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HSP50214

Programmable Downconverter

Intersil 

HSP50214

AGC LOOP FILTER

SERIAL

16

OUT

MSB = 0

16

µP

MSB = 0

LIMIT

DET

AGC ERROR SCALING

(RANGE = 0 TO 2.18344)

13

+

MANTISSA

EXP

4

4

AGCGNSEL

UPPER LIMIT †

LOWER LIMIT †

AGC

ERROR

DETECTOR

13

∆

13

4 EXP=2NNNN

9

MANTISSA =

01.XXXXXXX(XXXXXXX)††

LIMIT

DET

MAGNITUDE

(RANGE = 0 TO 2.3)

(RANGE = 0 TO 1)

26

18

IFIR

26

18

QFIR

18

IAGC

LIMIT

DET

18

QAGC

RE-SAMPLING

FIR FILTERS

AND

INTERPOLATING

HALFBAND

FILTERS

CARTESIAN

TO

POLAR

COORDINATE

CONVERTER

(G = 1.64676)

AGC MULTIPLIER/SHIFTER

† Controlled via microprocessor interface.

† † Indicates additional resolution of the A version.

FIGURE 23. AGC BLOCK DIAGRAM

Using AGC loop gain, the AGC range, and expected error

detector output, the gain adjustments per output sample for

the loop filter section of the Digital AGC can be given by:

AGC Slew Rate = 1.5dB(THRESH – (MAG *1.64676) ) ×

(ML

G

)(

2–4

)

2–(

15

–

EL

G

)



(EQ. 18)

The loop gain determines the growth rate of the sum in the

loop accumulator which, in turn, determines how quickly the

AGC gain traces the transfer function given in Figures 21

and 22. Since the log of the gain response is roughly linear,

the loop response can be approximated by multiplying the

maximum AGC gain error by the loop gain. The expected

range for the AGC rate is ~ 0.00004 to 1.23dB/symbol time

for a threshold of 1/2 scale. See the notes at the bottom of

Table 9 for calculation of the AGC Response times. The

maximum AGC Response is given by:

AGC ResponseMax = Input(Cart/Polar Gain)(Error Det Gain)(AG C

Loop Gain)(AGC Output Weighting)

(EQ. 19)

Since the AGC error is scaled to adjust the gain, the loop

settles asymptotically to its final value. The loop settles to

the mean of the signal.

20

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