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HSP50214BVC

Programmable Downconverter

Intersil 

HSP50214B

Additionally, the Programmable FIR filter provides for

decimation factors, R, from 1 to 16. The processing rate of

the Filter Compute Engine is PROCCLK. As a result, the

frequency of PROCCLK must exceed a minimum value to

ensure that a filter calculation is complete before the result is

required for output. In configurations which do not use

decimation, one input sample period is available for filter

calculation before an output is required. For configurations

which employ decimation, up to 16 input sample periods

may be available for filter calculation.

For real filter configurations, use Equations 11A and 11B to

calculate the number of taps available at a given input filter

sample rate.

TAPS = (floor[PROCCLK ⁄ (FSAMP ⁄ R) – R] )(1 +

SYM) – [(SYM)(ODD#)]

(EQ. 11A)

for real filters, and

TAPS = floor[(PROCCLK ⁄ (FSAMP ⁄ R) – R) ⁄ 2]

(EQ. 11B)

for complex filters, where floor is defined as the integer

portion of a number; PROCCLK is the compute clock; fS AMP

= the FIR input sample rate; R = Decimation Factor; SYM =

1 for symmetrical filter, 0 for asymmetrical filter; ODD# = 1

for an odd number of filter taps, 0 = an even number of taps.

Use Equation 12A to calculate the maximum input rate.

FSAMP = (PROCCLK) (R) ⁄ [R + [floor[(Taps) +

(SYM)(ODD#)] ⁄ (1 + SYM)]]

(EQ. 12A)

for real filters, and

FSAMP = [(PROCCLK)(R)] ⁄ [R + floor[(Taps)(2) ] ] (EQ. 12B)

for complex filters, where floor[x], PROCCLK, fSAMP, R =

Decimation Factor, SYM, and ODD# are defined as in

Equation 11A.

Use Equation 13 to calculate the maximum output sample

rate for both real and complex filters.

FFIR OUT = (FSAMP) ⁄ R

(EQ. 13)

The coefficients are 22-bits and are loaded using writes to

Control Words 128 through 255 (see Microprocessor Write

Section). For real filters, the same coefficients are used by I

and Q paths. If the filter is configured as a symmetric filter

using Control Word 17, Bit 9, then coefficients are loaded

starting with the center coefficient in Control Word 128 and

proceeding to last coefficient in Control Word 128+n. The

filter symmetry type can be set to even or odd symmetric,

and the number of filter coefficients can be even or odd, as

illustrated in Figure 20. Note that complex filters can also be

realized but are only allowed to be asymmetric. Only the

coefficients that are used need to be loaded.

C0

CN-1

COEFFICIENT

NUMBER

EVEN SYMMETRIC

EVEN TAP FILTER

C0

CN-1

COEFFICIENT

NUMBER

ODD SYMMETRIC

EVEN TAP FILTER

CN-1

C0

COEFFICIENT

NUMBER

EVEN SYMMETRIC

ODD TAP FILTER

CN

C0

COEFFICIENT

NUMBER

ODD SYMMETRIC

ODD TAP FILTER

C0

CN-1

C0

CN-1

COEFFICIENT

NUMBER

COEFFICIENT

NUMBER

ASYMMETRIC

EVEN TAP FILTER

ASYMMETRIC

ODD TAP FILTER

REAL FILTERS

CQ

CQ(0)

CQ(N-1)

COEFFICIENT

NUMBER

CI(N-1)

CI(0)

REAL COEFFICIENT VALUECI

COMPLEX FILTERS

Definitions:

Even Symmetric: h(n) = h(N-n-1) for n = 0 to N-1

Odd Symmetric: h(n) = -h(N-n-1) for n = 0 to N-1

Asymmetric: A filter with no coefficient symmetry.

Even Tap filter: A filter where N is an even number.

Odd Tap filter: A filter where N is an odd number.

Real Filter:

A filter implemented with real coefficients.

Complex Filters: A filter with quadrature coefficients.

FIGURE 20. DEMONSTRATION OF DIFFERENT TYPES OF

DIGITAL FIR FILTERS CONFIGURED IN THE

PROGRAMMABLE DOWNCONVERTER

Automatic Gain Control (AGC)

The AGC Section provides gain to small signals, after the

large signals and out-of-band noise have been filtered out, to

ensure that small signals have sufficient bit resolution in the

Resampling/Interpolating Halfband filters and the Output

Formatter. The AGC can also be used to manually set the

gain. The AGC optimizes the bit resolution for a variety of

input amplitude signal levels. The AGC loop automatically

adds gain to bring small signals from the lower bits of the 26-

bit programmable FIR filter output into the 16-bit range of the

20

FN4450.4

May 1, 2007

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