HSP50214 데이터 시트보기 (PDF) - Intersil

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HSP50214

Programmable Downconverter

Intersil 

HSP50214

Automatic Gain Control (AGC)

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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

Re-Sampling/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

output section. Without gain control, a signal at -72dBFS =

20log10(2-12) at the input would have only 4 bits of resolu-

tion at the output (12 bits less than the full scale 16 bits). The

potential increase in the bit resolution due to processing gain

of the filters can be lost without the use of the AGC.

Figure 23 shows the block diagram for the AGC Section. The

FIR filter data output is routed to the Re-Sampling and Half-

band filters after passing through the AGC multipliers and

shift registers. The outputs of the Interpolating Halfband fil-

ters are routed to the Cartesian to Polar coordinate con-

verter. The magnitude output of the coordinate converter is

routed through the AGC error detector, the AGC error scaler

and into the AGC loop filter. This filtered error term is used to

drive the AGC multiplier and shifters, completing the AGC

control loop.

The AGC Multiplier/Shifter portion of the AGC is identified in

Figure 23. The gain control from the AGC loop filter is sam-

pled when new data enters the Multiplier/Shifter. The limit

detector detects overflow in the shifter or the multiplier and

saturates the output of I and Q data paths independently.

The shifter has a gain from 0 to 90.31dB in 6.021dB steps,

where 90.31dB = 20log10(2N), when N = 15. The mantissa

provides an additional 6dB of gain in 0.0338dB steps where

6.004dB = 20log10[1+(X)2-8], where X = 28-1. Thus, the

AGC multiplier/shifter transfer function is expressed as:

AGC Mult/Shift Gain = 2N[1 + (X)2–8 ],

(EQ. 14)

where N, the shifter exponent, has a range of 0>N>15 and

X, the mantissa, has a range of 0>X>(28-1).

Equation 14 can be expressed in dB,

(AGC Mult/Shift Gain)dB = 20log10(2N[1 + (X)2–8 ] ) (EQ. 14A)

The full AGC range of the Multiplier/Shifter is from 0 to

96.314dB (20log10[1+(28-1)2-8] + 20log10[215] = 96.314).

Figure 21 illustrates the transfer function of the AGC multi-

plier versus mantissa control for N = 0. Figure 22 illustrates

the complete AGC Multiplier/Shifter Transfer function for all

values of exponent and mantissa control.

5

4

G (dB)

3

2

G (LINEAR)

1

0

0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240

AGC CONTROL MANTISSA VALUES (TIMES 256)

FIGURE 21. AGC MULTIPLIER LINEAR AND dB TRANSFER

FUNCTION

100

90

80

70

60

50

40

30

20

10

0

0

N = 15

N = 14

N = 13

N = 12

N = 11

N = 10

N=9

N=8

N=7

N=6

N=5

N=4

N=3

N=2

N=1

N=0

64

128

192

AGC CONTROL WORD (MANTISSA x 256)

FIGURE 22. AGC GAIN CONTROL TRANSFER FUNCTION

The Cartesian to Polar Coordinate converter accepts I and Q

data and generates magnitude and phase data. The magni-

tude output is determined by the equation:

r = 1.64676 I2 + Q˙2.

(EQ. 15)

where the magnitude limits are determined by the maximum

I and Q signal levels into the Cartesian to Polar converter.

Taking fractional 2’s complement representation, magnitude

ranges from 0 to 2.329, where the maximum output is

r = 1.64676 (1.0)2 + (1.˙0)2 = 1.64676 × 1.414 = 2.329.

The AGC loop feedback path consists of an error detector,

error scaling, and an AGC loop filter. The error detector sub-

tracts the magnitude output of the coordinate converter from

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