Why are ultrasonic filters set so high?

[nightanole]

It seems most people put their ultra sonic filter in the 300k-400khz range. I have some left over (and very expensive) 820pf mica caps id like to use, but with a 1k resistor that puts me at 200khz.

[sregor]

Don't know - but can make some guesses. First, to minimize loading of the input source (the load will start to go down when the capacitor impedance equals the input resistance, until it gets to the series resistance at which point the frequency starts rolling off). Second - convenience of values. Third - most people try to get the frequency as high as possible. The rule of thumb I've always heard was to make it at least 10 times the maximum frequency you want it to pass which would be 200 Khz if you want to have no affect at 20 Khz. Just my 2 cents.

[janneman]

Even at 200kHz, if it is a single pole roll off, there is a minuscule phase shift at 20kHz. Some people worry about that, and that they cut out some usefull content at say 50kHz.


jan didden