In order to simplify the transmitter design, we've used the new pll circuit from Motorola :the 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.

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.

VCO/PLL Schematic PDF File