HSP50214VC datasheet(21/54 Pages) INTERSIL

21 / 54 page

For example, if MLG = 0101 and ELG = 1100, the AGC

Loop Gain = 0.3125*2-7. The loop gain mantissas and

exponents are set in the AGC Loop Parameter control reg-

ister (Control Word 8, bits 0-15). Two AGC loop gains are

provided in the Programmable Down Converter, for quick

adjustment of the AGC loop. The AGC Gain select is a con-

trol input to the device, selecting Gain 0 when AGCGNSEL

= 0, and selecting Gain 1 when AGCGNSEL = 1.

Resampler/Halfband Filter

The Resampler is an NCO controlled polyphase ﬁlter that

allows the output sample rate to have a non-integer relation-

ship to the input sample rate. The ﬁlter engine can be viewed

conceptually as a ﬁxed interpolation ﬁlter, followed by an

NCO controlled decimator.

The prototype polyphase ﬁlter has 192 taps designed at 32

times the input sample rate. Each of the 32 phases has 6 ﬁl-

ter taps (6)(32) = 192. The stopband attenuation of the pro-

totype ﬁlter is greater than 60dB, as shown in Figure 24. The

signal to total image power ratio is approximately 55dB, due

to the aliasing of the interpolation images. The ﬁlter is capa-

ble of decimation rates from 1 to 4. If the output is at least 2x

the baud rate, the 32 interpolation phases yield an effective

sample rate of 64x the baud rate or approximately 1.5%,

(1/64), maximum timing error.

Following the Resampler are two interpolation halfband ﬁl-

ters. The halfband ﬁlters allow the user to up-sample by 2 or

4 to recover time resolution lost by decimating. Interpolating

by 2 or 4 gives 1/4 or 1/8 baud time resolution (assuming 2x

baud at the Resampler output). The halfband ﬁlters use the

same coefﬁcients as HB3 and HB5 from the Halfband Filters

Section. If one halfband is used, the 23-tap ﬁlter is chosen. If

two are used, the 23-tap ﬁlter runs ﬁrst followed by the

15-tap ﬁlter operating at twice the ﬁrst halfband’s rate. The

23-tap ﬁlter requires 7 multiplies, and the 15-tap ﬁlter

requires 5 multiplies to complete a ﬁlter calculation.

Using the interpolation halfband ﬁlters allows for reduction in

the FIR ﬁlter sample rate. This optimizes the use of the pro-

grammable FIR ﬁlter by allowing the FIR output sample rate

to be closer to the Nyquist rate of the desired bandwidth.

Optimizing the FIR ﬁlter performance provides better use of

the programmable FIR taps. Table 10 details the maximum

clocking rates for the possible re-sampling and interpolation

halfband ﬁlter conﬁgurations of this section of the PDC. Con-

trol Word 16, bits 2-0 identify the ﬁlter conﬁguration. Control

Word 16, bit 3 is used to bypass the polyphase Resampler

ﬁlter.

For proper data output from the interpolation ﬁlters, the data

ready signal must account for the re-sampling and interpola-

tion processes. Figure 25 illustrates the insertion of addi-

tional data ready pulses to provide sufﬁcient pulses for the

new output sample rate. The Resampler Output Pulse Delay

parameter is set in Control Word 16, bits 4-11. These bits set

the delay between the output samples when interpolation is

utilized. Program this distance between pulses using

A value of at least 5 is required to have sufﬁcient time to

update the output buffer register. (Writing 5 samples

requires 5 clock cycles) A value of at least 16 is required for

proper serial output from the part. (Conversion from 16-bit

parallel to serial) The value is programmed in numbers of

PROCCLK’s.

TABLE 7. AGC LOOP GAIN BINARY MANTISSA TO GAIN

SCALE FACTOR MAPPING

BINARY

CODE

(MMMM)

SCALE

FACTOR

BINARY

CODE

(MMMM)

SCALE

FACTOR

0000

0.0000

1000

0.5000

0001

0.0625

1001

0.5625

0010

0.1250

1010

0.6250

0011

0.1875

1011

0.6875

0100

0.2500

1100

0.7500

0101

0.3125

1101

0.8125

0110

0.3750

1110

0.8750

0111

0.4375

1111

0.9375

TABLE 8. AGC LOOP GAIN BINARY EXPONENT TO GAIN

SCALE FACTOR MAPPING

BINARY

CODE

(EEEE)

SCALE

FACTOR

BINARY

CODE

(EEEE)

SCALE

FACTOR

0000

215

1000

0001

214

1001

0010

213

1010

0011

212

1011

0100

211

1100

0101

210-

1101

0110

1110

0111

1111

(EQ. 20)

PRO CCLK/fOUT

() 1 ]

–

[

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