If you don't match impedance you get reflections, loss of signal integrity and all sorts of other horrible things.
Any matching cannot be said on this filter because the load is not obvious.The filters are generally designed with source and load impedances ( in fact resistances ) and the filter coefficients are found as a function of many parameters including source and load impedances/resistances.So, you should try to find a appropriate filter which satisfy your requirements.Can I have the question more specific?
I am really frustrated because I asked two engineers on my site but they reply me the same answer regarding termination and
I need to match termination. What's wrong with the circuit that I miss consideration.
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I don't exactly understand your problem. The filter can be designed for any termination, including complex ones. The only requirement is that either source or load have a sufficient real part. But the termination must be constant, otherwise the filter characteristic changes as well.By varying the source impedance from 50 Ohm, 120 Ohm to 1K Ohm, I obtained 3 difference AC response as below. Can I use this circuit or not because I don't have source impedance matching?
By varying the source impedance from 50 Ohm, 120 Ohm to 1K Ohm...
I didn't yet hear a clear statement about source and load impedance of the application of interest. Post #5 e.g. suggests real source and reactive load impedance.You need a "single terminated filter"
Can be solved with a diplexer, but this is a very elaborate design.but it's only achievable in the filter pass band. Out of band signals will be mostly reflected.
If you have a AC coupling with 5pF it's hard to build a "true" low pass filter (cut off frequency). I think you can go with a bandpass filter?? Give us some information about the passband: lower cut-off frequency and matching (Return Loss, VSWR?).
A source follower as a load might be hard to match to. I'd recommend adding a dummy load on the filter output in the range of 100-1000ohms. In theory the achievable insertion loss will increase, but it will make it far easier to get the desired frequency response.
Normally before starting such a design you should be equipped with specific measurements of the input and output impedance across the bandwidth of interest. Describing the load as "a source follower" doesn't really help.
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