TSSP: List Archives

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.