power could do better with better antenna. Download Project Domumentation.

Source: Very stable FM transmitter based on PLL synthesizer