PLL FM Transmitter Circuit

Posted on Feb 6, 2014

The PLL transmitter exciter has the functions of providing a stable, low noise, frequency-selectable RF signal and amplify it to a controllable output power sufficient to drive the power amplifier. It uses a PLL frequency synthesizer built with MC145151, which covers the FM band in 100kHz steps. The VCO covers only a few MHz without readjustment.

PLL FM Transmitter Circuit
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The pll transmitter output power is controllable from zero to 4 watts. A PLL unlock detector is included, to shut down the transmitter in the event of a malfunction. The Colpitts VCO is powered from a local 9V regulator, and has the frequency controlled by two back-to-back varactors, resulting in minimal loading and thus ultra low phase noise. The output of the VCO goes through an emitter follower buffer stage, then through a broadly tuned class A amplifier, followed by a class B driver and a class C power amplifier, which use medium-Q tuned impedance matching networks. These last two stages are powered from a separate input, so that the output power of the Phase-Locked Loop transmitter can be controlled from zero to 4 W by adjusting this voltage from zero to 15V. Note that the output of this pll module does not have enough harmonic filtering to connect it directly to an antenna. If you want to use this exciter as a stand-alone low power transmitter, you should add a low pass filter. The Phase-Locked Loop transmitter is built on a double sided PCB, which has its top side copper left mostly undisturbed as a ground plane. The copper is removed only around non-grounded pins. The ground connections are soldered on the top side, so it`s not necessary to have plated-through holes. This drawing shows the two sides of the PCB, so that you can print it and fold it in the middle to see how the two parts align. You will have to invert...

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