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500mW FM PLL transmitter

500mW FM PLL transmitter

This transmitter is PLL controlled and the frequency is very stable and can be programmed digitally. The transmitter will work from 88 to 108 MHz and the output power is up to 500mW. With minor changes the frequency can be set from 50 to 150 MHz.
PLL and Microcontroller The oscillator is made to work as “Voltage Controlled Oscillator” VCO. To control the frequency a synthesizer circuit LMX 2306 has been added. The PLL circuit has a pickup coil (L2) connected to pin 6. This coil should be put close to the L1 coil for picking up some of the oscillating energy. The PLL in the LMX2306 will then use this frequency to regulate the VCO and lock it to desired frequency. The regulating system also need an external reference crystal. In this case use 12.8 MHz. At pin 2 of MX2306 you will find a PLL filter to form the Vout which is the regulating voltage of the VCO. The PLL try to regulate the Vout so the oscillator keeps the frequency locked to desired frequency. The desired frequency is programmed into the PIC EEPROM and is clocked into the synthesizer (LMX2306) at power up. The author here explains how to program the EEPROM for different frequencies. At pin14 of the synthesizer you have a control output. At this output you will find the reference frequency for testing. (He must warn you because the signal is not symetrical in shape. The positive pulse are only a few microsecond so you will have difficult to see it at oscilloscope.) He solved it by connecting it to a 74HC4020 (14-stage Binary Counter) to pin 10 Clock input. At Q0 (pin 9) you will have a symmetrical square wave with half frequency since the circuit is a counter. At Q1 pin 7 it will be divided by 4, see datasheets for more info.


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