Question about noise in complex(polyphase) filter

[moh_hafez]

complex polyphase filter

I have made a 3rd order complex filter , when performing noise analysis using cadence , i put noise sources at both the inputs of I and Q channel , and saw the output noise at the I or Q channel , it was 45nV/sqrt(hz) . when i measured the input referred noise at the I or Q channel , it was about 100nV/sqrt(hz) although the gain is 0dB .
So does this may be bec. of Cadence when calculating Noise , as it doesn't see the phase difference bet. the noise sources of the I and Q channel (as it doesnt care for the data in ac sources when performing noise analysis) and hence it doesnt see the 0dB gain which depends on the phase shift bet. I and Q inputs .
So is the solution that i get the output noise using cadence and then calculate the input referred noise based on my understanding that the input will be the same as the output .

 

[Megh]

Hello hafez,
I am designing the polyphase filter and am stuck at the exactly same problem as yours.
Cadence recommends following way to do the noise simulation in Complex filters: -

Problem
You are trying to do a Spectre noise simulation of a circuit with multiple inputs and/or outputs.
However, the noise setup has the following restriction: only one port/source and only one output node/port can be specified.Is it possible to use SpectreRF in this case? If yes, how?

Solution
The answer is - yes: you can solve your problem using "principle of superposition".
For example, you have a complex filter with two inputs Iin, Qin and two outputs Iout and Qout in your LowIF receiver:
According to principle of superposition you can calculate the output spectra as follows.
1. Set ac-amplitudes of both input ports: portI = 1, portQ = 0.
2. Using AC analysis calculate the I- and Q - output voltages and save as the transfer functions HII(f) and HIQ(f) respectively.
3. Set ac-amplitudes of both input ports: portI = 0, portQ = 1.
4. Using AC analysis calculate both I- and Q - output voltages and save as the transfer functions HQI(f) and HQQ(f) respectively.
5. Calculate both I- and Q- output spectra:
Iout(f) = HII(f) * Iin(f) + HQI(f) * Qin(f);
Qout(f) = HQQ(f) * Qin(f) + HIQ(f) * Iin(f).

After step 2 you can calculate output noise (V**2/Hz) when only portI is active:
- at Iout: Ioutn_I(f);
- at Qout: Qoutn_I(f).

Note: Spectre calculates 'SSBnoise' = 2 * 'DSB noise'.

After step 3 you can 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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Now even after doing all this I see that input referred noise is double that of output referred noise. I don't know why this is happening. Please let me know if you have got the answer.
Thanks
Megh