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Re: Scoping of Secondaries

From: "SROYS"  <SROYS-at-radiology.ab.umd.edu>

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 
********************************************************************


Steve,

	There is a method I use which makes the signal generator look like 
a zero impedance source but it takes more time: 

As many people do, I base drive the secondary with the signal generator 
and use a capacitive pick-up 2ft or more from the top of the coil. I 
measure both the voltage at the pick-up and at the base of the coil.

First I find the resonant frequency f0 and set the signal generator to some 
reasonable level so that the voltage at the base of the coil is say 0.1 volt.

Next, I adjust the frquency so that I get about .707 of the original 
voltage amplitude produced by the pick-up. But note that now the base 
voltage has changed because of the finite inpedance of the generator. So I 
now readjust the signal generator amplitude so that I get the original 0.1 
volts at the base. After repeating these steps a few times, you can home-in 
on the f- and f+ frequencies. 

This process of manually adjusting the signal generator output simulates a 
zero impedance source.


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Another simpler method for simulating a zero impedance source:

	Place a 1ohm (or something small) resistor in series with 
the ground connection and the coil. Then connect your signal generator 
across the resistor. AS far as the coil is concerned, it is now being 
driven by a 1ohm source. The disadvantage is that your signal generator may 
not be able to handle the 1ohm load or may not be able to drive sufficent 
current the the 1ohm resistor to get 0.1 volts or so.

-Ed Harris