From: Paul
Date: Fri, 16 Feb 2001 15:03:44 +0000
Subject: Re: [TSSP] Top voltage testing
Hi Terry,
The situation is looking a little better with the raised signal level,
is this once again the HP reading in error with small signals?
> The reason the current is dropping with the probed coil is due to
> the Q of the coil dropping with the probe in place.
Ah, I haven't been bothering to take account of the resistive part of
the probe, since other losses are not well quantified - I assumed it
wouldn't make much difference. In fact, I put in a 10 Meg load resist-
ance alongside the probe capacitance, and the probed coil Q fell to 102
compared with 195 for the bare coil. Previously, without the allowance
for the 10 Meg, I got a Q around 170 for the probed coil. Your base
current readings have highlighted the fact that I must take the probe
resistance into account too.
Adjusting the model's ctop to match your new f1, we now get:
measured modeled error
f1: 310.9 kHz 310.9 kHz 0.0% Ctop adjusted to match: 3.145 pF
f3: 847.8 kHz 828.4 kHz -2.3%
Zft: 34450 ohms 34877 ohms +1.2%
Les: 17.64 mH 17.85 mH +1.2%
Lfac: 79.8% 80.8%
You mention that the coil is not 'perfectly well known', and we have
a 2% discrepancy in the Ldc, with 2.3% in f3, so maybe more refined
dimensions would resolve the remaining error.
Having got a reasonable match on Zft, perhaps we should have a quick
look at some of the Q-dependent measurements. Looking at your sweep
charts, the bandwidth of the probed coil f1 looks to be around the
3 kHz region, so maybe the Q predicted at 102 is not too far from
reality. For the probed coil, the model predicts:
Q: 102
Rin: 293 ohms
Vgain: 119
So far it's still dry and sunny outside, so the prospects for tests
on a system with Lfac=110% are encouraging. It's also time that I
collated the various 'top voltage testing' measurements and put
together another web page.
Cheers,
--
Paul Nicholson,
Manchester, UK.
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.