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Re: free standing coil

> The method I use to determine Q is the 1/2 power bandwith method: 
> 1) find the resonant frequency f0 of your coil with/without terminal
> using a sine wave signal generator and note either the peak current 
> into the coil

How do you measure the current into your coil?  I don't think you can 
use a regular meter since the frequency response drops off as you go 
above 60Hz (at least mine does - at kHz frequencies, my DVOM is 
substantially down compared to my VTVM).   Dave Sharpe at the 
Richmond Teslathon made a comment about using the two reverse 
biased led's to find f0 (a la Duane Bylund) and wiring in couple of 
regular (silicon, zener???) diodes somehow to be able to measure the 
current directly with a DC (I presume) meter.  For a real no-brainer, once 
you find f0, you would simply adjust the output of your signal generator 
to give a 10mA (or some other convenient power of 10) reading  on your 
meter.  You then change your frequency until the meter reads 7mA (or 
.7 x your initial reading).  Assuming that f1 and f2 are symmetric wrt/f0, 
this will give you a df value directly to compute Q=f0/(2*df).

Any idea how you would wire the regular diodes up to be able to read 
the AC current directly on a DC meter movement?  As an alternative, 
and since I like playing with my o'scope, I was going to hook up a 
resistor in series with my signal generator feeding the coil base and 
measure the voltage across it to get the current (i=v/r).  Any comments 
on this technique, either?


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