hsp50214a Intersil Corporation, hsp50214a Datasheet - Page 19

• Manufacturer Part Number: hsp50214a
• Description: Programmable Downconverter
• Manufacturer: Intersil Corporation

Definitions:
Even Symmetric: h(n) = h(N-n-1) for n = 0 to N-1
Odd Symmetric: h(n) = -h(N-n-1) for n = 0 to N-1
Asymmetric:
Even Tap filter:
Odd Tap filter:
Real Filter:
Complex Filters: A filter with quadrature coefficients.
FIGURE 20. DEMONSTRATION OF DIFFERENT TYPES OF
C0
C0
C0
EVEN SYMMETRIC
EVEN TAP FILTER
EVEN SYMMETRIC
ODD TAP FILTER
EVEN TAP FILTER
ASYMMETRIC
DIGITAL FIR FILTERS CONFIGURED IN THE
PROGRAMMABLE DOWNCONVERTER
C
Q
C
A filter with no coefficient symmetry.
A filter where N is an even number.
A filter where N is an odd number.
A filter implemented with real coefficients.
Q(0)
CN-1
CN-1
COEFFICIENT
NUMBER
COEFFICIENT
NUMBER
CN-1
NUMBER
COEFFICIENT
COMPLEX FILTERS
C
REAL FILTERS
I(0)
C0
C0
C0
ODD SYMMETRIC
EVEN TAP FILTER
ODD SYMMETRIC
ODD TAP FILTER
ODD TAP FILTER
C
ASYMMETRIC
I
C
Q(N-1)
C
COEFFICIENT
NUMBER
I(N-1)
CN-1
CN-1
CN
COEFFICIENT
NUMBER
COEFFICIENT
NUMBER
COEFFICIENT
NUMBER
HSP50214A
19
Automatic Gain Control (AGC)
The AGC Section provides gain to small signals, after the
large signals and out-of-band noise have been filtered out, to
ensure that small signals have sufficient bit resolution in the
Resampling/Interpolating Halfband filters and the Output
Formatter. The AGC can also be used to manually set the
gain. The AGC optimizes the bit resolution for a variety of
input amplitude signal levels. The AGC loop automatically
adds gain to bring small signals from the lower bits of the 26-
bit programmable FIR filter output into the 16-bit range of the
output section. Without gain control, a signal at -72dBf
20log
tion 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 Half-
band 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 sam-
pled 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
provides an additional 6dB of gain in 0.0338dB steps where
6.0204dB = 20log
AGC multiplier/shifter transfer function is expressed as:
where N, the shifter exponent, has a range of 0>N>15 and
X, the mantissa, has a range of 0>X>(2
Equation 14 can be expressed in dB,
The full AGC range of the Multiplier/Shifter is from 0 to
96.331dB (20log
Figure 21 illustrates the transfer function of the AGC multi-
plier 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 gain, 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 gain to zero.
AGC Mult/Shift Gain
(AGC Mult/Shift Gain)dB
10
(2
-12
) at the input would have only 4 bits of resolu-
10
10
[1+(2
=
[1+(X)2
2
N
=
10
15
1
20log
(2
+
-1)2
-15
N
X 2
), when N = 15. The mantissa
10
-15
], where X = 2
–
2
15
] + 20log
N
,
1
+
15
X 2
-1).
10
-15
[2
15
15
-1. Thus, the
] = 96.331).
(EQ. 14A)
(EQ. 14)
S
=