FM Transmitter with PLL


Posted on Jun 13, 2012

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.


FM Transmitter with PLL
Click here to download the full size of the above Circuit.

Power Output 200mW ajustable Frequency Output 87.5MHz to 108 MHz Frequency Step 10KHz Phase Noise -102dBc/Hz Frequency Stability +/- 300Hz Harmonic Filtering <-60dBc Spurious Emissions <-70dBc 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. Modulation Direct Frequency Modulation MPX Input Response +/-0.5dB 30Hz to 80KHz MPX Input Level 300mV RMS Min Power Supply 14 to18V




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