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 Post subject: 3mW FM Transmitter based on Franklin Oscillator
PostPosted: Aug Sat 07, 2021 5:53 pm 
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Joined: Mar Thu 01, 2018 1:30 am
Posts: 736
Franklin VFO based FM TX. It has not been tested in real life but it should work as drawn. Po is 3mW. Running it into a dummy load is fine. Oscillator output is taken via a 1k resistor and fed to 10dB gain buffer comprising T3 and T4. It's built entirely using LtSpice.

The coil in the LC tank circuit has no DC current flowing through it. This enhances frequency stability. It is coupled to the two oscillator transistors via 3.3p low value capacitors and lightly loaded.

Three 5.6V zener diodes are wired in series for varicap function. It provides better thermal stability than a varactor diode as temperature coefficient of 5.6V zener is close to zero! The variable pot should provide 1.5MHz of fine tuning.

The final stage operates in common base mode and isolates the antenna from rest of the circuit. It has good reverse isolation. The Franklin oscillator is powered by a 5V regulator. Rest of the circuit runs on 12V.

Frequency stability is going to be excellent. The trick is to draw as little power as possible from the tank circuit. C7, C2, and C4 should be NP0 capacitors. The coil is only 30nH in value and adds to the overall frequency stability.


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