Message-ID: <008801c177ee$3561acc0$8c94153e@w8k3f0>
From: "Dick Rollema" <d.w.rollema@freeler.nl>
To: rsgb_lf_group@blacksheep.org
References: <5.1.0.14.0.20011121102933.00a7b730@gemini.herts.ac.uk> <001401c176b3$3c5f8100$b6a1153e@w8k3f0> <002001c17798$650befe0$8ae086d4@ericadodd>
Subject: Re: LF: Measuring Q
Date: Wed, 28 Nov 2001 10:22:25 +0100
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Dear OM,

John, G3LDO wrote:

> Your three Q measurements indicate that your measurement techniques are
> sound - unlike mine!
> It occurs to me that I could use my HP vector impedance meter to measure Q
> using the series resonant method. Unfortunately it only goes down to
500kHz
> so cannot measure Q at the operating frequency. If I did the measurement
at
> a higher frequency would the reactance of the series resonating capacitor
be
> significant and give a higher value of perceived resistance?
> I could also use my 3M bridge (page 86 of LF book), which can measure R at
> the operating frequency (136kHz and 73kHz). The resonance curve could then
> be plotted using the S/W shown on Fig 6.12.
> More experimenting to be done when the weather gets dryer.

The charm of the M0MBU method is that the only critical part is a signal
generator with a well defined internal resistance; the actual value of that
resistance could be 50, 75 or whatever ohms, as long as it is known.

If you are not sure about the source resistance you could insert a resistive
pad of, say, 20dB or so to obtain a well defined value.
Another nice feature of Jim's method is that loading of the circuit by the
voltmeter hardly plays a role as it is in parallel with some tens of ohms at
the most.

Q is rather dependent on frequency so a measurement at a frequency different
from 137kHz may not be meaningful.

73, Dick, PA0SE