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.