pll fm transmitter circuit schematic

PLL FM Transmitter Circuit

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.

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.

PLL transmitter circuit diagram

pll fm transmitter circuit schematic

Phase-Locked Loop transmitter PCB

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 the image to print it for making the board, so that the ink gets in contact with the copper. This PCB is fitted with soldered shields all around and between stages, on both sides of the board. They are best installed before populating it.
phase-locked loop transmitter pcb

Pll fm transmitter adjustments

First you set all trimmers to mid range and program the frequency. For this task, you simply add the switch weights: The least significant switch produces 100kHz, the second adds 200kHz, the next 400kHz, and so on, until the eighth, which adds 12.8 MHz. The ninth actually connects to two inputs of the PLL chip, so it adds 76.8 MHz, with the tenth switch adding 102.4MHz. To calculate switch settings for a given frequency, you simply decompose it into its binary components, and set the proper switches. Note that a switch that is ON is NOT adding its frequency contribution! For example, if you want to transmit on 96.5 MHz, you would set switches 9, 8, 7, 3, and 1 to OFF, the others to ON.

Now you should connect a 15V power supply to the main power input only, with a voltmeter at the output of U3, and a frequency counter at the collector of Q4. If you get the correct frequency, you are in big luck and should go and play lottery! Usually the VCO will be out of capture range. If the voltmeter reads around 14V, it means the frequency is too low. If it reads close to zero, it means the frequency is too high. The frequency counter should agree with this.
You need to adjust the VCO center frequency to bring it into range. For this task you have two adjustment points: One is C20, the other is bending L4! Usually the trimmer alone does not give enough range, so feel free to bend the coil. When you have adjusted the VCO roughly right, the PLL will lock in, and you will get a stable output frequency, very close to the one you want. Adjust L4 and C20 so that the voltmeter reads roughly 9V. Such a relatively high varactor voltage is convenient for best noise performance, because it keeps the varactors from entering conduction at the RF peaks . Ideally you should adjust the coil so that the trimmer is near center range with the voltage at 9V. This gives you easiest correction later.

Now you can set the reference crystal to the precise frequency, by adjusting C12 so that the frequency on the counter is exactly the correct one.

Let’s go to the power stages: Connect an RF power meter and a 50 ohm dummy load to the output, and apply a few volts to the variable voltage input. Adjust C28, C32, C37 and C38 for highest power. If you run out of range in any trimmer, correct that by bending the coils connected to it: L5, L7, L11, L10. Now increase the voltage and retouch these trimmers. You should get 4 to 5 watts output at 15V of supply voltage.

This excellent pll transmitter project is part of 80W fm stereo transmitter.


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