Re: Building the 'Minimod' AM modulator-questions!
This does highlight a worst-case scenario when using a simple LC oscillator to generate RF. My bigger modulators use a crystal, but as outlined in the article, obtaining one for an MW channel is very expensive. Cheap 1 MHz crystals and other non-channel MW crystals are about, but if crystal stability is chosen it might as well be on a channel. As Mike has fond it's not really possible to set a precise frequency by adjusting L1 - it's too coarse.
But the whole point of the MiniMod was do design a very simple, comparatively cheap low power modulator, hence the L/C VFO. And after all, the vintage radios it will likely be used with have an L/C VFO/LO and most don't have as much drift as Mike is experiencing with his MiniMod circuit.
It's odd - and probably significant - that it drifts in one direction even when unpowered.
BTW, if you're using a bigger inductor for L2 (output), then if you remove the 390R resistor you might get more output power. I added it for two reasons; it keeps the power down to a 'safe', very local range, and it stops the DC through the 90uH coil from saturating the small core on modulation peaks.
Ian
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