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HSP50214B
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
22
21
20
19
18
17
16
22
21
20. 2-1 2-2 2-3 2-4 2-5 2-6 2-7 2-8 2-9 2-10
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 20 to 2-15.
Together the mantissa and exponent define the loop gain as
given by,
AGC Loop Gain = M LG 2
–4
2
– ( 15 – E LG )
(EQ. 16)
where MLG is a 4-bit binary value ranging from 0 to 15, and
ELG is a 4-bit binary value ranging from 0 to 15. Table 7 and
8 detail the binary values and the resulting scaling effects of
the AGC scaling mantissa and exponent. The composite
(shifter and multiplier) AGC scaling Gain range is from
0.0000 to 2.329(0.9375)20 = 0.0000 to 2.18344. The scaled
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 Equation 17:
AGC Gain Limit = ( 1 + m AGC 2
–9
)2
e
(EQ. 17)
( AGC Gain Limit )dB = ( 6.02 ) ( eeee ) + 20 log ( 1.0 + 0.mmmmmmmm )
(EQ. 17A)
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 through
6C. The bit weightings for the AGC Loop Feedback elements
are detailed in Table 9A.
22
TABLE 6A. AGC LIMIT EXPONENT vs GAIN
GAIN(dB)
EXPONENT
MANTISSA
96.332
15
255
90.309
15
0
84.288
14
0
78.268
13
0
72.247
12
0
66.227
11
0
60.206
10
0
54.185
9
0
48.165
8
0
42.144
7
0
36.124
6
0
30.103
5
0
24.082
4
0
18.062
3
0
12.041
2
0
6.021
1
0
0.000
0
0
TABLE 6B. AGC LIMIT MANTISSA vs GAIN
GAIN(dB)
EXPONENT
MANTISSA
6.000
0
255
5.750
0
240
5.500
0
226
5.250
0
212
5.000
0
199
4.750
0
185
4.500
0
173
4.250
0
161
4.000
0
149
3.750
0
138
3.500
0
127
3.250
0
116
3.000
0
105
2.750
0
95
2.500
0
85
2.250
0
75
2.000
0
66
1.750
0
57
1.500
0
48
1.250
0
39
1.000
0
31
0.750
0
23
0.500
0
15
0.250
0
7
0.020
0
1
FN4450.4
May 1, 2007