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the AGC THRESHOLD value (Control Word 8, Bits 16-28) is
shown in Table 5. Note that the MSB is always zero. The range
of the AGC THRESHOLD value is 0 to +3.9995. The AGC Error
Detector output has the identical range.
TABLE 5. AGC THRESHOLD (CONTROL WORD 8) BIT
WEIGHTING
28
27
26
25
24
23
2
1
0
-1
-2
-3
.
2
2
2
2
2
2
The loop gain is set in the AGC Error Scaling circuitry, using
the two programmable mantissas and exponents. The
mantissa, M, is a 4-bit value which weights the loop filter
input from 0.0 to 0.9375. The exponent, E, defines a shift
factor that provides additional weighting from 2
Together the mantissa and exponent define the loop gain as
given by,
4
–
AGC Loop Gain
M
2
=
LG
where M
is a 4-bit binary value ranging from 0 to 15, and
LG
E
is a 4-bit binary value ranging from 0 to 15. Table 7 and
LG
8 detail the binary values and th
the AGC scaling mantissa and exponent. The composite
(shifter and multiplier) AGC scaling Gain range is from
0
0.0000 to 2.329(0.9375)
2
gain error can range (depending on threshold) from 0 to
2.18344, which maps to a "gain change per sample" range
of 0 to 3.275dB/sample.
The AGC Gain mantissa and exponent values are
programmed into Control Word 8, Bits 0-15. The PDC
provides for the storing of two values of AGC Scaling Gain
(both exponent and mantissa). This allows for quick
adjustment of the loop gain by simply asserting the external
control line AGCGNSEL. When AGCGNSEL = 0, then AGC
GAIN 0 is selected, and when AGCGNSEL = 1, AGC Loop
Gain 1 is selected. Possible applications include
acquisition/tracking, no burst present/burst present, strong
signal/weak signal, track/hold, or fast/slow AGC values.
The AGC loop filter consists of an accumulator with a built in
limiting function. The maximum and minimum AGC gain
limits are provided to keep the gain within a specified range
and are programed by 12-bit Control Words using the
following the equation:
(
AGC Gain Limit
=
1
+
m
AGC
(
)dB = 6.02
(
) eeee
(
AGC Gain Limit
where m is an 8-bit mantissa value between 0 and 255, and e
is the 4-bit exponent ranging from 0 to 15. Control Word 9,
Bits 16-27 are used for programming the upper limit, while bits
0-11 are used to program the lower threshold. The ranges and
format for these limit values are shown in Tables 6A - C. The
bit weightings for the AGC Loop Feedback elements are
detailed in Table 9.
22
21
20
19
18
17
-4
-5
-6
-7
-8
-9
2
2
2
2
2
2
0
-15
to 2
(
)
–
15 E LG
–
2
(EQ. 16)
resulting scaling effects of
e
= 0.0000 to 2.18344. The scaled
–
9
e
)2
(EQ. 17)
2
)
(
20
log
1.0
0.mmmmmmmm
+
+
(EQ. 17A)
3-22
HSP50214B
TABLE 6A. AGC LIMIT EXPONENT vs GAIN
GAIN(dB)
96.332
90.309
84.288
78.268
16
72.247
-10
2
66.227
60.206
54.185
48.165
42.144
.
36.124
30.103
24.082
18.062
12.041
6.021
0.000
TABLE 6B. AGC LIMIT MANTISSA vs GAIN
GAIN(dB)
6.000
5.750
5.500
5.250
5.000
4.750
4.500
4.250
4.000
3.750
3.500
3.250
3.000
2.750
2.500
2.250
2.000
1.750
)
1.500
1.250
1.000
0.750
0.500
0.250
0.020
EXPONENT
MANTISSA
15
255
15
0
14
0
13
0
12
0
11
0
10
0
9
0
8
0
7
0
6
0
5
0
4
0
3
0
2
0
1
0
0
0
EXPONENT
MANTISSA
0
255
0
240
0
226
0
212
0
199
0
185
0
173
0
161
0
149
0
138
0
127
0
116
0
105
0
95
0
85
0
75
0
66
0
57
0
48
0
39
0
31
0
23
0
15
0
7
0
1
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