Q Factor and coupling between resonant circuits

[doneirik]

Dear forum,

I am conducting some tests on coupling between resonant circuits. Some of the results I cannot really explain, could anyone be of help.

Problem:
Main LCR circuit is tuned to 13.56 MHz/50 ohm and driven by a 5V (peak to peak) signal.
Secondary circuit is a closed circuit of 2 coils and a capacitor tuned to resonate at 13.56 MHz. One of the coils is coupled to the coil in the main circuit the other is the "output" coil (to which I want to couple a third circuit).

I measure the Q-Factor of the main circuit to be 18. Q-factor of the secondary circuit alone is measured to be 7.
However, the Q-Factor of the main and secondary circuit together is more than 40!!

According to the theory I have read, a coupling between two resonant circuit will always result in a Q-Factor that is lower than the Q-Factor of the single circuits. Why do I get a much higher Q-Factor??



regards
doneirik

 

[Warpspeed]

I suppose it depends on how you measure and define circuit operating Q.

If it is pure voltage or current magnification at resonance for a given input power, then adding extra tuned circuits to it will most likely increase insertion losses and reduce Q.

But if you are talking about half power bandwidth and selectivity, then more tuned circuits, provided they are below critical coupling, will sharpen the selectivity, and by definition possibly increase circuit Q.