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HSP50214

Programmable Downconverter

Intersil 

HSP50214

Halfband Decimating Filters

The Programmable Down Converter has five halfband filter

stages, as shown in Figure 17. Each stage decimates by 2

and filters out half of the available bandwidth. The first half-

band, or HB1, has 7 taps. The remaining halfbands; HB2,

HB3, HB4, and HB5; have 11, 15, 19, and 23 taps respec-

tively. The coefficients for these halfbands are given in Table

4. Figure 18 shows the frequency response of each of the

halfband filters with respect to normalized frequency, FN.

Frequency normalization is with respect to the input sam-

pling frequency of each filter section. Each stage is activated

by their respective bit location (15-20) in Control Word 7. Any

combination of halfband filters may be used, or all may be

bypassed. Figure 19 details the halfband alias profiles with

respect to normalized frequency FN.

Since each halfband filter section decimates by 2, the total

decimation through the halfband filter is given by:

DECHB= 2N

(EQ. 9)

where N = Number of Halfband Filters Selected (1 - 5).

HALFBAND

FILTER INPUT

FN = fIN

fIN = fS

†

HALFBAND FILTER 1

CONTROL WORD 7, BIT 15

FN = FHB1

01

FHB1 = fS OR fS/2

†

HALFBAND FILTER 2

CONTROL WORD 7, BIT 16

FN = FHB2

01

FHB2 = FHB1 OR FHB1/2

†

HALFBAND FILTER 3

CONTROL WORD 7, BIT 17

FN = FHB3

01

FHB3 = FHB2 OR FHB2/2

†

HALFBAND FILTER 4

CONTROL WORD 7, BIT 18

FN = FHB4

01

FHB4 = FHB3 OR FHB3/2

†

HALFBAND FILTER 5

CONTROL WORD 7, BIT 19

01

F5 = FHB4 OR FHB4/2

HALFBAND

FILTER OUTPUT

† Each halfband section decimates by 2

FIGURE 17. BLOCK DIAGRAM OF HALFBAND FILTER

SECTION

0

-6dB BANDWIDTH

-20

-40

-60

-80

-100

HALFBAND FILTER 5

HALFBAND FILTER 4

HALFBAND FILTER 3

HALFBAND FILTER 2

HALFBAND FILTER 1

-120

0.125

0.25

0.375

0.5

NORMALIZED FREQUENCY (FN)

FIGURE 18. HALFBAND FILTER FREQUENCY RESPONSE

0

ALIAS

PROFILES

-20

-6dB BANDWIDTH

-40

-60

-80

-100

-120

HALFBAND FILTER 5

HALFBAND FILTER 4

HALFBAND FILTER 3

HALFBAND FILTER 2

HALFBAND FILTER 1

0.125

0.25

0.375

0.5

NORMALIZED FREQUENCY (FN)

FIGURE 19. HALFBAND FILTER ALIAS CONSIDERATIONS

15

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