From: Paul
Date: Wed, 24 Jan 2001 21:58:42 +0000
Subject: Re: [TSSP] Top voltage testing
Terry,
Thanks for the further Q and V/I measurements.
It seems that no matter how carefully you take the measurements, the
'Qi' always wants to come out larger than the 'Qv', the difference
being rather higher with the unloaded coil.
I'll focus on the final set of bare coil results, as these exhibit
the Q discrepancy the most. Adjusting the model Q to match the Qv,
we get
Meas Model Error
Q: 68.3 68.3 Adjusted to fit Qv
Vgain: 71.6 74.5 +4.1%,
Zft: 42.6K 44.7K +4.9%
[The Vgain of 71.6 comes from the measured ratio 75.65, reduced by the
cal factor 14.571/15.4 of the scope. I'm assuming the base voltage is
indicated by the sig gen output setting, which agrees with the HP
meter, and that the B210 is on the same vertical gain range when
measuring top volts as it was when you compared it with the HP. The
Zft is Vtop/Ibase with Vtop adjusted for the scope cal factor.]
Four things are of note. First the voltage gain is significantly
lower than predicted for the given Qv, and if I were to set the model
loss to match Qi instead, the shortfall in voltage gain would be even
worse. Secondly the Qi is significantly higher than Qv. Thirdly, the
Qv readings seem to be fairly repeatable and the Qi varies quite wildly.
And fourthly, the transimpedance is too low. This last one is quite
significant, bear in mind that Zft is virtually independent of Q
factor. We need to get a match on Zft before anything else, since if
this is out, everything else will be too.
Since you've checked the scope cal against the HP, and I'll assume
that the calibration isn't affected by a scope range change, then we
might look a bit more closely at the current measurement.
You mention the 1 ohm resistor is right at the generator output.
The current reading will therefore include current shunted to ground
through the feedline capacitance and thus never reaching the coil,
which will make the reading artificially high. The coil input
impedance is around 600 ohms, and the feedline C necessary to raise
the current by 5% is around 120pF, which corresponds to 3m of coax.
Perhaps you can move the current shunt to the coil end of the
feedline?
Also, if the conducting loop formed by the 1 ohm current shunt,
the HP probe leads, and the HP meter, is picking up an EMF induced
from the coil B-field, this would add a component (leading or trailing
by 90 deg, depending on loop orientation) to the voltage developed
across the shunt resistor, and the RMS readings from the HP would be
artificially high. Do you get a lower current reading when the HP
probe leads are twisted together?
Any instability in these two possible factors might also account for
the variability of the Qi readings.
Perhaps you could have a closer look at the current readings and see
if Zft can be brought into line.
Cheers,
--
Paul Nicholson,
Manchester, UK.
--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.