From: "Terrell W. Fritz"
Date: Fri, 26 Jan 2001 15:58:39 -0700
Subject: Re: [TSSP] Top voltage testing
Hi Paul, I measured the current for the 501.88 ohm resistor at many frequencies with the following results: F(kHz) I(mARMS) VRMS(in) (meter) Rcalc 1 1.361 0.67798 497.148 2 1.353 0.67343 496.731 4 1.354 0.67495 497.486 10 1.352 0.67435 497.780 20 1.351 0.67568 499.133 40 1.341 0.67721 504.004 100 1.309 0.67710 516.265 105* 1.290 0.68130 527.140 200 1.157 0.67928 586.105 400 0.814 0.68102 835.634 1Meg 0.0848 0.71268 8403.23 *I took this measurement later and the level drifted somewhere. Need that HP 33120 :-)) 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 :-)) Probably the only multimeter in the world that can do this stuff). > 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 I would submit the measurement error is found and corrected and all is well with Zft :-)) We are working at the limits of the equipment (wish the Tek scope was closer to the HP**) but the errors all seem to be converging to the model's predictions eventually. If I win that HP signal generator on Ebay, it will help a lot to determine error since it can act as a very accurate calibration source for this stuff. If I get out bid, I will just buy one... I will try to get the metering to simply work right in the first place. This correction table can be applied to my last post... **The HP is AC coupled true RMS while the scope is AC+DC True RMS. That may explain the error a little... But when I switch the scope to AC coupling nothing changes... Perhaps the far higher scope bandwidth is picking up high frequency noise or it is sampling error in it's rather course vertical digital sampling... From the HP comparison it appears to be high, but it has always seemed to give the right answers... Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.