TSSP: List Archives

From: "Terrell W. Fritz"
Date: Thu, 25 Jan 2001 21:24:37 -0700
Subject: Re: [TSSP] Top voltage testing

Hi Paul,

At 09:58 PM 1/24/2001 +0000, you wrote:
>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.

That does seem to be a "real" observation.

>
>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.

I took the generator to work and the HP readings agree dead on.  The scope
(or probe) seems to add a little gain at higher frequencies.  The ranges
were the same and the Tek scope seems real good at range to range consistancy.

>
>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?

Good point!!  Pics of the setup are at:
http://hot-streamer.com/temp/P1210001.jpg
http://hot-streamer.com/temp/P1210002.jpg


>
>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?

They are only two inches long.  Probably don't pick up too much.

>
>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.

Very possibly the shunt capacitance.  Unfortunately, when I took the
generator to work I also "drop tested" it :-(  I'll have to recheck it to
see if it still works.  However,  I am working on replacing it with a new
15 MHz HP33120a arbitrary function generator.  Hey! gotta have it :-))
I'll try to evaluate the shunt C loading if the old generator still works.
It should be dry by now (why not drop it in water if your gonna drop it...:-))

>
>Cheers,
>--
>Paul Nicholson,
>Manchester, UK.
>--


Maintainer Paul Nicholson, paul@abelian.demon.co.uk.