Pentek 6230 Operating Manual page 180

GC4016 MULTI-STANDARD QUAD DDC CHIP
7.11.2 Oversampling Using the Resampler
This example assumes the input sample rate is equal to 6*N*B, where N is the decimation in the CIC filter (See Section 5.6)
and B is the IS95 symbol rate (1.2288 MHz). The PFIR filter output rate is 1.5 samples per symbol (1.5X). The outputs are two
complex samples per symbol (2.4576 MHz) if the resampler is set to interpolate by 4/3. The output rate can be 4X or 8X as
desired by changing the resampler ratio. The resampler uses the configuration res_6x64_60. (See Section 7.7). This
configuration introduces -58dB inband noise, not enough to effect the BER. The spectral effect of the resampler is included in
Figure 22.
The resampler ratio for two samples per symbol (2.4576MHz) is 0x03000000, for four samples per symbol (4.9152MHz) it
is 0x01800000, and for eight samples per symbol (9.8304MHz) it is 0x00C00000. Note that the resampler can only handle two
channels at eight samples per symbol.
Sample rates other than 6*N*B can be accepted. To use these rates one needs to customize the PFIR filter to match the
bandwidth when the sample rate has been decimated by 4*N, and then the resampling ratio needs to be adjusted to match the
desired output rate. Call Graychip for details.
7.11.3 Gain
The example configuration assumes a CIC decimation of N=8, which corresponds to an ADC clock rate (CK) of 58.9824MHz.
The values of SCALE and BIG_SCALE must be chosen to satisfy:
(
and SHIFT=4, so SCALE
of N are chosen, then SCALE and BIG_SCALE need to be modified as necessary. The overall gain is adjusted using FINE_GAIN
and FINAL_SHIFT. The overall gain is:

(
5
( )N
 1
---------------------
GAIN = 2
 NZEROS 1 +
where NZEROS=0, N=8, SHIFT=4, SCALE=1, BIG_SCALE=7, COARSE=0, CFIR_SUM=83001, PFIR_SUM=89373,
RES_SUM=134786 and NDELAY=64. Because of the loss of 1/2 when converting real data to complex, the desired gain is 2.0.
This can be achieved by setting FINE_GAIN to 1153 and FINAL_SHIFT equal to 4.
© GRAYCHIP,INC.
This document contains information which may be changed at any time without notice
× ) ≤
+ 6 BIG_SCALE 43 , which is satisfied by setting SCALE=1 and BIG_SCALE=7. If other values

× )
SHIFT + SCALE + 6 BIG_SCALE – 62


(
SHIFT + SCALE
COARSE
( ) ( ) (
CFIR_SUM PFIR_SUM
------------------- ------------------ -
2
65536 65536
- 67 -
DATA SHEET REV 1.0
× ) ≤ (
+ 6 BIG_SCALE 62 5log
FINAL_SHIFT
) ( ) ( ) 2 (
FINE_GAIN RES_SUM
--------------------
--------------------------------------------- -
×
32768 NDELAY
1024
August 27, 2001
)
– N . N is 8
2
)