On Wed, 5 Sep 2012, paul wrote:
So, you can get two Q measurements here: one by the ratio V_top / V_base and another from the bandwidth. However, measuring the top voltage is tricky because directly attaching a 10Meg scope probe will also spoil the Q. Some ingenuity may be required here.
The only way I see to accomplish this directly would be a capacitative divider; unfortunately, for a calculated DC capacitance (per JavaTC) of 11pF, I doubt I can make one that won't disturb the Q -- much less produce reliable measurements.
On the other hand, I may use this as an excuse to finally make some Terry Fritz noncontact voltage / plane-wave antenna probes:
http://www.pupman.com/listarchives/1998/August/msg00357.html This is very similar to the method that JavaTC recommends: http://www.classictesla.com/java/script/outQ.html
If you have a calibrated CT on the base current, then you can measure the base input impedance at the resonant frequency, Z_in = V_base / I_base and this is equal to the total loss resistance, from which you can calculate Q. This may disturb the resonator less than trying to get directly at the top volts.
When you say V_base, are you talking about the voltage from the coil base to RF ground? I have a feeling it will be immeasurably low, assuming an I_base of ~140mA during operation and 1 ohm of resistance in the lead from base to the cold-water pipe that serves as ground.
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