GC4016 Datasheet, PDF (18/83 Pages) Texas Instruments – MULTI-STANDARD QUAD DDC CHIP

GC4016 MULTI-STANDARD QUAD DDC CHIP
DATA SHEET REV 1.0
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:
RATIO
=
226


N----N-D---D-E----EL----AC----Y--^
=
226


O----I-U-N----TP---P-U---U-T----T--S---S-A---A-M---M--P---P-L---LE----E--R---R--A---A-T---TE----E--^
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 F_S-B/2. The user designs this
filter assuming a sample rate equal to NDELAY*F_S. 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.
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*F_S, where F_S is
the sample rate into the resampler. The second spectrum
shows the shape of the QTAP filter which must be applied to
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.
NDELAY Images added by zero padding
Power
...
-3F_S
-2F_S
ImabgyeFsilrteedriuncged
-F_S
-F_S/2
F_S/2 F_S
a) Spectrum after zero padding
QTAP Filter
Frequency Response
Power
...
-3F_S
-2F_S
-F_S
-F_S/2
F_S/2 F_S
b) Spectrum after filtering
In-band aliasing caused by decimation by NDEC
Power
...
Freq
2F_S
3F_S
...
Freq
2F_S
3F_S
-F_O
-F_O/2
F_O/2
Freq
F_O=(NDELAY/NDEC)F_S
c) Final Spectrum after decimating by NDEC
Figure 13. The Resampler’s Spectral Response
© GRAYCHIP,INC.
- 13 -
August 27, 2001
This document contains information which may be changed at any time without notice