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2.1.5

Filter Chain

GC5318

HIGH-DENSITY DIGITAL UPCONVERTER

SLWS187 – JULY 2006

GC5318 transmit filtering is performed in three stages:

• Interpolate by two pulse-shape filtering using the programmable FIR filter (PFIR)

• Interpolate by two compensation filtering using the programmable compensating FIR filter (CFIR)

• High-rate interpolation (4 to 32) using the six stage cascade-integrate comb filter (CIC)

Figure 2-8. DUC Filter Chain

The purpose of the transmit filter chain, shown in Figure 2-8, is to interpolate the input signal data up to

the mixer clock rate, nominally 122.88 MHz. The following table provides two examples of how the

interpolation can be allocated among the three different filters for both CDMA and UMTS.

Table 2-6. Example UMTS and CDMA2000 DUC Transmit Modes

Input Rate Rate

PFIR Interpolation

CFIR Interpolation

CIC Interpolation

Overall Interpolation

CDMA

1.2288 MSPS

100

UMTS

3.84 MSPS

2.1.5.1 Programmable FIR Filter

The 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 GC5318 clock rate and input sample rate and

is limited by the number of coefficient memory registers. The number of taps available in CDMA mode

ranges from 31 to 63. In UMTS mode it ranges from 15 to 63. In both cases, the number of taps is one

less than a multiple of four (31, 35, ... 59, 63).

Subject to the above range, the maximum number of taps available in each mode is:

• UMTS Mode: 2 × ( txclk × input sample rate)

• 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 channels 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)

× 2^[cfir_gain-19]

GC5318 Operation

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