From: Paul
Date: Sat, 27 Jan 2001 01:04:48 +0000
Subject: Re: [TSSP] Top voltage testing
Terrell W. Fritz wrote: > I measured the current for the 501.88 ohm resistor at many > frequencies ... Quite some variation of the small signal HP response with frequency, yet it's spot on at higher voltages. Most odd - wonder how it manages it? > So the higher voltage is good but tiny voltages start to loose it above > 20kHz. Fortunately, the loss is predictable (this is when the extra $$ for > the HP meter pays off :-)) Yes. Interestingly, the procedure you're using to calibrate the current readings against a known resistance is essentially what you'd be doing if you were using a CT instead of a current shunt. > > 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% > > > So at 105kHz, 527.14 / 501.88 = 1.05033 > Zft = V/R or corrected = V / (R / 1.05033) = 42.6 x 1.05 = 44.73K Which explains why we only saw 1.6% error in Zft in your NSVPI bare coil results, rather than 4 or 5%, the scope error is compensated by an almost equal HP small signal HF error. Wicked. > I would submit the measurement error is found and corrected and all > is well with Zft :-)) Possibly, although we're not quite out of the woods yet. A final (final) crosscheck might be to turn up the generator voltage, say 10V instead of 1V peak across the 500 ohm. If you're right, you'll get something nearer to 501.88 than the 527.14. At the same time, you could afford to increase the current shunt size, > a current shunt with a higher value R like 10 ohms yes, maybe higher still, 50 or 100. With 100 ohms and 10V drive you will have 1000 times the voltage to sample, and the R should converge then to 501.88. If it doesn't we might have to conclude that the resistance of the 500 ohm is higher than nominal at HF, and we are back to square one with Zft. Significant that you got different values for R on two different occasions - shows that something is uncontrolled somewhere. The resonant frequencies f3 and f5 are a little out, -2.7% and -4.2%, I may be pushing the limits of the capacitance program, as it's setup for speed rather than accuracy right now. Too tired tonight to do any more but tomorrow I'll fiddle the caps values to see just how much difference 4% in f5 makes to Zft. Thanks for coming back so quickly with these extra checks, it'll be good to pin this Zft down firmly before moving onto the trickier stuff. Cheers, -- Paul Nicholson, Manchester, UK. --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.