Datasheet

LM97593

SNWS019B –JULY 2007–REVISED APRIL 2013

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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 compensates for a slight droop induced by the CIC

filter while removing 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 complement numbers. Unity gain will

be achieved through the filter if the sum of the 21 coefficients is equal to 2

. If the sum is not 2

, then F1 will

introduce a gain equal to (sum of coefficients)/ 2

. The 21 coefficients are identified as coefficients h

(n), n =

0, ..., 20 where h

(10) 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 passband which is flat (0.01 dB ripple) over 95%

of the final output bandwidth with 70dB of out-of-band rejection (see Figure 66). 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

Figure 66. F1 STD frequency response

The second set of coefficients (GSM set) are intended for applications that need deeper stop bands or need

oversampled outputs. These requirements are common in cellular systems where out of band rejection

requirements can exceed 100dB (see Figure 67). 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

Figure 67. F1 GSM frequency response

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