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Dear OM,
On second thought the simple method of measuring Q of a coil
described by James Moritz can even be performed when the internal resistance of
the signal generator is unknown. If you don't possess a signal generator you
could even use your 136kHz or 73kHz transmitter, provided its output can be
adjusted to a low value and the harmonic content of the output signal is
low.
First connect the coil in series with a variable capacitor to the
signal generator, which is tuned to 137kHz . Read the output voltage of the
generator with a voltmeter or oscilloscope. Carefully tune the
circuit for series resonance where the voltage reaches a minimum.
Record that value. Now replace coil and capacitor by a low value variable
resistor. Adjust the resistor until the voltage has the
same value as the one recorded before. Next measure the value of
the variable resistor, for instance with a digital multimeter. The value so
found is equal to the series resistance Rs of the coil.
The inductance of the coil must be known. If not it must
be measured. You could also measure the value of the variable capacitor that
resonated the coil on 137kHz. Calculate the reactance of the coil: XL = 2
* pi * f * L (or of the capacitor: XC = 1/(2 * pi * f * C) with f =
137kHz.
Now find Q = XL/Rs or XC/Rs.
This substitution method has another advantage, apart from the fact
there is no need to know the internal resistance of the generator. The voltmeter
or oscilloscope is only used as an indicator to set the voltage with
the variable resistor to the same value as the one with the coil +
capacitor at resonance. So there is no need for calibration, good linearity or a
high resistance. Even a simple diode, followed by a multimeter could do the
job, provided the voltages to be indicated are sufficiently large to provide a
reliable indication using that simple arrangement.
73, Dick, PA0SE
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