# Series-parallel tank circuit resonance condition and impedance

[STRIKE]Thank you very much for taking the time to analyze the circuit. I still don't quite understand, and maybe I am missing something, but your graphs only confirm what I have said (was this your intention?). I should also clarify a few things:

1. I am not measuring S11, rather (1+S11)/(1-S11) * 50 which gives me impedance.
2. abs shouldn't matter when looking for resonance, the imaginary (reactance) will be 0 at the resonant frequency, so when looking for a series resonance minimum (as vk6kro suggested), it is sufficient to look only at the resistance.
3. The graphs you attached show a peak around 2.25 Mhz. I produced the same result with the MATLAB code I c/p earlier, only I changed the cap values to 511 and the inductance to 9.4*10^3 nH. It seems we are in agreement.
[/STRIKE]
Scratch everything I just said. I discovered semilog plotting in MATLAB -_-.
Thank you!
If you don't mind me asking:
1. does this mean that this circuit has 2 resonant modes?
2. Say I run a signal through this circuit at some carrier frequency. If I want to tune and match this circuit, which resonance frequency (if there are 2) would I tune it to. And if I were to match it to a 50 ohm coax, would I need to do this at or near this resonance frequency, or can I set the resonance frequency higher then my carrier and then match it elsewhere (away from peak) to 50 ohms.

In short, does my tank circuit need to be tuned at the carrier for minimal power loss. I have heard stories of no power loss away from the impedance peak if everything is matched. Is this true?
I hope my questions make sense. Thank you again.

Last edited:
The Electrician
Gold Member
1. Of course, this depends on just is meant by "resonant mode". There are 2 frequencies where the phase angle of the impedance is zero--where the imaginary parts of the impedance are zero. Sometimes the term "antiresonance" is used to describe the high impedance peak.

2. I can't answer this in a single sentence. It sounds like you are concerned with impedance matching. You should consult a book on RF design for information about matching networks.