GC5316_14 Datasheet, PDF (33/76 Pages) Texas Instruments – HIGH-DENSITY DIGITAL DOWNCONVERTER AND UPCONVERTER

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GC5316
SLWS154A − JANUARY 2004 − REVISED MARCH 2004
3.1.5.1 Transmit Programmable FIR Filter
The transmit programmable FIR filter (PFIR) pulse shapes the baseband signal data and interpolates by a fixed factor
of two.
The PFIR length is programmable. This permits turning off taps and saving power if short filters are appropriate. The
maximum PFIR filter length is a function of GC5316 clock rate and input sample rate and is limited by the number
of coefficient memory registers. The number of taps available ranges in CDMA mode ranges from 31 to 63, in UMTS
mode it ranges from 15 to 63. Both in increments of four taps.
Subject to the above range, the maximum number of taps available is:
D UMTS Mode: 2 x ( txclk ÷ input sample rate)
D CDMA Mode: txclk ÷ input sample rate
Assuming a txclk of 122.88 MHz, both UMTS (3.84 MSPS) and CDMA (1.2288 MSPS) modes provide 63 taps. The
same PFIR coefficients are used for both the A and B signals in CDMA mode.
The PFIR filter consists of 32 forward and reverse data RAM cells each 36 bits in width. The coefficient memory
provides storage for up to 63 unique 18-bit taps. A 19 x 18 multiplier and full-precision accumulator form the filter
convolution. An optional (pfir_gain) up-shift of one follows. Finally, the output is hard-limited. The PFIR gain is:
Gain = sum(coefficients) x 2^{pfir_gain − 18}
Table 7. Programming
VARIABLE
crastsrttap_pfir(4:0)
cdma_mode
symmetric_pfir
DESCRIPTION
Number of DUC PFIR filter taps is (2 x crastarttap) +1
When set, puts the CFIR and PFIR blocks in CDMA2000 mode.
Set to 1 if filter is symmetric. This saves a modest amount of power.
The PFIR filter’s 18-bit coefficients are loaded by the software cmd5316. The user
must provide a coefficient file with one integer coefficient per line. Note that the PFIR
filter coefficients are shared by the A and B signals in CDMA mode.
3.1.5.2 Transmit Compensating FIR filter
The transmit CFIR filter interpolates by a fixed factor of two and is usually programmed to compensate for the CIC
filter’s roll-off.
The CFIR filter length is programmable. This permits turning off taps and saving power if short filters are appropriate.
The maximum CFIR filter length is a function of GC5316 clock rate and input sample rate and is limited by the number
of coefficient memory registers. The number of CFIR taps in CDMA mode is 31 to 47, while in UMTS mode it is 15 to
31. The number of taps may be increased in increments of four taps.
Subject to the above minimum, maximum, and increment values, the maximum number of taps available is:
D UMTS Mode: txclk ÷ input sample rate
D CDMA Mode: 0.5 x (txclk ÷ input sample rate)
Assuming a txclk of 122.88 MHz, UMTS (at 3.84 MSPS) mode would provide 31 taps and CDMA (1.2288 MSPS)
mode provides 47 taps.
The CFIR coefficients are shared by the A and B signals in CDMA mode. CFIR gain is:
Gain = sum(coefficients) x 2^{cfir_gain−19}
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