hsp50214 Intersil Corporation, hsp50214 Datasheet - Page 18

• Manufacturer Part Number: hsp50214
• Description: Programmable Downconverter
• Manufacturer: Intersil Corporation

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
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 =
20log
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 = 20log
provides an additional 6dB of gain in 0.0338dB steps where
6.004dB = 20log
AGC multiplier/shifter transfer function is expressed as:
AGC Mult/Shift Gain
where N, the shifter exponent, has a range of 0>N>15 and
X, the mantissa, has a range of 0>X>(2
Equation 14 can be expressed in dB,
(AGC Mult/Shift Gain)dB
The full AGC range of the Multiplier/Shifter is from 0 to
96.314dB (20log
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.
10
(2
-12
) at the input would have only 4 bits of resolu-
10
10
=
[1+(2
[1+(X)2
2
N
=
10
8
1
20log
-1)2
(2
+
-8
N
X 2
], where X = 2
), when N = 15. The mantissa
-8
10
] + 20log
–
2
8
N
,
1
+
8
-1).
X 2
10
[2
–
8
8
15
-1. Thus, the
] = 96.314).
(EQ. 14A)
(EQ. 14)
HSP50214
18
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:
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
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
FIGURE 21. AGC MULTIPLIER LINEAR AND dB TRANSFER
r
r
=
FIGURE 22. AGC GAIN CONTROL TRANSFER FUNCTION
6
5
4
3
2
1
0
100
=
90
80
70
60
50
40
30
20
10
1.64676
0
0
1.64676 I
0
16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
AGC CONTROL MANTISSA VALUES (TIMES 256)
FUNCTION
1.0
2
AGC CONTROL WORD (MANTISSA x 256)
+
2
64
Q
˙
+
2
.
1.0
˙
2
G (dB)
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
1.64676 1.414
128
G (LINEAR)
192
=
2.329
.
(EQ. 15)