Building and tinkering with crystal radios goes back to my childhood of the late 1960's and coming round again.
Back a few years ago I stepped up my game and wired up a DSB double sideband tranceiver that actually worked quite well and I made daily net contacts on 75 meters for almost a year with it. The circuit is the Wee Willy and can be found online. Output was about 1 watt.
Well, you could call the oscillator I am using a BFO or a VFO ( Actually both ). Depends on how you look at it. I have used this circuit and actual build for many years without any problems. Not really state of today's art and it sure is not stable compared to a lot of other circuits out there but it works with just a hand full of parts and never quits.
Here is the circuit followed by the web page link.
I have made a couple of changes to this circuit. First I replaced the upper left 2N3053 with a 22PF NPO cap. I also replaced the lower left 2N3053 with 2 IN914 switching diodes in parallel. I also made this circuit with just a single band in mind so did not need the switch S1a/b
Well, this is a cleaned up setup. At the rear is a perf board crystal controlled oscillator that can be modulated by the laptop if needed and is loose coupled to the radio. The circuit on the left is the RF BFO and is also loose coupled via the slug tuned coil. On the right are a few of the crystals used on the signal generator. Not shown is the small earphone I use to listen to the audio output but that just clips onto the radio. The BFO is varicap tuned on its front panel and has a 15 KHz range from about 7.03 to 7.045 MHz.
The BFO circuit is at least 15 years old and was part of a much older crystal radio receiver for 40M CW. In this case the crystal radio becomes a very basic Direct Conversion receiver. Still no RF or AF amplification but I might add a LM386 audio amp for speaker listening.
Well, I have finished the construction of the radio and since I have yet to put up a decent antenna, I want to at least test to see if the tuned circuit and detector will sniff out a RF source of some kind.
I do have a simple crystal test circuit wired up on perf board from years ago and if I have it oscillate at say 7.04 MHZ and modulate the RF, I should be able to hear the modulation in the earphone.
Going old school and brute force method and modulate the Oscillator by placing a small audio transformer in the 9 volt power lead then feed the transformer with a audio source from the laptop. Not the best way to do it but it is what it is :-)
Two Hours Later.......
Success, I was able to generate a modulated RF signal at 6, 7 and 11.155 MHZ and calibrate a rough dial. The tuning was fairly sharp and the variable coupler works like it should as well :-)