GC4016 datasheet(74/83 Pages) TI

74 / 83 page

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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

7.12.1 UMTS Filter Response

Figure 23 shows the overall response using specially tuned CFIR and PFIR filters., The PFIR filter is a specially optimized

root-raised-cosine 63 tap filter with an alpha of 0.22. The filter is designed to have less than -50dB of inter symbol interference

(ISI) noise.

These coefficients are available by e-mail or over the web as cfir_umts.taps and pfir_umts.taps. The CFIR_SUM for

cfir_umts.taps is 58827. The PFIR_SUM for pfir_umts.taps is 61821.

7.12.2 Oversampling Using the Resampler

This example assumes the input sample rate is equal to 4*N*B, where N is the decimation in the CIC filter (See Section 5.6)

and B is the UMTS symbol rate (3.84 MHz). The outputs are two complex samples per symbol (7.68MHz) with the resampler set

to interpolate by unity (no interpolation). The output rate can be doubled or quadrupled as desired by changing the resampler

ratio. The resampler uses the configuration res_8x64_60. (See Section 7.7). This configuration introduces 0.03dB of passband

ripple and -62dB inband noise, neither being large enough to effect UMTS demodulation.

The resampler ratio for two samples per symbol (7.68MHz) is 0x04000000 and for four samples per symbol (15.36MHz) it

is 0x02000000.

Sample rates other than 4*N*B can be accepted by adjusting the PFIR filter to match the desired RRC filter and by using the

resampler to adjust the final sample rate to be exactly 2X or 4X the symbol rate.

7.12.3 Gain

The example configuration assumes a CIC decimation of N=4, which corresponds to an ADC clock rate (CK) of 61.44 MHz.

The values of SCALE and BIG_SCALE must be chosen to satisfy:

. N is 4

and SHIFT=4, so

, which is satisfied by setting SCALE=5 and BIG_SCALE=7, which are their

maximum values. If other values of N are chosen, then SCALE and BIG_SCALE need to be modified as necessary. The overall

gain is adjusted using COARSE, FINE_GAIN and FINAL_SHIFT. The overall gain is:

where NZEROS=0, N=4, SHIFT=4, SCALE=5, BIG_SCALE=7, COARSE=1, CFIR_SUM=58827, PFIR_SUM=61821,

Figure 23. Frequency Response for the Example UMTS Application

2.5MHz

SHIFT

SCALE6

BIG_SCALE

(

)

5log

–

(

)

≤

SCALE6

BIG_SCALE

(

) 48

≤

GAIN

NZEROS

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

(

SHIFT

SCALE6

BIG_SCALE

–

(

)













COARSE

(

) CFIR_SUM

65536

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

(

) PFIR_SUM

65536

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

(

) FINE_GAIN

1024

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

(

)

RES_SUM

32768

NDELAY

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

(

) 2

FINAL_SHIFT

(

)

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