GC4016 datasheet(18/83 Pages) TI

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August 27, 2001

GC4016 MULTI-STANDARD QUAD DDC CHIP

DATA SHEET REV 1.0

This document contains information which may be changed at any time without notice

channels, or as two segments of 32 complex words each to

support 2 resampler channels, or as a single segment of 64

complex words to support a single resampler channel. The

number of segments is set by NCHAN in address 16 of the

resampler control page.

The interpolation filter zero pads the input data by a

factor of NDELAY and then filters the zero padded data using

a QTAP length filter. The output of the QTAP filter is then

decimated by a factor of NDEC.

The resampling ratio for each channel is determined by

setting the 32 bit RATIO controls in addresses 16 through 31

of the resampler ratio page. The value of RATIO is defined

as:

Up to four ratios can be stored within the chip. A ratio

map register (address 23) selects which ratio is used by each

channel.

The three spectral plots shown in Figure 13 illustrate the

steps required to resample the channel data. The first

spectral plot shows the data just after zero padding. The

sample rate after zero padding is NDELAY*FS, where FS is

the sample rate into the resampler. The second spectrum

shows the shape of the QTAP filter which must be applied to

the zero padded data in order to suppress the interpolation

images. The last spectrum shows the final result after

decimating by NDEC.

3.5.2 The Resampler Filter

Figure 13(b) illustrates the spectral shape requirements

of the QTAP filter. If the desired signal bandwidth is B, then

the filter’s passband must be flat out to B/2 and the filter’s

stop band must start before FS-B/2. The user designs this

filter assuming a sample rate equal to NDELAY*FS. Section

7.7 contains example resampler filters coefficient sets. Other

passband and stopband responses can be used, such as

root raised cosine receive filters, as desired. The resampler

filter can also be used to augment the CIC, CFIR and PFIR

filters’ spectral response.

The number of filter coefficients, QTAP, is equal to

NMULT*NDELAY, where NMULT is the number of multiplies

available to compute each resampler output, and NDELAY is

either 4, 8, 16, 32 or 64 as will be described later. The

maximum filter length is 512. The user specifies NMULT in

address 17 of the resampler control page.

The filter can be symmetric, or non-symmetric, as

selected by the NO_SYM_RES control in address 17 of the

resampler control page. The symmetric filter is of even length

which means the center tap repeats.

RATIO

NDEC

NDELAY

------------------------









INPUT SAMPLE RATE

OUTPUT SAMPLE RATE

--------------------------------------------------------------------









-FS/2

FS/2

-FS

NDELAY Images added by zero padding

QTAP Filter

a) Spectrum after zero padding

Images reduced

by Filtering

Power

Frequency Response

In-band aliasing caused by decimation by NDEC

2FS

3FS

Freq

...

-2FS

-3FS

...

-FS/2

FS/2

-FS

b) Spectrum after filtering

Power

2FS

3FS

Freq

...

-2FS

-3FS

...

-FO/2

FO/2

-FO

FO=(NDELAY/NDEC)FS

c) Final Spectrum after decimating by NDEC

Power

Freq

Figure 13. The Resampler’s Spectral Response

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