iVenky
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Posts: 93
Silicon Valley

I have a very basic question regarding S21 when there is an impedance mismatch with the characteristic impedance.
I have attached a figure of the test circuit.
Assume that the Transmission line is loseless and has an effective length of λ/4 at Frequency=200GHz. When I plot the S11 at DC, it's simply
S11= (1M10)/(1M+10)~0.99998 (pretty close to 0dB, it's complete reflection at the input). S21 obtained is around 44 dB (based on simulation on ADS/Cadence). Is this because, S11^{2}+S21^{2}=1 for a lossless transmission line? What I don't get is the following.
At DC, the gain of the above network is 1M/(1M+10) ~ 1, on the other hand, S21 is much below (i.e. 44 dB). What am I missing? I thought S21 is an indication of insertion loss but this result is counter intuitive. I am so confused...
*Note that I am not interested in transmission line sparameters because in that case I have to use 50 ohm load. I am more interested in reflection coefficient and insertion loss when there is an impedance discontinuity and understand what s21 means in that case.
