Electronic – 7.2 MHz CW reciever

If you have a signal: $$x(at)$$ the fourier transform will be: $$F[x(at)]=\dfrac{1}{\lvert{a}\rvert} X(\dfrac{f}{a}) $$ this is known as the scaling property of Fourier transforms. Which also relates the inverse relationship between the time domain and the frequency domain.

So if you sample the broadcast signal at very high frequencies and then play it back (or down sample via decimation) the entire bandwidth will be compressed/scaled.

the challenge here will be to have enough Bandwidth on the front end, a high enough sampling rate and sufficient gain (that pesky \$ \dfrac{1}{a}\$ factor). However, it need not be digital, just sampled.

What you have here is a straightforward minimal component crystal oscillator.

Your best bet is going to be to follow it up with an amplitude-modulated amplifier stage, along with the necessary microphone amplifier and signal conditioning.

Best resource will be OLD copies of the ARRL Handbook. Hit your libraries.

If you can find a copy of DeMaw & Hayward's "Solid State Design for the Radio Amateur", it will give you a lot of help, although not much on AM. Copies OCCASIONALLY show up on Amazon for reasonable prices.

--de John WB5YOO (Amateur Extra)