Help for PHS system simulation in ADS DSP environment.

I use ADS ptolemy to simulate PHS RF system with DSP. I create PI/4 DQPSK modulator and demodulator, RF superheterodyne receiver, BER block. When I perform sensitivity simulation, I found an odd phenomenon.
When I reduce bandwidth of the image rejection filter in RX according to the real filter performance ,BER performance become worse. While when
I increase bandwidth, BER performance become better. Even when I remove the image rejection filter, BER performance become very good.
It seems that filter performance in superheterodyne receiver is very sensitive to BER. I also use SAW filter in circuit envelope simulation to cosimulate in Ptolemy, the results become very worse than that has no SAW filter.I feel very confused now. Another problem is about MixerRF in
Ptolemy.I use MixerRF block to model two mixers in superheterodyne RX.
in simulation, when the 2nd LO is low injected(RF freq>LO), BER is very good, while when I change the 2nd LO to high injection, BER increases to about 50%.why? Does anyone perform RF superheterodyne receiver system with DSP block in ADS?

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