[Fwd: LF: Measuring Q ?]

Andre' Kesteloot akestelo@bellatlantic.net
Sun, 06 Dec 1998 09:32:38 -0500

vernall wrote:

> > G0MRF@aol.com wrote:
> >
> > > However, we've had difficulty trying to assess its performance. All attempts
> > > to measure Q with only a sig gen and an oscilloscope have been inconsistant.
> > > Does anyone know a reliable technique that doesn't load the circuit???
> >
> > I use a low capacitance probe for my oscilloscope to measure the voltage across
> > the coil, and feed RF from the generator through a 47kohm resistor in series
> > between the generator and the top the coil.  Then vary the frequency until I get
> > the two -3dB points.  That arrangement works well for me
> > 73
> > Andre' N4ICK
> The signal generator can also be injected by "low impedance" in series
> with the circuit under test.  Use a high permeability toroid with say 20
> to 30 turns on it, and connect that winding to the signal generator.
> Couple to the circuit under test by "1 turn", by simply running a wire
> through the toroid, in series with the circuit under test.  Using a high
> impedance probe, find the -3 dB response points, as described by
> Andre'.  I have a selective level meter with a digital frequency
> readout, which provides a "one box Q meter" with the toroid and high
> impedance (scope) probe.  The digital readout is good to have for LF, as
> otherwise a few hundred Hertz readout errors in the -3 dB frequencies
> can give erroneous Q readings.
> 73
> Bob ZL2CA