Hello, I'm reading "RF Circuit Design" by Chris Bowick and a question arose in the resonant circuits chapter. The book says that you can couple capacitively two or more parallel LC tanks in order to get a better factor form (i.e. to get a steeper skirt when considering the amplification graph). The question is, why don't parallel directly the LC tanks without the capacitor? Why is it necessary to place a capacitor between them? Any thoughts?

Thanks!

 

Because two tank circuits in parallel would not be two tank circuits at a selected frequency but would instead be a single tank circuit at a completely different frequency. To see this you need to review parallel combinations of inductors (they work just like resistors) and parallel combinations of capacitors (they don't work like resistors).

 

Ok, I know that in the case that the resonators had a different resonance frequency you would end up with a complete different one, but in the context of the book I understood that the two resonators are equal (two resonators in parallel make a resonator of the same resonance frequency and twice the quality factor ignoring the component losses). Does the capacitor make any difference if they are equal?

 

Listen very carefully. From a signal flow point of view, you want the two resonators in series with a low impedance coupling capacitor between them. this gives you a four pole filter. Cascading two circuits with the same resonant frequency does not change the center frequency but it does change the rolloff. Two circuits in parallel with equal values of L and C is indistinguishable from a single circuit, and gets you only a 2-pole rolloff.

 

Thank you for the reply! Somehow I related the quality factor with the form factor but using poles/zeros it's clear now.