From: "Terrell W. Fritz"
Date: Thu, 28 Dec 2000 12:27:50 -0700
Subject: Re: [TSSP] coil measurements
Hi Marc, At 04:08 AM 12/28/2000 -0500, you wrote: > >i would now like to do the capacatance tests like terry has just shown >and any other testing that can be of help, readings with topload, or i >would love to make Q and coupling measuremnts on some of these if anyone >could be so kind as to offer an explenation? >hope some of this helps and i will keep pluging away blindly as time >goes on. >marc > The coupling can be measured with the info at: http://www.pupman.com/listarchives/1997/november/msg00898.html I guess there may be a small error in that method in light of yesterday's stuff but its the best "I" know of at the moment. For capacitance, I just hook one side of the meter to the otherwise unconnected primary and the other meter lead to the otherwise disconnected secondary and read what the meter says. For Q, hook the coil to a signal generator and find the frequency that gives the peak reading on the scope antenna or whatever (just like finding Fo). Then find the two frequencies (one higher and one lower) that give a signal that is 70.71% of the peak value you read before. This gives your three readings. The peak, the lower 71% of peak frequency, and the higher 71% of peak frequency. Q then equals: Q = Fo / (Fh - Fl) Where: Q = Quality factor. Fo = Center resonant frequency. Fh = The upper frequency where the signal level is 71% of the peak. Fl = The lower frequency where the signal level is 71% of the peak. So if: Fo = 100 kHz Fl = 95 kHz Fh = 104 kHz Q = 100 / (104 - 95) = 11.11 71.71% is actually 1 / SQRT(2) or the 1/2 power point. If you drop the voltage by 71% (70.710678118654752440084436210485...% ;-)), the power into say a resistor is 1/2. Using a little magic the above formula gives Q based on this. (Fh - Fl) is also known as the bandwidth, a term that is used often in all kinds of electronic things. So, Q can also be termed the center frequency divided by the bandwidth. That should get you started. Ask away if you have questions, here or just to me as you feel best. Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.