From: Paul
Date: Fri, 26 Jan 2001 20:18:37 +0000
Subject: Re: [TSSP] Top voltage testing
Hi Terry, All,
Glad the generator is still OK after its bath!
Terrell W. Fritz wrote:
> I disconnected the coil and the current fell to 3.4uArms. I
> disconnected the shunt resitor for the generator and got 5.3uArms.
> So the shunt C and noise seem to be insignificant. I also tried
> holding the wires and such but the current held steady.
Thanks. I see from the photos that you're using an open wire to
feed the coil base. That'll introduce much less capacitance than
a coax would - say 10 to 30 pF, or about 5uA RMS for 10pF.
As a final check, you could terminate the feed at the coil
base with a 600 ohm resistor, or thereabouts. A check of
the current reading, and a voltage reading with the scope probe
across the 600 ohm would confirm that all is well.
This 5% discrepancy in Zft is quite big, so I think we should focus
on Zft measurements and forget Q and voltage gain for the time
being. The ratio of top volts to base current is, for a high Q
coil, more or less independent of Q and is a function of the
reactances only. Failure of the model to get this prediction
right is pretty fundamental. It's not clear whether its a problem
with the theory, modeling, or measurements, so now's the time
for others on the list to step in with some measurements.
Terry, with your bare coil with the scope probe attached, with an
f1 of around 105.06 kHz, can you measure f3 and f5? This will
help to determine if the model is taking good account of the coil's
distributed capacitance. Eg, for the bare coil without probe, we
get
measured modeled error
f1 148.4kHz 146.5kHz -1.3%
f3 353.4kHz 349.6kHz -1.1%
f5 513.8kHz 502.0kHz -2.3%
which is reasonable.
I adjusted the probe capacitance in the model to get a match on
f1, but we haven't checked f3 and f5. If these are wrong, then
the Zft predictions will also be wrong.
The corresponding predictions for the probed coil are
measured modeled error
f1 105.06kHz 105.05kHz 0.0% Adjusted to match
f3 ??????kHz 325.4kHz
f5 ??????kHz 482.9kHz
Perhaps you could check these?
All,
As long as we're concentrating on Zft, a low source resistance is not
so important, providing the resulting Q is reasonable, say 50 or more.
This means that a series resistor of say 50 or 100 ohms can be used,
rather than 1 ohm. Those with signal generators which have a fairly
high output impedance will not be at a disadvantage when making Zft
measurements. The top voltage is easy, just apply the scope probe,
and calibration is straightforward - since we're after a ratio rather
than an absolute value, the voltage and current readings can be referred
back to one instrument, eg the HP in Terry's case, and we do not need to
know the absolute calibration of that instrument. The only fiddly bit
is measuring the base current. A CT can be built easily from a flyback
core with a single turn primary and about a dozen turns on the
secondary.
Meanwhile, I'd better take another look at how I'm adjusting for Terry's
probe capacitance and Q factor. The loss factor will affect Zft a little
but the model should be taking that into account already. Maybe the
effect on Zft varies depending on how the loss is applied - coil
resistance,
E-field loss factor, or top load impedance. I'll do some trial runs
at this end, but I'll be surprised if Zft is affected by more than 1%.
In short, Zft is a reasonably easy measured ratio, and is also a fairly
basic prediction of the model. If we can't get them to match then
something is well up the creek and it would be a major setback.
Cheers,
--
Paul Nicholson,
Manchester, UK.
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.