Re: Measuring secondary Q Re: Success Re[TCML] Measuring secondary current

[paul <tcml88@xxxxxxxxxxx>]

Andreas wrote:

> This is a matter of driving the primary with a
> signal generator and watching the secondary's
> output with a scope as I vary the frequency?

You must pad the output impedance of the sig gen down to a
low value, otherwise its 50 ohm output impedance will add in
series with the resonator loss resistance and destroy the Q.

For example, use a simple resistive pad

 http://abelian.org/tmp/ah120905a.png

The resistance values are not critical, use whatever you have
to hand.   Make the measurement with the top load and primary
in place, but with the primary open circuit at its 'hot' end.

The top voltage is approximately equal to Q times the base
voltage.

Measure the frequency width between the half-power points
(where the amplitude is 1/sqrt(2) = 71% of the peak response).

Then Q = F_peak / F_width

Sometimes it is easier to measure the half-amplitude points
instead of the half-power, in which case

 Q = F_peak * sqrt(3) / F_width

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.

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.

--
Paul Nicholson
--

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