Mikek,
?? ??? ??? ??? ? 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
Sure, the capacitance of the resistor is in parallel with the Q meter tuning cap.
??? ??? ??? ??? 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?
Correct !? As long as nothing saturates.? Just monitor the output with a scope.
You may test the amplifier with the capacitive divider that will be used in the final circuit.? This is where you want a high Rp.
Note that with a minimum freq of 1 MHz, Q=1000 and Cmin. =20 pF, the corresponding Rp is 8 Mohms.
So if the amplifier Rp is 80 Mohms, the Q error is 10 %,? and 1% with 800 Mohms.
As the frequency increases, the source resistance will start to give appreciable error.? See my attached doc.
Jacques
VE2AZX
