I am designing a Polyphase filter as the main block for my Quadrature phase generator.
I am using the 1st order polyphase filter to check how much insertion loss can be expected before I fix the design. After the SP analysis I got an insertion loss of (S21 = 7.6dB)
I have attached the circuit I used and the testbench. What can do to improve my testbench to get a lower insertion loss (~ 4dB)?
There is not really enough information to provide a thorough response to your question as to improving the insertion loss from you reported value of -7 dB. The insertion loss and phase quadrature characteristics of a first order polyphase filter are highly dependent on the input frequency, the values of the RC components in the filter, the source driving resistors, and the output load. I might recommend you initially sweep the input frequency and observe the insertion loss as a function of frequency. It is possible that you have not chosen the poles of your filter to provide the required quadrature phase relationship at your desired frequency. Since the principal of the circuit is to provide a +/-45 degree phase contribution of a combined LPF/LHF to the input signal, there will be an inherent loss in the filter. It is also worth noting that the source and load impedances impact the HPF and LPF corner frequencies differently. Hence, you may need to adjust their values separately to properly account for the source and load impedances.
I am sorry I can not provide an exact answer to your question, but there are too many unknowns about your design for me to provide a specific answer.