[Lowfer] Still looking for a capacitance dissapation factor test

Thu, 20 Nov 2003 09:39:48 +1300

Bill

 Yes- I'm finding that measuring Q when it's higher than about 500 or so is
quite challenging- and I've had similar issues with a Tek probe as well. I
guess as long as one can characterise all the test gear correctly it's
possible to compensate readings but it seems to be bordering on the
uincertainty principle sometimes!

73
Dave
ZL3FJ

----- Original Message ----- 
From: "Bill Ashlock" <[email protected]>
To: <[email protected]>
Sent: Wednesday, November 19, 2003 12:22 PM
Subject: Re: [Lowfer] Still looking for a capacitance dissapation factor
test

> >I connect the scope to the transformer primary and adjust the
> > > turns in the transformer so that at resonance the voltage dips to 1/2.
I
> > > then calculate the what the Rac load on the secondary must be by
> >dividing
> > > 180 ohms by the square of the number of turns. For example, if the
turns
> > > count is 30: 180/900 = 0.2 ohms. The combined Q is the XL of the loop
> >(or
> > >Xc  of the capacitance) divided by 0.2.
>
> >How does this loaded Q figure compare to one derived from a measurement
of
> >the -3dB points (with the same setup)?
>
> Hi Dave,
>
> As far as I can tell they are close but it's really hard to find the -3 db
> points with the Wavetek when the loop Q is running up to 800 (remember
this
> is 1700k, not 185k or 137k). It's very easy to tune for the dip in voltage
> that I mention above. I'm finding that I can't trust the normal ways of
> measuring loop current. Even with a descent Tek current probe the loop Q
> falls to considerably less than its actual value because of the insertion
> loss (actually less than 0.1 ohms).
>
> Bill A
>
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