HSP50214B_07 Datasheet, PDF (21/62 Pages) Intersil Corporation – Programmable Downconverter

HSP50214B
output section. Without gain control, a signal at -72dBFS =
20log₁₀(2^-12) at the input would have only 4-bits of
resolution at the output (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 23 shows the Block Diagram for the AGC Section.
The FIR filter data output is routed to the Resampling and
Halfband filters after passing through the AGC multipliers
and Shift Registers. The outputs of the Interpolating
Halfband filters are routed to the Cartesian to Polar
coordinate converter. The magnitude output of the
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 23. The gain control from the AGC loop filter is
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 = 20log₁₀(2^N), when N = 15.
The mantissa provides an additional 6dB of gain in
0.0338dB steps where 6.0204dB = 20log₁₀[1+(X)2^-15],
where X = 2¹⁵-1. Thus, the AGC multiplier/shifter transfer
function is expressed as:
AGC Mult/Shift Gain = 2^N[1 + (X)2^–15 ],
(EQ. 14)
where N, the shifter exponent, has a range of 0<N<15 and X,
the mantissa, has a range of 0<X<(2¹⁵-1).
Equation 14 can be expressed in dB,
(AGC Mult/Shift Gain)dB = 20log₁₀(2^N[1 + (X)2^-15 ] ) (EQ. 14A)
The full AGC range of the Multiplier/Shifter is from 0 to
96.331dB (20log₁₀[1+(2¹⁵-1)2^-15] + 20log₁₀[2¹⁵] = 96.331).
Figure 21 illustrates the transfer function of the AGC
multiplier versus mantissa control for N = 0. Figure 22
illustrates the complete AGC Multiplier/Shifter Transfer
function for all values of exponent and mantissa control.
The resolution of the mantissa was increased to 16-bits in
the A Version, to provide 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.
For fixed gains, either set the upper and lower AGC limits to
the same value, or set the limits to minimum and maximum
gains and set the AGC loop gain to zero.
6
5
4
G (dB)
3
2
G (LINEAR)
1
0
0 16 32 48 64 80 96 112 128 144 160 176 192 208 224 240
AGC CONTROL MANTISSA VALUES (TIMES 256)
FIGURE 21. AGC MULTIPLIER LINEAR AND dB TRANSFER
FUNCTION
100
N = 15
90
N = 14
N = 13
80
N = 12
N = 11
70
N = 10
60
N=9
N=8
50
N=7
N=6
40
N=5
30
N=4
N=3
20
N=2
N=1
10
N=0
0
0
64
128
192
AGC CONTROL WORD (MANTISSA x 256)
FIGURE 22. AGC GAIN CONTROL TRANSFER FUNCTION
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 15:
r = 1.64676 I² + Q².
(EQ. 15)
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 (1.0)² + (1.0)² = 1.64676 × 1.414 = 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 bit weighting of
21
FN4450.4
May 1, 2007