In a two-port system where a square signal passes through a via we can get a HIGH Q resonance structure which could ruin our S21.
I know that resonance is a standing half-wave pattern, so what what causes S21 to plunge?
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what what causes S21 to plunge?
S21 is the ratio of power leaving port 2 relative to the power into port 1 so, if S21 plunges, the (effective) input impedance must also plunge in order to have real power entering port 1. So, in this respect you can't really model the via like a parallel tuned circuit because it wouldn't alter S21 at all (output power ratio to input power would be the same but, power input would be very small.
However, thinking of the via as a parallel tuned circuit is convenient for explaining the loss of power reaching the output port. But, you also have to consider the via as a short length of unmatched transmission line. This creates an input impedance to signal source related to the reflected power at the via and, interfere with the S21 measurement.
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\$\begingroup\$ Hello Andy, can i think that the resonator causes the signal to be trapped in the medium going back and forth and not reaching the destination? thanks. \$\endgroup\$– Josh23Commented Jan 3 at 21:46
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\$\begingroup\$ You can but it's more standard to think of it as a blocking impedance. But, a blocking impedance isn't sufficient to explain a plunging S21 hence, we need to think of the via as creating a transmission line discontinuity also. \$\endgroup\$– Andy akaCommented Jan 3 at 22:22
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\$\begingroup\$ Hello Andy, Yes I understand the VIA is a discontinuity, but in resonance the energy is stuck in the middle so its not absorbed in none of the ports straight away.so with using the discontinuity point could you please explain how that the energy is stuck in the middle with an example so i could simulate? Thanks. \$\endgroup\$– Josh23Commented Jan 4 at 4:27
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\$\begingroup\$ It isn't sufficient to regard it this way as I have pointed out in my previous comment and my answer. \$\endgroup\$– Andy akaCommented Jan 4 at 9:17
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\$\begingroup\$ Hello Andy ,i'll be glad if there is some transmission line example i would see your good explanantion. \$\endgroup\$– Josh23Commented Jan 4 at 12:29