Other Electronic Projects
I have considered crystal filters to be out of reach for hobby projects. That is, until I found this excellent video on YouTube: Nick (M0NTV) documents the design, build and testing of a 4-pole Crystal Ladder Cohn Filter. https://www.youtube.com/watch?v=vVioj-QkaVo I have a small bag of 20MHz crystals and managed to find four that matches within less than 1kHz. Turns out, they were not exactly 20MHz, rather like 19996-ish. Here's how it looks ... Picture of the finished filter. All capacitors are 100pF, which gave about 6kHz bandwidth. Schematic: A short video to give an idea about the selectivity achieved: (VOA1 on 49m band, broadcasting from southern Africa I think) It may be too narrow for good fidelity sound, but I think it is about right for short wave, where stations are more tightly packed. The filter works surprisingly well, I think. Selectivity is much better than the 455kHz IF-transformers I’ve used so far. An advantage is that the high IF frequency removes any image frequencies without needing more than a single tuned antenna circuit. One downside might be that the VFO operates at more than 25MHz, which probably needs a digitally controlled oscillator to be stable enough, like an Arduino controlled VFO. But even that turned out to be quite doable, even for an old bugger like me, although I don’t understand much of the Arduino code. Luckily, several smart and generous people have posted their projects online, complete with code and schematics. All in all, I found this project very rewarding and I will definately experiment more with crystals.