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Old 24th Dec 2019, 10:41 pm   #3
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Another pantry transmitter

Well its easy enough from looking at the photo to determine its basic features good and maybe not so good.

It uses an L/C oscillator which is not really stable enough for a good transmitter but might appear ok most of the time.

The incoming audio is sent to a power amplifier IC, presumably the output of which amplitude modulates the supply rail to the RF output transistor . (Two power devices visible, probably one is the RF output the other the modulator).

While this works there is usually fairly significant modulation distortion and the change of the loading, being reflected to the L/O output and adding FM components in the transmission.

Going in its favor, unlike many AM transmitter kits I have seen, it has a decent sized RF power output device so that is a big plus.

Generally is better for pantry transmitters to have a crystal based or synthesized L/O from an xtal source, have a linear amplitude modulator circuit, RF buffers then an RF output stage with correct matching to the type of antenna which is different for loops & wires. Also ideally the audio is level compressed prior to the modulator and there is soft peak clipping to prevent over modulation of the carrier.

Having said that, because this type of modulator likely has limited capability to modulate the carrier completely out, it may well be helpful in the absence of more elaborate circuitry. So it would be worth a punt buying one and trying it out.
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