strabush
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Filter impedance
The filter frequency response depends on the impedance it sees. If you designed the filter to be loaded by 4K at both sides and then you connect it to a circuit that shows it 400 Ohm at one side, it will not work as intended and its frequency response will be distorted. You can put a 3.3K resistor in series of the filter input in order for the filter and RFIC to see the right impedances, but then you will have a 5 - 6 dB loss at the passband. Is the load of the filter 4K too? What RFIC are you using?
The filter frequency response depends on the impedance it sees. If you designed the filter to be loaded by 4K at both sides and then you connect it to a circuit that shows it 400 Ohm at one side, it will not work as intended and its frequency response will be distorted. You can put a 3.3K resistor in series of the filter input in order for the filter and RFIC to see the right impedances, but then you will have a 5 - 6 dB loss at the passband. Is the load of the filter 4K too? What RFIC are you using?