From: "Terrell W. Fritz"
Date: Fri, 16 Feb 2001 20:02:15 -0700
Subject: Re: [TSSP] Top voltage testing
At 03:03 PM 2/16/2001 +0000, you wrote: >Hi Terry, > >The situation is looking a little better with the raised signal level, >is this once again the HP reading in error with small signals? Maybe, I will have to run some calibrations... > >> The reason the current is dropping with the probed coil is due to >> the Q of the coil dropping with the probe in place. > >Ah, I haven't been bothering to take account of the resistive part of >the probe, since other losses are not well quantified - I assumed it >wouldn't make much difference. In fact, I put in a 10 Meg load resist- >ance alongside the probe capacitance, and the probed coil Q fell to 102 >compared with 195 for the bare coil. Previously, without the allowance >for the 10 Meg, I got a Q around 170 for the probed coil. Your base >current readings have highlighted the fact that I must take the probe >resistance into account too. > >Adjusting the model's ctop to match your new f1, we now get: > > measured modeled error > f1: 310.9 kHz 310.9 kHz 0.0% Ctop adjusted to match: 3.145 pF > f3: 847.8 kHz 828.4 kHz -2.3% > > Zft: 34450 ohms 34877 ohms +1.2% > Les: 17.64 mH 17.85 mH +1.2% > > Lfac: 79.8% 80.8% > >You mention that the coil is not 'perfectly well known', and we have >a 2% discrepancy in the Ldc, with 2.3% in f3, so maybe more refined >dimensions would resolve the remaining error. > >Having got a reasonable match on Zft, perhaps we should have a quick >look at some of the Q-dependent measurements. Looking at your sweep >charts, the bandwidth of the probed coil f1 looks to be around the >3 kHz region, so maybe the Q predicted at 102 is not too far from >reality. For the probed coil, the model predicts: > > Q: 102 > Rin: 293 ohms > Vgain: 119 With the signal generator cranked up, I get using the transformer/resistor box. Ibase = 1.175mA Vbase = 0.48885V Vtop = 40.80 Qtopv = 72.5 QIbase = 70.98 So Rin = 0.48885/1.175 = 416.0 Vgain = 40.8/0.48885 = 83.46 This coil uses white PVC for the form that is 1/16 inch thick. The diameter is 4.250. The length is 26.125 The turns are 1175.5 Rdc = 33.40 ohm I tried hooking the generator directly to the base of the coil and got over 1kV peak to peak on the top but the Q was still around 70. I tried using the amplifier but that only raised the Q a small amount. However, I raised the coil about 2.5 inches off the ground plane and the Q did go up significantly. I will have to clean up the setup to know how much but perhaps eddy currents in the foil are causing to low Q. If I we to make a radial counterpoise out of #24 wire, do you have any idea how many radials I would need and their length for an effective plane? That would eliminate a lot of ground plane effects. I could also raise it off the floor to hide test stuff under it. I could probably only build one big enough for the 26 inch coil. Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.