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You also have to deal with the input and output capacitance of the gate, along with the distributed capacitance in the circuit board. It's also possible that he took the photo from a hand built prototype.

I used to work with two stage Sallen-Key filters. They used 14, 1% components, plus either a wideband voltage amplifier, or a current amplifier for the higher frequencies. Both the ripple and the 3dB points had to fall within 5%, but they often needed components changed to get them to pass. Sometimes the resistance of the reflowed solder was enough to made it fail. Just a fraction of an ohm. Other times, swapping the pair of 100pF SMD caps in a stage would pull i into spec. There were 16 identical filters on these video boards, but minor variations in the distributed capacitance would cause major problems. I wrote a program to suggest new resistor values, based on how far out of spec a stage was.

Other people would just randomly select a new part, and try dozens of combinations to get it to pass. These filters ranged for 10 KHz to 20 MHz and they were in top of the line Telemetry receivers. One was put aboard the ISS as part of a kU band communications system. The multiple filters allowed the use of voice, video or wideband data uploads.


Michael A. Terrell