I have created a differential channel with Transmission line and interconnects for simulation. In the real channel, there is 100nF capacitor in between 2 transmission lines. Now the problem is when I put this capacitors in series while connecting the transmission line (TL), I see very steep transition of S21 as well as S11 at low frequency (around kHz level). but If I remove the capacitors the transition is not there at those very low frequencies. I am not sure why is this transition happening at low frequencies while capacitors are placed? I have given the pictures below:

Simple block channel with capacitors: enter image description here

S21 and S11 for the channel with capacitors: enter image description here

Simple block channel without capacitors: enter image description here

S21 and S11 for the channel without capacitors: enter image description here

  • \$\begingroup\$ The capacitor's impedance is very high at low frequencies, in terms of S-parameters this means large reflections at low frequencies (S11 -> 1 or S21 -> 0). Just the normal circuit behavior though. \$\endgroup\$ – Captainj2001 Jul 12 '18 at 14:14
  • \$\begingroup\$ This is not a transmission line affect rather just a “lumped element” RC High pass filter but you can the effects on transfer function and input impedance s21 & s11 \$\endgroup\$ – Tony Stewart EE75 Jul 12 '18 at 14:28

Your transmission line has an impedance of (possibly) 50 ohm and it is terminated in 50 ohm hence the two capacitors (net value 50 nF because they are in series) form a high pass filter with the 50 ohm load of the t-line.

The 3 dB point of 50 nF and 50 ohm is about 63.7 kHz and pretty much occurs where the slope is in your first graph.

  • \$\begingroup\$ thanks for nice explanation. I have a confusion. As my channel is differential and shouldn't it be 100 ohm? I have terminated in both the ends of n channel with 50 ohm and did the same for p channel. So it should be overall 100 ohm isn't it? \$\endgroup\$ – aguntuk Jul 13 '18 at 12:31
  • \$\begingroup\$ I just guessed at 50 ohm and used that as an example. If it's 100 ohm then the 3 dB point will be at 31.8 kHz and still pretty much in that transition frequency. \$\endgroup\$ – Andy aka Jul 13 '18 at 12:34
  • \$\begingroup\$ aha now make sense. thanks a lot for clearing it up :) \$\endgroup\$ – aguntuk Jul 13 '18 at 12:36

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