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HSP50110
(Rev.:1999)

Digital Quadrature Tuner

Intersil 

HSP50110

A

B

C

D

fS

fS

fS

2R

2

FIGURE 12.

To create the alias profile, a composite response, the

components of which are shown in the”D” portion of

Figure 12, is made from the sum of all the alias elements.

The primary use of an alias profile is used to determine what

bandwidth yields the desired suppression of unwanted

signals for a particular application.

Reviewing Figures 9 through 11, note the following

observations:

1. The uncompensated I&D (1st order CIC) filter yields

about 12dB of alias suppression at fS/16R. This usable

bandwidth is considerably narrower than the 3dB filter

bandwidth. The I&D filter is the matched filter for square

wave data that has not been bandlimited.

2. The compensated I&D filter offers a flatter, wider band-

width than just the I&D alone. This filter compensates for

the frequency roll off due to the A/D converter.

3. The uncompensated 3rd order CIC filter yields over 60dB

of alias suppression at fS/16R. Typical application is

found in tuners, where the DQT is followed by a very nar-

row band filter.

4. The 3rd order CIC with compensation yields alias sup-

pression comparable to the 3rd order CIC, but with the

flatter, wider passband. This filter is selected for most

SATCOM applications.

Which filter is selected, is dependent on the application. It is

important to utilize these alias responses in calculating the

filter to be used, so that the signal suppression prediction will

accurately reflect the digital filter performance.

Noise Equivalent Bandwidth

The noise equivalent bandwidth (BN) performance of the

channel filter is dependent on the combination of Decimation

Filter and Compensation Filter chosen. For configurations

using the Integrate and Dump filter, BN is constant

regardless of decimation rate. However, for configurations

which use the third order CIC filter, BN converges to a

constant for decimation factors of over ~50. A summary of

equivalent IF BN’s for different filter configurations and

decimation rates is given in Table 3. These noise bandwidths

are provided so that output SNR can be calculated from input

SNR. In detection applications this bandwidth indicates the

detection bandwidth.

TABLE 3. DOUBLE SIDED NOISE EQUIVALENT BANDWIDTH

FOR DIFFERENT FILTER CONFIGURATIONS AND

OUTPUT SAMPLE RATES

DEC

INTEGRATE/

INTEGRATE/ DUMP W/

DUMP

x/sin(x)

2

1.0000

1.3775

10

1.0000

1.3775

18

1.0000

1.3775

26

1.0000

1.3775

34

1.0000

1.3775

42

1.0000

1.3775

50

1.0000

1.3775

58

1.0000

1.3775

66

1.0000

1.3775

74

1.0000

1.3775

82

1.0000

1.3775

90

1.0000

1.3775

98

1.0000

1.3775

106

1.0000

1.3775

114

1.0000

1.3775

122-

4096

1.0000

1.3775

3RD

ORDER

CIC

0.6250

0.5525

0.5508

0.5504

0.5502

0.5501

0.5501

0.5501

0.5501

0.5500

0.5500

0.5500

0.5500

0.5500

0.5500

0.5500

3RD

ORDER

CIC W/

[x/sin(x)]3

1.3937

1.0785

1.0714

1.0698

1.0691

1.0688

1.0687

1.0686

1.0685

1.0684

1.0684

1.0684

1.0684

1.0684

1.0683

1.0683

Re-Sampler

The Re-Sampler sets the output sample rate by controlling

the sample rate of the decimation filters (see Low Pass Filter

Section). The output sample rate may be fixed or adjusted

dynamically to synchronize with baseband waveforms. The

reduction in sample rate between the Low Pass Filter input

and output represents the decimation factor.

The Decimating filter output is sampled by the programmable

divider shown in Figure 13. The divider is a counter which is

decremented each time it is clocked. When the divider reaches

its terminal count, the output of the decimating filter is sampled.

The divider may be programmed with a divisor of from 1 to

4096 (see Table 11 Decimating Filter Configuration Register).

One of two internal clock sources are chosen for the divider

based on whether a fixed or adjustable sample rate is

desired. For fixed output sample rates, a clock equal to the

input sample rate is selected (see Decimating Filter

Configuration Table 11). For adjustable output sample rates,

a clock generated by the carry out from the Re-Sampler

NCO is chosen.

3-238

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