Filter Output Quantization in Digital Down Converter

Hi, I am using DDC from Xilinx CoreGen. My application requires a 6MHz sampled signal (freq = 2MHz, BW = 2MHz) to be down converted to baseband,resampled at 2 MHZ (ultimately the signal will be at baseband with 2 sided bandwidth of 2MHz). My input bit width is 4. I have configured my DDS mixer to 8 bit output with SFDR of 25dB. I am not using CIC filter since downconversion ratio is small (by 3) and also cutoff fc is high (1/2) w.r.t. lower sampling rate of 2MHz. I am using CFIR filter as my decimation filter in each of the I and Q arm for restricting the signal BW to 1MHz. I am using 15 tap filter with normalized 8 bit filter coefficients. When I generate the core, what I see is, the required output bit width is 18. What I want is my output bit width of the DDC should be 4 bits (with less degradation in performance of the filter). That means I have to quantize the filter output. If I quantize uniformly(4 MSBs), I am obtaining wrong results. My question is this. Is it possible to quantize such an output to 4 bits? If not, How can I calculate & justify the number of quantization bits required at the output? Any specific references?

Regards, Nagaraj

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Nagaraj
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