CLC5903 datasheet(15/29 Pages) NSC

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mum headroom through the CIC filter. For optimal noise per-

formance the SCALE value is set to increase this level until

the CIC filter is just below the point of distortion. A value is

nor mally calculated and loaded for SCALE such that

. The actual gain of the CIC

filter will only be unity for power-of-two decimation values. In

other cases the gain will be somewhat less than unity.

Channel Gain

The gain of each channel can be boosted up to 42 dB by

shifting the output of the CIC filter up by 0 to 7 bits prior to

rounding it to 21 bits. For channel A, the gain of this stage is:

, where GAIN_A ranges from 0 to 7. Over-

flow due to the GAIN circuit is saturated (clipped) at plus or

minus full scale. Each channel can be given its own GAIN

setting.

First Programmable FIR Filter (F1)

The CIC/GAIN outputs are followed by two stages of filtering.

The first stage is a 21 tap decimate-by-2 symmetric FIR filter

with programmable coefficients. Typically, this filter compen-

sates for a slight droop induced by the CIC filter while remov-

ing undesired alias images above Nyquist. In addition, it

often provides stopband assistance to F2 when deep stop

bands are required. The filter coefficients are 16-bit 2’s com-

plement numbers. Unity gain will be achieved through the fil-

ter if the sum of the 21 coefficients is equal to 216. If the sum

is not 216, then F1 will introduce a gain equal to (sum of coef-

ficients)/216. The 21 coefficients are identified as coefficients

where

is the center tap. The

coefficients are symmetric, so only the first 11 are loaded

into the chip.

Two example sets of coefficients are provided here. The first

set of coefficients, referred to as the standard set (STD),

compensates for the droop of the CIC filter providing a pass-

band which is flat (0.01 dB ripple) over 95% of the final out-

put bandwidth with 70dB of out-of-band rejection (see Figure

21). The filter has a gain of 0.999 and is symmetric with the

following 11 unique taps (1|21, 2|20, ..., 10|12, 11):

29, -85, -308, -56, 1068, 1405, -2056, -6009,

1303, 21121, 32703

The second set of coefficients (GSM set) are intended for

applications that need deeper stop bands or need oversam-

pled outputs. These requirements are common in cellular

systems where out of band rejection requirements can

exceed 100dB (see Figure 22). They are useful for wideband

radio architectures where the channelization is done after the

ADC. These filter coefficients introduce a gain of 0.984 and

are:

-49, -340, -1008, -1617, -1269, 425, 3027, 6030,

9115, 11620, 12606

Second Programmable FIR Filter (F2)

The second stage decimate by two or four filter also uses

externally downloaded filter coefficients. F2 determines the

final channel filter response. The 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 216. If the

sum is not 216, then the F2 will introduce a gain equal to

(sum of coefficients)/216.

The 63 coefficients are identified as

where

is the center tap. The coefficients are sym-

metric, so only the first 32 are loaded into the chip.

An example filter (STD F2 coefficients, see Figure 23) with

80dB out-of-band rejection, gain of 1.00, and 0.03 dB peak to

peak passband ripple is created by this set of 32 unique

coefficients:

-14, -20, 19, 73, 43, -70, -82, 84, 171, -49, -269,

-34, 374, 192, -449,

-430, 460,751, -357, -1144, 81, 1581, 443, -2026,

-1337, 2437, 2886,

-2770, -6127, 2987, 20544, 29647

A second set of F2 coefficients (GSM set, see Figure 24)

suitable for meeting the stringent wideband GSM require-

ments with a gain of 0.999 are:

-536, -986, 42, 962, 869, 225, 141, 93, -280,

-708, -774, -579, -384,

-79, 536, 1056, 1152, 1067, 789, 32, -935, -1668,

-2104, -2137, -1444,

71, 2130, 4450, 6884, 9053, 10413, 10832

The filter coefficients of both filters can be used to tailor the

spectral response to the user’s needs. For example, the first

can be loaded with the standard set to provide a flat

GAIN

SHIFTUP GAI NCIC

⋅

≤

GAIN

GAIN_A

1 n

() n

… 20

1 10

()

Figure 21. F1 STD frequency response

0.1

0.2

0.3

0.4

0.5

−100

−90

−80

−70

−60

−50

−40

−30

−20

−10

Frequency Response of F1 Using STD Set

Frequency Normalized To Filter Input Sample Rate

Figure 22. F1 GSM frequency response

0.1

0.2

0.3

0.4

0.5

−120

−100

−80

−60

−40

−20

Frequency Response of F1 Using GSM Set

Frequency Normalized To Filter Input Sample Rate

2 n

()

… 62

2 31

()

Detailed Description (Continued)

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