Pentek 6210 Operating Manual page 72

range for the AGC rate is ~ 0.000106 to 3.275dB/output
sample 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 Response = Input(Cart/Polar Gain)(Error Det Gain) AGC
Max
Loop Gain)(AGC Output Weighting)
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.
TABLE 7. AGC LOOP GAIN BINARY MANTISSA TO GAIN
SCALE FACTOR MAPPING
BINARY
CODE SCALE
(MMMM) FACTOR
0000 0.0000
0001 0.0625
0010 0.1250
0011 0.1875
0100 0.2500
0101 0.3125
0110 0.3750
0111 0.4375
TABLE 8. AGC LOOP GAIN BINARY EXPONENT TO GAIN
SCALE FACTOR MAPPING
BINARY
CODE SCALE
(EEEE) FACTOR
15
0000 2
14
0001 2
13
0010 2
12
0011 2
11
0100 2
10-
0101 2
9
0110 2
8
0111 2
For example, if M = 0101 and E
LG
-7
Gain = 0.3125*2 . The loop gain mantissas and exponents
are set in the AGC Loop Parameter Control Register (Control
Word 8, Bits 0-15).
Two AGC loop gains are provided in the Programmable Down
Converter, for quick adjustment of the AGC loop. The AGC
Gain select is a control input to the device, selecting Gain 0
when AGCGNSEL = 0, and selecting Gain 1 when
AGCGNSEL = 1.
3-24
HSP50214B
(
(EQ. 19)
BINARY
CODE SCALE
(MMMM) FACTOR
1000 0.5000
1001 0.5625
1010 0.6250
1011 0.6875
1100 0.7500
1101 0.8125
1110 0.8750
1111 0.9375
BINARY
CODE SCALE
(EEEE) FACTOR
7
1000 2
6
1001 2
5
1010 2
4
1011 2
3
1100 2
2
1101 2
1
1110 2
0
1111 2
= 1100, the AGC Loop
LG
In the HSP50214, a reset event (caused by SYNCIN2 or
CW25) would clear the AGC loop filter accumulator. In the
HSP50214B, if Control Word 27, Bit 15 is set to zero, the
AGC loop filter accumulator will clear as in the original
HSP50214. If Control Word 27, Bit 15 is set to a one, the
backend reset (from CW25) will not clear the AGC loop filter
accumulator.
In the HSP50214, the settling mode of the AGC forces the
mean of the signal magnitude error to zero. The gain error is
scaled and used to adjust the gain up or down. This
proportional scaling mode causes the AGC to settle to the
final gain value asymptotically. This AGC settling mode is
preferred in many applications because the loop gain
adjustments get smaller and smaller as the loop settles,
reducing any AM distortion caused by the AGC.
With this AGC settling mode, the proportional gain error
causes the loop to settle more slowly if the threshold is
small. This is because the maximum value of the threshold
minus the magnitude is smaller. Also, the settling can be
asymmetric, where the loop may settle faster for "over range"
signals than for "under range" signals (or vice versa).
In some applications, such as burst signals or TDMA signals,
a very fast settling time and/or a more predictable settling
time is desired. The AGC may be turned off or slowed down
after an initial AGC settling period.
To minimize the settling time, a median AGC settling mode
has been added to the HSP50214B. This mode uses a fixed
gain adjustment with only the direction of the adjustment
controlled by the gain error. This makes the settling time
independent of the signal level.
For example, if the loop is set to adjust 0.5dB per output
sample, the loop gain can slew up or down by 16dB in 16
symbol times, assuming a 2 samples per symbol output
sample rate. This is called a median settling mode because
the loop settles to where there is an equal number of
magnitude samples above and below the threshold. The
disadvantage of this mode is that the loop will have a wander
(dither) equal to the programmed step size. For this reason,
it is advisable to set one loop gain for fast settling at the
beginning of the burst and the second loop gain for small
adjustments during tracking.
The median settling mode is enabled by setting Control
Word 27, Bit 16 to a logic one. If Control Word 27, Bit 16 is
zero, the mean loop settling mode is selected and the loop
works identically to the HSP50214.
In the median mode, the loop works as follows:
The sign of the true gain error selects a fixed gain error of
0010000000000 or 1110000000000
b
These gain error values are scaled by the programmable
AGC loop gains to adjust the data path gain.
.
b