TSSP: List Archives

From: "Malcolm Watts"
Date: Mon, 19 Feb 2001 09:23:12 +1300
Subject: Re: [TSSP] Top voltage testing

On 16 Feb 01, at 15:03, Paul 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?
> 
> > 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. 

I did mention it a while ago didn't I? Again you have to consider the 
transmission line equations and the effect of Zft^2 on one end with a 
load of some sort at the other. A simple calc reveals this starkly. 
This problem reared its ugly head back in '94 when I was researching 
my article and it was from this that I decided Q measurements were 
best taken with the probe at a distance. I understand of course that 
the modelling will take this resistance into account for direct probe 
connection.

Regards,
malcolm

 
> 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 
> 
> So far it's still dry and sunny outside, so the prospects for tests on
> a system with Lfac=110% are encouraging. It's also time that I
> collated the various 'top voltage testing' measurements and put
> together another web page.
> 
> Cheers,
> --
> Paul Nicholson,
> Manchester, UK.
> --
> 



Maintainer Paul Nicholson, paul@abelian.demon.co.uk.