ISL5216_05 INTERSIL [Intersil Corporation], ISL5216_05 Datasheet - Page 24

• Manufacturer Part Number: ISL5216_05
• Description: Four-Channel Programmable Digital DownConverter
• Manufacturer: INTERSIL [Intersil Corporation]

24-bit programmable FIR filter output into the range of 20-bit
and shorter words in the output section. Without gain control,
a signal at -72dBFS = 20log
only 4 bits of resolution at the output if a 16 bit word length
were to be used (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 4 shows the Block Diagram for the AGC Section. The
FIR filter data output is routed to the Cartesian to polar
coordinate converter after passing through the AGC
multipliers and shift registers. The magnitude output of the
Cartesian to polar 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 4. The gain control from the AGC loop filter is
†
Controlled via microprocessor interface.
(11 MANTISSA
4 EXPONENT)
IFIR
QFIR
SERIAL
OUT
µP
16
19
24
24
MSB = 0
MSB = 0
10
4
24
24
EXP=2
(2
AGC MULTIPLIER/SHIFTER
24
-12
18
NNNN
) at the input would have
EN
AGC LOOP FILTER
LOAD
AGC
FIGURE 4. AGC FUNCTIONAL BLOCK DIAGRAM
16
M
U
X
MANTISSA =
01.XXXXXXXXXXXXXX
LIMIT
DET
LIMIT
DET
µP
LOWER LIMIT
LIMIT
UPPER LIMIT
DET
(RANGE = -2.18344 TO 2.18344)
IAGC
24
ISL5216
+
QAGC
24
†
†
sampled 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
N = 15. The mantissa provides up to an additional 6.02dB of
gain. The gain in dB from the mantissa is:
20log
mantissa interpreted as an unsigned integer ranging from 0
to 2
Thus, the AGC multiplier / shifter transfer function is
expressed as:
AGC Mult/Shift Gain = 2
where N, the shifter exponent, has a range of 0<N<15 and X,
the mantissa, has a range of 0<X<(2
14
10
- 1.
AGCGNSEL
28
[1+(X)2
AGC ERROR SCALING
EXP
-14
], where X is the fractional part of the
(RANGE = 0 TO 1)
4
N
(RANGE = 0 TO 2.32887)
[1+ (X)2
MANTISSA
4
MAGNITUDE
COORDINATE
CONVERTER
(G = 1.64676)
CARTESIAN
-14
POLAR
TO
]
14
10
-1).
(2
DETECTOR
ERROR
N
AGC
∆
) when
16
16
July 8, 2005