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Re: Making a Q-meter /

 

On Thu, 4 Aug 2022 at 10:13, Kenneth Greenough via <g8beqglossop=[email protected]> wrote:
Re' a Q meter.
This is only a thought idea, not tested hi.
Have a small enclosure with sockets on opposite sides. each socket has a
single turn loop attached with sufficient distance between the two loops
to allow the coil under test to be positioned between them. one turn is fed
from your sig' gen' and the other feeds a scope or RF meter.? A sufficient
level from the sig gen may require an amplifier. With the coil to be tested
placed between the loops tune the sig gen for maximum throughput, ie
resonance. Then tune the sig gen lower until the throughput is 3dB
down, ie 0.71 reduction in voltage or 1/2 power. Repeat by tuning the sig gen
higher than resonance for a similar drop in throughput. the resonant
frequency divided by the bandwidth just ascertained is the Q of the coil.
The coil being tested can, of course, have a capacitor in parallel if wished.
73 Ken G8BEQ
?
Apologies for not replying earlier. Yes, this is a known method, although it seems more common to use a VNA now and measure S21 as that makes sweeping the frequencies much easier.

Both inductive and capacitive coupling can be done. That measures the loaded Q. I think I am right in saying that the coupling factor at each end of the DUT needs to be the same in order that it computes the unloaded Q, which is what is required. See particularly section 3 of

/g/Test-Equipment-Design-Construction/files/Q-factor%20%28Q%20is%20the%20inverse%20of%20dissipation%20factor%20DF%29./Q%20factor%20measurements,%20analog%20and%20digital%20by%20Darko%20Kajfez.pdf

A good practice guide from NPL does this, but fits the transmission curve, not just a measurement of 3 places.

/g/Test-Equipment-Design-Construction/files/Q-factor%20%28Q%20is%20the%20inverse%20of%20dissipation%20factor%20DF%29./Q-factor%20measurement%20by%20using%20a%20Vector%20Network%20Analyser.%20NPL%20Report.%20MAT%2058%20by%20Gregory,%20A%20P%20%282021%29/Gregory,%20A%20P%20%282021%29%20Q-factor%20measurement%20by%20using%20a%20Vector%20Network%20Analyser.%20NPL%20Report.%20MAT%2058.pdf

Dave

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