From: "Terrell W. Fritz"
Date: Sun, 21 Jan 2001 21:44:56 -0700
Subject: Re: [TSSP] Top voltage testing
At 09:40 AM 1/17/2001 +0000, you wrote: >Terrell W. Fritz wrote: snip... >> What is odd is that the bandwidth Q and the voltage rise Q are >> always the same in the simple lumped element models. > >When the current is non-uniform the voltage gain is no longer equal to >Q, as expected with an idealised lumped coil. The voltage gain is >given by > > Vgain = Q * 1/(w * sqrt(Lee * Cee)) > >It's the factor 1/(w * sqrt(Lee * Cee)) that makes the difference. >For a TC with average or large h/d ratio, we have > > Lee < Les, >and > Cee < Ces, > >and for all coils, w = 1/sqrt( Les * Ces) > >which means that the factor 1/(w * sqrt(Lee * Cee)) is a little larger >than unity for medium to large h/d. For small h/d the factor can be >smaller than unity, ie gain lower than Q. Adding topload pulls the >gain towards unity from either side. This stuff was in the section I >deleted from pn2511 - there was a sentence I couldn't prove so I've >deferred it to the next installment of 'theory notes'. > >> Bandwidth Q Vtop = 74.055 >> Bandwidth Q Iin = 78.36 > >> For the last Q measurement I used the current input multiplied >> by 0.7071 to find the Q but got a different answer than using the >> top voltage. > >> Do you think using the input current as a measure of Q is valid >> and should it be equivalent to the top voltage Q? > >They should be the same. The transimpedance ratio Vtop/Ibase varies >very slowly - I checked this with the model and the two methods >gave the same Q to within 0.5%. I think we need to take a closer >look at the base drive arrangement in order to understand this >discrepancy. We might also need to look at the harmonic content of >your signal generator output at some point too. > >> here is a 0.995 surface mount resistor in series with the >> generator output, so the driving impedance is around 1 to 2 ohms. > >I guess that's the resistor across which you sample the current. Is >that done with the scope probe? Which end takes the scope ground? >Something must be floating here. Is there a shunt resistor to the >groundplane upstream of the series resistor? Perhaps you could tell us >a bit more about what's connected to where. > >> YT5060 through ~1 meter 30ga. wire >> Fo = 95.309 kHz > >Well it seems the thin wire does nothing but add capacitance. >I thought using the wire might take a bite out of the extra C apparent >in the toroided case - presumably C between the toroid and probe body. >In the bare-coil case you don't seem to have any extra 'probe body' C >to begin with. In view of this I'll focus on the directly connected >105.05 kHz results. > >If I fiddle the loss factor to obtain a match with your measured >Q=69.8, we get > > Measured Modeled >Vgain 77.2 76.0 -1.6% >Zft 49.95 Kohm 44.7 Kohm -10.5% >Rin 647 ohms 588 ohms -9.1% > >We should concentrate on the major Zft discrepancy to begin with, as >this is virtually independant of Q, ie it's a function of the react- >ances only. The weak area is the input current measurement. To fix >this discrepancy we need to register a higher current, ie a higher >drop across the 0.995 ohm series resistor, which leads me back to >wondering where the scope probe ground connects to - we may be loosing >some V from that reading. Getting to the bottom of this Zft discrep- >ancy may make your two methods of Q reading converge, since Zft is >heavily involved with the difference. > >Cheers, >-- >Paul Nicholson, >Manchester, UK. >-- I retook the Q measurements for a top loaded coil again twice with the following results: Test F Ibase Vtop Fo 70955 1.590 51.9 Fl 70482 1.124 36.7 Fh 71482 36.7 Fh 71449 1.124 Q 73.4 71.0 Test F Ibase Vtop Fo 70958 1.595 52.1 Fl 70472 1.128 36.8 Fh 71455 36.8 Fh 71317 1.128 Q 84.0 72.2 Voltage gain Q = 52 / (0.7071 - 0.0016) = 73.7 I then went back for a third time trying my best...: Test F Ibase Vtop Fo 70964 1.592 51.9 Flv 70486 1.134 36.7 Fhv 71482 1.068 36.7 Fli 70475 1.125 36.7 Fhi 71440 1.125 38.1 Q 73.5 71.3 Voltage gain Q = 51.9 / (0.7071 - 0.0016) = 73.6 This was with the 8 x 22 toroid 46 inches (center line) off the floor I did notice that the Fl measurement for current was more sensitive than the Fh measurements. So to make a long story short. The voltage Q seems slightly low but the voltage and current Q do seem to match. The errors seem to be due to the sensitivity of this measurement and were probably related to my movement near the coil's fields. I have to work fairly close to the coil and I think I moved too much in some of the other measurements. The last one should be the best by far since I tried really hard not to move around. the voltage Q does seem consistently low but that is probably due to calibration error in the scope's relatively course level measurements which tended to bounce around a lot. So all seems well... So I removed the toroid and redid the measurements in light of being carful not to move or disturb anything during the test. Test F Ibase Vtop Fo 105060 1.179 53.4 Flv 104304 0.833 37.8 Fhv 105843 0.730 37.8 Fli 104304 0.833 37.8 Fhi 105724 0.833 40.9 Q 74.0 68.3 Voltage gain Q = 53.4 / (0.7071 - 0.0012) = 75.65 So the voltage Q still seems a little low... So I hooked the HP meter to the bare coil top along with the scope probe... The HP has like a 1 Meg ohm and 100pF load along with the long wire. but I tuned it and I got: HP 34401a 14.571 VACrms TEK B210 15.4 VACrms So the scope appears to be reading about 5.69% high. Unfortunately, that would make the Q reading for voltage even lower instead of higher :-p I hit the self recalibration button on the scope and the reading dropped to 15.3. I then put another probe on the meter's inputs and read 15.4. So the scope and the meter are reading a bit different values. They calculate RMS a bit differently in that the meter is AC coupled. I put the scope on AC coupling but nothing changed. so I am not sure what is going on there. The meter should have an ACrms accuracy of 0.7% at this range and frequency... the scopes has a 256 bit vertical resolutions so it is pretty course. It's accuracy is around ~4% (the "spec" reads like a Florida ballot :-)) The probe adds some error to but it is not specked. Since the signal generator has a very accurate output I measured it with both instruments at a variety of frequencies. The HP meter seemed deadly accurate up to almost 1MHz while the scope seemed to vary a bit. they read exactly the same at 1MHz but the scope reads a bit higher at higher frequencies. The scope also has a little offset and such as well as low level noise. However, all that would tend to make it read higher not lower... So the accuracy problem (if there is one) is probably the scope (or probe) but it would appear that the voltage Q is too low. I am at the limit of my equipment here to get any more detail on that. BTW - I measure current though a 1 ohm resistor right at the signal generator output. The HP meter is connected across the resistor. The signal generator has about 0.25 ohm impedance at these frequencies. It has very low THD. I hope all this helps rather than adds to the confusion :-)) Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.