Low Power Stable FM TX

Sound is distorted. Why so? Low oscillator current? Low feedback cap value? If I remove the 10K base to ground resistor then oscillator sounds fine but drift increases. The 10K to gnd keeps drift to minimum but audio quality suffers. Not using varicap diode definitely improves stability. This was supposed to be an ultra stable fm tx for beginners.

Hi Joy,
Yes, I could do this, but I would need to see the pictures and data. I think it best I create a page for your circuits, rather than mixing them with mine :-)

Unfortunately my computer will not allow me to download the pictures since the site certificate has expired.

BR Harry

Schematic: https://pasteboard.co/ITdjYVT.jpg

Prototype: https://pasteboard.co/ITdncbL.jpg


Four stage FM transmitter. Audio is fed directly to the base of PN2222A oscillator. It's followed by a three transistor RF amplifier which boosts 0.3mW from oscillator stage to over 4mW (+6dBm) after the Butterworth low pass filter.

TX is mega stable and long term frequency stability is good. After 12 hours of operation at normal room temperature, the drift was measured at 3KHz. Proper shielding of the oscillator section with copper clad boards from all sides and on top will insulate it from air currents and minimize drift.

With the given component values, the operating frequency should lie somewhere between 91MHz - 93MHz. This TX shows that excellent frequency stability can be obtained even at VHF frequencies if proper design procedures are followed.

No varactor diode or trimmer capacitors have been used. The 2n3904 works well as an RF amplifier (Ft = 300MHz) for frequencies up to 94MHz. Gain falls sharply after that. After connecting a quarter wavelength wire antenna, if the PA stage self oscillates and any kind of instability is noticed, then connect a 220 - 470ohm resistor at the output between antenna terminal and ground. Use 12V regulated power supply.

Can you publish this circuit on your website under transmitters section?

Thanks,
Joy