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Re: Scoping of Secondaries

After reading your post from the other day, I went home and played 
around a little more with my scope and coil.  I found out a few things...

1. You should turn all your equipment on and let it warm up for at least 
1/2 to 1 hour before using it.  I was sitting there watching my frequency 
generator change frequency by a few Hz every couple seconds for 
quite a while after I turned it on.

2. Series resistance does make a difference.  I have a 0-10 mHz 
Hewlett Packard signal generator that has 75 ohm and 600 ohm 
outputs.  When I used either the 600 ohm output from the signal 
generator or the cable that has the 1K resistor wired in series , I 
measured Q's in the neighborhood of 30-50.  When I used the 75 ohm 
output with a regular RG59 coax cable and no series resistor, I got a Q 
of 200.

This raises some other questions.  I'm assuming that a signal 
generator with an even lower impedence would give an even more 
accurate value for Q, but since I have what I have, is there any way to 
compensate for this resistance?  Also, rather than just believing the 
label that says 75 ohms by the output jack, what can I do to confirm or 
compute the actual impedence of my signal generator?  Would I be 
able to measure the open circuit voltage and the short circuit current (at 
say, 60Hz) and use these values to calculate the internal resistance, or 
is there something more complicated involved?


Steven Roys (sroys-at-radiology.ab.umd.edu)