DDC Functional Description

The DDC is a decimating quadrature downconverter. It receives samples of the IF from the ADC at a rate of 4 8-bit real samples per cycle at 200MHz, and outputs 1 16-bit complex (8-bit real, 8-bit imaginary) sample of the baseband signal per cycle. The incoming IF is mixed down with 275MHz, put through a decimating symmetric low-pass filter. The filter downsamples by 4 and has a -6dB cutoff frequency of 80MHz.

Filter specs:

Filter type: Windowed FIR

Filter order: 63

Cutoff frequency: 80MHz; -6dB

Windowing: Tukey, alpha=0.5

Figure 1: DDC low-pass filter frequency response

Figure 2: DDC low-pass filter transition band

Filter coefficients:

0.0000 0.0000 -0.0001 -0.0008 -0.0017 -0.0022 -0.0012 0.0017

0.0056 0.0086 0.0085 0.0038 -0.0044 -0.0129 -0.0176 -0.0155

-0.0063 0.0068 0.0191 0.0255 0.0224 0.0094 -0.0104 -0.0303

-0.0424 -0.0396 -0.0179 0.0219 0.0736 0.1273 0.1717 0.1967

0.1967 0.1717 0.1273 0.0736 0.0219 -0.0179 -0.0396 -0.0424

-0.0303 -0.0104 0.0094 0.0224 0.0255 0.0191 0.0068 -0.0063

-0.0155 -0.0176 -0.0129 -0.0044 0.0038 0.0085 0.0086 0.0056

0.0017 -0.0012 -0.0022 -0.0017 -0.0008 -0.0001 0.0000 0.0000

0.0000	0.0000	-0.0001	-0.0008	-0.0017	-0.0022	-0.0012	0.0017
0.0056	0.0086	0.0085	0.0038	-0.0044	-0.0129	-0.0176	-0.0155
-0.0063	0.0068	0.0191	0.0255	0.0224	0.0094	-0.0104	-0.0303
-0.0424	-0.0396	-0.0179	0.0219	0.0736	0.1273	0.1717	0.1967
0.1967	0.1717	0.1273	0.0736	0.0219	-0.0179	-0.0396	-0.0424
-0.0303	-0.0104	0.0094	0.0224	0.0255	0.0191	0.0068	-0.0063
-0.0155	-0.0176	-0.0129	-0.0044	0.0038	0.0085	0.0086	0.0056
0.0017	-0.0012	-0.0022	-0.0017	-0.0008	-0.0001	0.0000	0.0000