RF Circuits

Here's a simple FM transmitter design. It uses pll circuit from Motorola MC145170. This PLL includes the prescaler and a serial standard bus called SPI. We advise to use the P2 version that fixes some init issues during the Power Up.


PLL FM Synthesizer
This PLL circuit is definitely smaller than the old one, consequently we've added on the radio board, the power control circuit that allows to switch on, the RF signal when the lock detect is established. In fact a simple LM317 that supplies the last RF stage, is controled by the lock detect signal through the ADJ pin.

The oscillator and the RF driver stages have been improved to get a better harmonics rejection than the previous version. We have replaced the BB204 varactors by new ones: BB209. Consequently, the kvco is more stable on the FM band and the AF signal has been directly applied on the control voltage line coming from the PLL. A Cauer filter has been added to improve the second harmonic rejection.

Digital Board 16F84
The digital board uses a 16F84 Microcontroller to drive the FM transmitter equiped with the MC145170 pll circuit. The pic is in charge of controling the SPI Bus in order to provide the 205 channels of the FM band.

Two keys allow you to launch a scan frequency through the FM band with a 100KHz step frequency and also a storage operation into the EEPROM. When the frequency setting is achieved,the pic controls the lock detect signal to prevent radio hardware issues.

The schematic is quite simple ,consequently it's not really necessary to provide technical explanation concerning the design. The lcd display is a 2*16 caracters with backlight. Download FM Synthesizer Schematic (pdf) - Digital Board Schematic (pdf) -  Digital Board Hex.