Hi Jacques,
?I hope to get some closure on this, I have been a week on and off trying to understand the measurement.
The question is what causes the R to fall?
?It is my contention that on the Q meter that 0.2pf is resonated out, because after adding the resistor I need to retune to resonance.
?I'm using an 0805 smd resistor with 1" leads of #32 wire.
After adding that resistor to the resonate circuit, I need to adjust the tuning cap 0.12pf to get back to resonance.
?I appreciate any enlightenment you can give me.? Mikek
P.S. You mentioned 'high impedance' amplifiers, I don't recall if I said anything here about testing the input of high input impedance amps,
?but that is what I want to do. Put the amp input across a resonant circuit, see haw much I need to readjust the capacitance to get back to resonance and
see how much the Q dropped. Then do the math,? Rp = Q1 x Q2? /? 2 x pi x f x C x Delta Q.
?Do you see this as a reliable method to find the input impedance of a "high input impedance' amplifier?
I have two "high input impedance' amplifiers that I have tested at 1MHz. 0. pf and 180M¦¸ the other
tested 1.1pf and 200M¦¸, the later seems within the realm of the designers numbers, the other is
published 0.25pf and 100M¦¸, but no frequency :-)
?The Delta Q is small, not as easy to measure, leading to? errors, but for comparison it should work well.
?
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Thanks, Mikek
