Cadence recommends following way to do the noise simulation in Complex filters: -

According to principle of superposition noise can be calculated as follows:

1. Calculate output noise (V**2/Hz) when only portI is active:

- at Iout: Ioutn_I(f);

- at Qout: Qoutn_I(f).

2. Now calculate output noise (V**2/Hz) when only portQ is active:

- at Iout: Ioutn_Q(f);

- at Qout: Qoutn_Qf).

Now you can calculate the total otput noise (V**2/Hz) at both outputs:

Ioutn(f) = Ioutn_I(f) + Ioutn_Q(f);

Qoutn(f) = Qoutn_I(f) + Qoutn_Q(f);

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The complex BPF is made of 2 LPFs. Both of them have gain = 0dB. Also Gain is 0dB for complex out to complex input.

Now
I see that input referred noise is double that of output referred noise
even though gain is 0dB . I don't know why this is happening. Please
help.

Thanks

Megh