GC4116 datasheet(11/57 Pages) ETC

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GC4116 MULTI-STANDARD QUAD DUC CHIP

DATA SHEET REV 1.0

- 6 -

APRIL 27, 2001

This document contains preliminary information which may be changed at any time without notice

The interpolated signal is modulated by a sine/cosine

sequence generated by the NCO. In the real output mode the

real part (I-half) of the complex result is saved as the channel

output.

In the complex output mode the CIC interpolation is cut

in half and the NCO/mixer calculates both the I-half and

Q-half of the complex result. In this mode the complex output

sample rate is one-half the clock rate, with the I and Q halves

multiplexed together onto the same output bus.

3.3.1 The Programmable Interpolate By 2

Filter (PFIR)

The input samples are filtered by two stages of

interpolate by 2 filtering before they are interpolated by the

CIC filter. The first stage interpolate by two filter is a 63 tap

filter with programmable 16 bit coefficients. The PFIR will

accept either complex or real input data. If the input samples

are complex, the filter doubles the input rate by inserting

zeroes between each sample, and then low pass filters the

result. If the input samples are real (REAL in address 1 is

set), the filter translates the real samples down by FIN/4,

where FIN is the input sample rate, by multiplying them by the

complex sequence +1, -j, -1, +j,

…, and then lowpass filters

the result. This generates a single-sideband modulation of

the real input. Note that in the real input mode the data is

entered as pairs of samples packed into the complex input

word format (see Section 3.2). If double sideband real

upconversion is desired, then the chip should be operated in

the complex mode with the Q-half of each complex pair set

to zero.

The PFIR filter passband must be flat in the region of the

signal of interest, and have the desired out of band rejection

in the region that will contain the interpolation image. Figure

6 illustrates the passband and stopband requirements of the

filter. FIN is the input sample rate to the channel. 2FIN is the

output sample rate of the PFIR. A common use of the PFIR

is to pulse shape digital data. The PFIR will accept QPSK,

O-QPSK, PSK, PAM, OOK,

π/4-QPSK, or QAM symbols

and then filter them by the desired pulse shaping filter, which

is commonlt a root-raised-cosine (RRC) filter. The symbols

can be entered directly into the chip at the desired symbol

(baud) rate. The application notes in Section 7 describes

sample filter coefficients sets for common standards

(DAMPS, GSM, IS95, UMTS).

Each channel has its own PFIR coefficient memory, so

the same filter, or a different filter, can be used in each

channel.

The user downloaded filter coefficients are 16 bit 2’s

complement numbers. Unity gain will be achieved through

the filter if the sum of the 63 coefficients is equal to 65536. If

the sum is not 65536, then PFIR will introduce a gain equal

to:

, where PFIR_SUM is the sum of

the 63 coefficients.

The 63 coefficients are identified as coefficients h0

through h62, where h31 is the center tap. The coefficients are

assumed to be symmetric, so only the first 32 coefficients (h0

through h31) are loaded into the chip. A non-symmetric mode

(NO_SYM_PFIR in address 26) allows the user to download

a 32 tap non-symmetric filter as taps h0 through h31. The

newest sample is multiplied by h31 and the oldest is

multiplied by h01.

3.3.2 The Compensating Interpolate by 2

Filter (CFIR)

The second stage filter is a fixed coefficient 31 tap

interpolate by 2 filter. The second stage filter always

interpolates by a factor of two. The second filter has a

passband which is flat (0.01 dB ripple) out to 0.6FIN and

provides over 90dB of image rejection beyond 1.4FIN. The

second filter also compensates for the droop associated with

the CIC interpolation filter described in the next section. The

16 unique coefficients of the symmetric filter are:

-34, -171, -166, 403, 837, -317, -1983, -790,

2820, 3328, -1667, -6589, -4024, 7232, 20602,

26577

The passband of this filter is wide enough to upconvert

digital symbol data with excess bandwidths up to 0.35.

The CFIR output is scaled to have unity gain.

The output rate of the CFIR filter is 4FIN in the complex

input mode and is 2FIN in the real input mode. The CFIR

output rate relative to the clock rate is FCK/N

FIN

Frequency

-100 dB

0 dB

Signal of Interest Passband

(Typically 0.25 to 0.4 of FIN)

FIN/2

Image Reject Stopband

(Typically starts between

Narrow transition band

Power

Figure 6. Typical PFIR Specifications

0.6 and 0.75 FIN)

PFIR_GAIN

PFIR_SUM

65536

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