GC4016 Datasheet, PDF (69/83 Pages) Texas Instruments – MULTI-STANDARD QUAD DDC CHIP

GC4016 MULTI-STANDARD QUAD DDC CHIP
DATA SHEET REV 1.0
7.10 EXAMPLE IS-136 DAMPS APPLICATION
This section describes how to configure the chip to downconvert DAMPS signals, apply the root-raised-cosine receive filter,
and output samples at two times, four times or eight times the DAMPS symbol rate. The desired DAMPS Specifications are:
Table 31: Desired DAMPS Specifications
Specification
Clock (CK)
Input Sample Rate
Input Format
Spur free dynamic
range
Output Sample Rate
Passband Response
Stopband Rejection
Output Format
Value
62.208MHz =8*320*24.3KHz
Same as clock rate
A port, 14 Bit, 2’s Complement
> -110dB
Comment
Can be any value equal to 8NB, where N is the CIC decimation ratio and B is the DAMPS
symbol rate of 24.3KHz
Can be 1/2, 1/3, 1/4, ... 1/N of the clock rate by using the zero pad feature.
Can be modified as desired
The GC4016 NCO provides more than 115 dB of spur free dynamic range
48.6KHz, complex data (2X)
Can be 4X, 8X or any multiple of the symbol rate by changing the resampler ratios
RRC with alpha=0.35
Matches the required receive filter
Set by DAMPS Specification
See the DAMPS rejection mask in Figure 21
Four serial streams, 24 bit I and Q data Can be modified as desired
7.10.1 DAMPS Filter Response
Figure 21 shows the overall response using CFIR filter, cfir_68, and a set of RRC PFIR coefficients tuned for 2X oversampled
output data. The DAMPS out of band rejection mask is also shown.
Rejection Mask
Figure 21. Frequency Response for the Example DAMPS Application
The RRC PFIR coefficients are:
-12 -117 -113 8 127 112 -38 -175 -134 82 275 224 -71 -348 -307 61 398 286 -286 -761
-442 766 1954 1708 -660 -4042 -5641 -2533 6187 18177 28625 32767
These coefficients are available by e-mail or over the web as pfir_damps_2x.taps. The PFIR_SUM for this set of coefficients
is 119387.
7.10.2 Oversampling Using the Resampler
This example assumes the input sample rate is equal to 8*N*B, where N is the decimation in the CIC filter (See Section 5.6)
and B is the DAMPS symbol rate (24.3 KHz). The outputs are two complex samples per symbol (48.6 KHz) with the resampler
set to interpolate by unity (no interpolation). The output rate can be doubled or quadrupled as desired by changing the resampler
© GRAYCHIP,INC.
- 64 -
August 27, 2001
This document contains information which may be changed at any time without notice