From: "Terrell W. Fritz"
Date: Fri, 26 Jan 2001 14:54:55 -0700
Subject: Re: [TSSP] Top voltage testing
Hi Paul, At 08:18 PM 1/26/2001 +0000, you wrote: > >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. I connected a tiny 501.88 ohm resistor to the ends of the wires to the coil and read 1.275mArms with (0.7071-0.001275) volts in. V/I = R = 553.6 ohms. So something is messed up 10%. I bet the HP meter does not like the tiny voltages at such a high frequency. Just because it is a six digit meter is no garantee that those last digits are anything but noise in this case... Hopefully it is a measurement error instead of a modeling error. I'll try to figure this out and get it working. > >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. Yes. Let's get this Zft figured out before we trust the other stuff. > >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? I found that the current and voltage reading had two different peak frequencies at f3 and f5. Here is the data using the voltage readings from the scope. VoltsRMS AmpsRMS Frequency f0 54.3 1.120mA 104912 f3 10.7 423uA 334435 f5 4.49 38.5uA 504258 For the current peak on the HP meter: VoltsRMS AmpsRMS Frequency f0 54.1 1.123mA 104861 f3 9.79 465uA 333868 f5 3.00 206uA 501435 I will build a current shunt with a higher value R like 10 ohms and see if that helps things. Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.