Electronics > RF, Microwave, Ham Radio

How can I improve this cheap AM transmitter?

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--- Quote from: radiolistener on April 03, 2023, 06:36:44 pm ---Forgot to say, this transmitter doesn't have frequency stabilization with crystal oscillator and buffering, so it's frequency depends on impedance of your transmitting antenna.

What this means - if you connect some transmitting antenna and signal disappears from your receiver, it doesn't means that transmitter stopped to work due to overload, it may just change frequency. So, you're needs to check if transmitter works on different frequency before conclusion that it stops to work. Just scan entire AM band on your receiver if signal disappears after some change within circuit or antenna.

The good practice is to check signal with a small wire loop (RF sensor) connected to oscilloscope. But if you don't have oscilloscope, you can use your receiver, just scan entire AM band to find the carrier of your transmitter.

--- End quote ---

it depends...




Made yesterday the circuit, and fiddle with it this evening, while probing with an oscilloscope.  Doesn't look good.
- asymmetric carrier waveform
- strong parasitic FM modulation
- the AM modulation factor is too small
- at 6V supply the modulation factor is bigger, but it can only take about 0.1V audio, or else the oscillator will stop during the negative swing of the audio
- at 12V works a little better, but then the modulation factor is even smaller, so more hiss and noises at reception for the same audio volume
- frequency shifts a lot with the power supply voltage

First 3 pics are at 6Vdc power and 0.775Vrms/1kHz (0dB audio).  Normal zoom, then zoomed-in to see the RF carrier during peak/troughs.  Next 3 are the same thing for only 250mVrms, to get a smaller modulation factor.  All pics look very bad to me.

Will try to squeeze more from that schematic, but I'm afraid that schematic as it is would never work decent.  :-\


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