HSP50216_15 Datasheet, PDF (23/59 Pages) Intersil Corporation – Four-Channel Programmable Digital Downconverter

HSP50216
SERIAL
19
OUT
16
μP
(11 MANTISSA
4 EXPONENT)
AGC LOOP FILTER
μP
AGC ERROR SCALING
(RANGE = -2.18344 TO 2.18344)
MSB = 0
M
U
+
X
MSB = 0
EN AGC
LOAD
LIMIT
DET
UPPER LIMIT †
LOWER LIMIT †
28
EXP 4
AGCGNSEL
MANTISSA
4
AGC
ERROR
DETECTOR
16
Δ
18
24
IFIR
24
QFIR
4 _EXP=2NNNN
16 MANTISSA =
01.XXXXXXXXXXXXXX
LIMIT
DET
24
24
24
IAGC
LIMIT
DET
24
QAGC
MAGNITUDE
(RANGE = 0 TO 2.32887)
16
(RANGE = 0 TO 1)
CARTESIAN
TO
POLAR
COORDINATE
CONVERTER
(G = 1.64676)
AGC MULTIPLIER/SHIFTER
† Controlled via microprocessor interface.
FIGURE 1. AGC FUNCTIONAL BLOCK DIAGRAM
In dB, this can be expressed as:
(AGC Mult/Shift Gain)dB = 20 log₁₀(2^N[1 + (X)2^-14])
The full AGC range of the multiplier/shifter is from 0 dB to
20log ₁₀ [1+(2 ¹⁴ -1)2 ^-14 ] + 20log ₁₀ [2 ¹⁵ ] = 96.329 dB.
The 16 bit resolution of the mantissa provides a theoretical
AM modulation level of -96dBc (depending on loop gain,
settling mode and SNR). This effectively eliminates AM
spurious caused by the AGC resolution.
The Cartesian to polar coordinate converter accepts I and Q
data and generates magnitude and phase data. The
magnitude output is determined by the equation:
r = 1.64676 I² + Q²
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
r = 1.64676 1² + 1² = 1.64676x1.414 = 2.329
The AGC loop feedback path consists of an error detector,
error scaling, and an AGC loop filter. The error detector
subtracts the magnitude output of the coordinate converter
from the programmable AGC THRESHOLD value. The AGC
THRESHOLD value is set in IWA register *012h and is equal
to 1.64676 times the desired magnitude of the I1/Q1 output.
Note that the MSB is always zero. The range of the AGC
THRESHOLD value is 0 to +3.9999. The AGC Error
Detector output has the identical range.
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FN4557.8
October 5, 2015