Re: MMC dissipation factor measurement

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Paul Nicholson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <paul-at-abelian.demon.co.uk>

Terry wrote:
> measuring the dissipation
> factor of a poly MMC cap?  Like <0.001 dissipation factor...

Interesting.  And you'd want it to be the dissipation at the
TC operating frequency...

How about:  Put the 'test' MMC into a calorimeter and incorporate it
into a cap bank attached to a primary to obtain the normal resonance.  

Couple in a couple of hundred watts of CW at Fres.  The temperature
inside the calorimeter will rise over a period of time to settle at
some value.  Measure the calorimeter temperature, the room temperature,
and the RF input VA *to the test MMC*.

Then replace the test MMC in the calorimeter with a resistance carrying
DC.  Adjust the DC to obtain the same temperature difference above
ambient as you got from the MMC dissipation.

Then the DC power equals the RF power that was dissipated in the MMC
and you work can out the loss factor from that.

If you can measure to 1 degC and obtain a temperature rise of say 
10degC above ambient in the calorimeter, then you'll get 10% accuracy.
You can probably measure the MMC volts and current to 1% accuracy so
the temperature measurements will be the dominant source of error.

The difficulty might be to actually get enough power dissipated in the
MMC to get a decent temperature rise.  You might need to put a whole
cap bank's work of MMC's into the thing, rather than the one!

> I have a TEK 3012 scope.

The box it came in could provide the polystyrene for the
calorimeter :))

OK, you don't like that plan?  Alright, another then:

Make up an air spaced parallel plate capacitor to the same value as
the test MMC.  Resonate each with a large inductor to the TC frequency
and obtain the two Q values with the pinger and tcma.  Use the
difference in Q factor to calculate the extra ESR introduced by the
MMC.  This method has two problems: you have to assume the air spaced
cap has negligible loss, and the result depends on measuring the small
difference between two Q factors. 

And a third method that you could try is to measure that phase angle.
Capture the V and I of the test cap at the TC frequency in some
convenient setup and measure the time delay between the zero crossing
points of the two waveforms.  Calibrate out the phase error of the
scope by repeating the measurement again with the roles of the two
Y amplifiers exchanged.  This last I think would be the least
accurate with only 10,000 points per sweep.
--
Paul Nicholson
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