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.