In order to probe a crystal’s response with decent resolution, I need a gadget to step a decent-quality sine wave by 0.01 Hz across the 10-to-100 kHz range and a logarithmic front end with a decent dynamic range. That’s prompted by looking at crystal responses through the SA’s 30 Hz resolution bandwidth:
Mashing a cheap AD9850/AD9851 DDS board against an Arduino Pro Mini, adding a knob, and topping with a small display might be useful. A Raspberry Pi could dump the response data directly into a file via WiFi, which may be more complication that seems warranted.
The DDS boards come with absurdly high-speed clock generators of dubious stability; a slower clock might be better. A local 10 MHz oscillator, calibrated against the 10 MHz output of the HP 3801 GPS stabilized receiver would be useful. If the local oscillator is stable enough, a calibration adjustment might suffice: dial for 10 MHz out, then zero-beat with the GPS reference, so that the indicated frequency would be dead on to a fraction of 1 Hz.
The HP 8591 spectrum analyzer has a better-quality RF front end than I can possibly build (or imagine!), but, at these low frequencies, a simple RF peak detector and log amp based on the ADL5303 or ADL5306 should get close enough. One can get AD8302 / AD8310 chips on boards from the usual low-budget suppliers; a fully connectorized AD8310 board may be a good starting point, as it’s not much more than the single-connector version.
With frequencies from 10 kHz to 100 kHz coming from a local oscillator, one might argue for a synchronous detector, formerly known as a lock-in amplifier. A Tayloe Detector might be a quick-and-dirty way to sweep a tracking-filter-and-detector over the frequency range. Because it’s a tracking generator, the filter bandwidth need not be very tight.
At some point, of course, you just digitize the incoming signal and apply DSP, but the whole point of this is to poke around in the analog domain. This must not turn into an elaborate software project, too.