GC5316 datasheet(14/75 Pages) TI

14 / 75 page

GC5316

SLWS154A − JANUARY 2004 − REVISED MARCH 2004

www.ti.com

2.3.5 Receive CIC Filter

−1

−m1

−1

−m2

−m3

−m4

−m5

−m6

Shift

m1, m2, m3, m4, m5, m6 = 1 or 2

Decim ate

by 4−32

Round

Limit

Down

shift

5−36

Figure 13. Six Stage CIC Filter

The CIC filter provides the first stage of filtering and large-value decimation. The filter consists of six stages and

decimates over a range from 4 to 32.

I data and Q data are handled separately with two CIC filters. In addition, when in CDMA mode (two CDMA channels

processed within a single DDC), another pair of CIC filters handles the B-side channel.

The filter response is (Sin(x)/x)6 in character where the key attribute is that the resulting response nulls alias back

to dc when the signal is decimated. The aliasing rejection achieved depends on the bandwidth of the signal of interest

relative to the CIC output sample rate. A good rule of thumb is the signal of interest should be less than 25% of the

CIC output rate. This means that the CIC decimation value should be chosen so that the signal exiting the CIC filter

is oversampled by at least a factor of four. (Generally, it is close enough for digital signals that the CIC output rate

be at least four times the symbol rate).

The filter is equivalent to six stages of a FIR filter with uniform coefficients (six combined boxcar filter stages). Each

filter would be of length Ncic if m=1, or 2

×Ncic if m=2.

The filter is made up of six banks of 54-bit accumulator sections followed by six banks of 24-bit subtractor sections.

Each of the subtractor sections can be independently programmed with a differential delay of either one or two. A

shift block follows the last integration stage and can shift the 54 bit accumulated data down by 36−cic_scale (a

programmable factor from 0 to 31 bits).

The CIC filter exhibits a droop across its frequency response. This should be compensated in either the CFIR or PFIR

filters that follow. Typically, droop compensation is done in the CFIR but it is also possible to compensate for CIC

droop in the PFIR filter.

The gain of the receive CIC filter is: Ncic6

× 2(numberofstageswhereM=2)× 2(−36+CIC_SCALE) where CIC_SCALE is 0 to

31. There is no rollover protection internal to the CIC or at the final round so the user must guarantee no sample

exceeds full scale prior to rounding. For practical purposes, this means the CIC gain must be less than or equal to

one.

A fixed gain of 12 dB at the output of the CIC can also be programmed.

The post CIC gain is rollover protected. Post CIC gain = 2(cic_gain_ddc x 2).

GC5316 Datasheet(PDF) 14 Page - Texas Instruments

GC5316 Datasheet(HTML) 14 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75

Datasheet Download

Link URL