I have always admired what Don has done with his modulation system. That 3400 hertz brick wall filter is a rarity unless one uses high technology active circuitry such as active op-amps, switched capacitor filters, dsp, etc. I believe Don has a passive module that does a great job. I wonder where he got it?
It is a passive module. I picked it up in Washington, DC back in 1972 when I had stopped over there to visit a friend. We went to a little hole-in-the-wall military surplus outlet. I paid $5 for it. It is potted, about 1 1/2" X 1 3/4" X 2 ". It has an octal plug on the bottom. The impedance in/out is 10,000 ohms. I wasn't sure about it when I purchased it, but when I tried it out with a signal generator after returning home, I wished I had bought every one they had. It is flat up to about 3300~, and at 3500~ the signal is so far down as to be undetectable on the scope. You can simulate the envelope pattern of a cw signal by swishing the test oscillator about 100~!
The only thing like that I have ever seen at a hamfest was physically and electrically identical but had a useless cutoff frequency of 2000~.
Don K4KYV
