After using this a bit, it works decently well but there are some areas that could use improvement:
- The software can use some general polishing, most likely adding a state machine to avoid the error where decreasing the sweep size basically causes a seg fault.
- The two transistor amp could probably use some rework. A high speed op-amp may be more suitable. Just be wary of the DC biasing...
- I don't remember which specific inductors and capacitors I selected for the filter, but 72 MHz might be beyond their self resonant frequency. That's something I should have checked before ordering/soldering them.
All in all, I am quite pleased with how this works. It's a phenomenal tool for measuring crystals for selection/characterization for use in crystal ladder filters. I haven't tested it with amplifier circuits or other frequency selective things but I suspect that below 30 MHz it's great and over any 1-2 MHz bandwidth it is quite flat. It's only when the response is taken from 1-72 MHz that you see the lack of flatness, but it's still quite usable (especially since I added the calibration feature)!