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Okay the idea is to use the following band-pass active filter:

enter image description here

To this filter I want to add a switched-capacitor filter to replace the resistors with capacitors and switches. I'm using the formulas \$f_{c1}=\frac{1}{2\pi R_1C_1}\$ and \$f_{c2}=\frac{1}{2\pi R_2C_2}\$ with \$f_{c1}=2kHz\$. Using those formulas with \$R1=R2=2k\$ we get \$C_1=39.78n\$ and \$C_2=19.89n\$. Finally, we just need to replace the resistors. For that I want to use a clock with \$T_s=1MHz\$ and to calculate the needed capacitors I'm using \$C=\frac{1}{T_sR}\$. Then, the capacitors have the same value (because of the same resistors) and are qual to 0.5nF. The schematic is: enter image description here

All seems fine. However I find the following problems:

  1. The bandwidth isn't the desired (it should be 2k, but I get about 4k).
  2. The working of the circuits seems independant of the clock pulse. I tried changing it from 1Ghz to 1Hz and the response in frequency is the same. That is strange because clearly something should change if I change the clock.

enter image description here

So, what is going on here? What could be wrong? Am I ignoring something?

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How did you simulate the frequency response for the S/C circuit? Note that you must not apply the ac analysis in the frequency domain. This analysis is allowed only for linear - and time continuous - systems. That is the reason for not observing any influence of the clock frequency.

For finding/simulating the transfer function in the frequency domain you must replace all S/C blocks with time-continuous equivalent blocks.

For this purpose there exist two basic techniques as described in the relevant literature.

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  • \$\begingroup\$ The OP could use a simulator like SIMPLIS that can do frequency response on a transient model. But for PSPICE you need the averaged models. \$\endgroup\$ – John D Jun 28 at 18:22

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