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Re: free standing coil


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

Steven Roys writes:

SR>How do you measure the current into your coil?  I don't think you can 
SR>use a regular meter since the frequency response drops off as you go 
SR>above 60Hz (at least mine does - at kHz frequencies, my DVOM is 
SR>substantially down compared to my VTVM). 

Right, Most DMMs don't have good frequency resonse, though I have an HP 
which goes to about 100kHz. I actually measure the voltage at the top of 
the coil using a what amounts to a high impeadance capacitive divider. I
use a small probe wire connected to an ordinary scope probe and place it 
something like a foot above the top of the coil. To get consistent results 
though, you must make sure that you don't maove  any objects around beteen 
measurements, or else the capacitive coupling will change drastically. 


  Dave Sharpe at the 
SR>Richmond Teslathon made a comment about using the two reverse 
SR>biased led's to find f0 (a la Duane Bylund) and wiring in couple of 
SR>regular (silicon, zener???) diodes somehow to be able to measure the 
SR>current directly with a DC (I presume) meter.  For a real no-brainer, once 
SR>you find f0, you would simply adjust the output of your signal generator 
SR>to give a 10mA (or some other convenient power of 10) reading  on your 
SR>meter.  You then change your frequency until the meter reads 7mA (or 
SR>.7 x your initial reading).  Assuming that f1 and f2 are symmetric wrt/f0, 
SR>this will give you a df value directly to compute Q=f0/(2*df).

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

Personally, I'd go with the resistor-scope measurement. One note: don't 
forget that 1Mohm scope probes are only 1Mohm at DC, the impedance can be 
quite a bit lower at high frequencies. So I wouldn't use a very big 
resistor.


-Ed Harris