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detailed theory about fm transmitter


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A link to a review of the Veronca transmitter kit in Google doesn't work anymore, but I found it in Google's cache:
"Circuit Description
The transmitter is based on a novel double-ended free-running voltage controlled oscillator (VCO) operating at half the output frequency. Note stations based on a free-running VCOs are not licensable in the UK. The idea behind the VCO operating at half the output frequency is that it should be more resistant to having the centre frequency pulled by variations in loading further down the transmit chain.  The active devices in the VCO are a pair of BF494.  The mono audio input circuit is totally passive.  No pre-emphasis is provided. The audio input is a PCB mounting phono connector.  A small 5V voltage regulator provides a stabilised DC reference for the Varicap diode in the VCO.

The output device is a trusty old 2N4427 operating in class C.  This picks off the 2nd harmonic of the VCO and amplifies it up to 1W.  There are four adjustable capacitors.  The first sets the centre frequency of the VCO.  The second is on the input match to the 2N4427, the third and fourth form the capacitive legs of a pi matching circuit for the output match of the 2N4427. The only harmonic filtering will be by this pi circuit.  A simple push-on aluminium heatsink is fitted to the output device.  The RF output connector is a SO-259 (UHF) socket, mounted directly onto the PCB.  A simple rectifier/amplifier circuit drives an on-board LED which allows the circuit to be tuned without additional test-equipment, by adjusting the trimmers for maximum LED brightness.  The circuit is reverse polarity protected by a series diode in the supply.  This diode drops 0.78V with a 16V supply."

They measured a huge frequency change when the power supply voltage was changed and when something became near its antenna.

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