Hey Mike,
I have the Oscope, and am planning on getting a signal generator.
Icurrrently have a source of RF, but only at one frequency (for whatever
crystal I plug into it.) Please, speak! Knowledge is never wasted, and I
would like to hear what you have to say...
Thanks,
Dave
I thought I had this written up in a word file but couldn't find it,
probably lost
in a harddrive crash or upgrade.
First you need to resonate the inductor at the frequency you want to know
the Q.
You should use a good low loss capacitor, I usually use polystyrene.
This method uses the difference between the upper and lower 3db points.
The less loading of you resonate circuit the more accurate the result.
Let's use a 10uh inductor and a 1000pf capacitor as a parallel resonate
circuit.
This should resonate at about 1.59Mhz, however when you put the (X10)
scope probe across the circuit it will slightly change the resonance and
loading.
{More on that later.}
You need to lightly couple some energy from your signal generator into the
resonate circuit. This can be done by placing the generator wires near the
resonate circuit close enough to get the scope signal level you need but as
far
away as possible so you don't load the circuit. You can couple it with high
value resistors if desired, but this increases loading on the inductor.
Then I adjust coupling and signal generator output to get 7 units
on the scope. Why 7 units? I'm glad you ask! You want to move the signal
generator frequency up until the voltage on the scope drops to 5 units.
Some explanation; We want to measure the upper and lower frequency points
where the voltage drop is 3db or .707 times the resonate voltage.
So, back to the 7 units, .707 times 7 units equals 4.949 units or 5 units
when
I'm looking at my scope.
So we adjust the signal generator frequency to peak the waveform on the
scope.
Let's say the waveform peaks at 1,596,200hz.
To get the 7 units sometimes I adjust the generator drive and sometimes I
change
the scope variable attenuator.
I move the frequency up until the scope reads 5 units, Record this
frequency.
Lets say it's 1,600,200hz
Now move the frequency down until the scope reads 5 units, Record this
frequency.
Lets say this is 1,592,219hz
Do the math 1,600,200 - 1,592,219 = 7981
then using the resonate frequency of 1,596,200 / 7981 = 200
The Q of your inductor is 200
{More on that later.} Ok this is later, I have been known to isolate the
scope probe with a
100k or 1meg ohm resistor. This helps reduce the loading on the resonate
circuit, But this
can also induce some 60hz into the scope wave form.
I think I got this all correct, it has been a few years since I did this. I
used it a lot when I was
trying methods to improve the Q of some potcore inductors.
Let me know if you have questions.
Mike