ISL5416_14 Datasheet, PDF (12/71 Pages) Intersil Corporation – Four-Channel Wideband Programmable

ISL5416
SIGNAL
PN
GAIN REG
TABLE 1. PN GENERATOR BIT WEIGHTING
2^
0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19 -20 -21 -22 -23
SSSXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXX
PN Generator
After the mixers, a PN (pseudonoise) signal can be added to
the data. This feature is provided for test and to digitally reduce
the input sensitivity and adjust the receiver range (sensitivity).
The effect is the same as increasing the noise figure of the
receiver, reducing its sensitivity and overall dynamic range.
The one bit PN data is scaled by a 16 bit programmable
scale factor. The overall range for the PN is 0 to 1/4 full
scale. A gain of 0 disables the PN input. The bit weighting for
the gain is shown in table 1.
The minimum, non-zero, PN value is 1/2¹⁸ of full scale (-108
dBFS) on each axis (-105 dbFS total).
CIC Filter
Next, the signal is filtered by a cascaded integrator/comb
(CIC) filter. A CIC filter is an efficient architecture for
decimation filtering. The power or magnitude squared
frequency response of the CIC filter is given by:

 ^2N
P(f)
=


-^s---ⁱ-ⁿ----⁽---^π---^M------^f--^{)}


sin


π_-R---f_


where
M = Number of delays (1 for the ISL5416)
N = Number of stages
and R = Decimation factor.
The passband frequency response for 1^st (N=1) though 5^th
(N=5) order CIC filters is plotted in Figure 20. The frequency
axis is normalized to f_S/R, making f_S/R = 1 the CIC output
sample rate. Figure 19B shows the frequency response for a
5^th order filter but extends the frequency axis to f_S/R = 3 (3
times the CIC output sample rate) to show alias rejection for
the out-of-band signals. Figure 19A provides the amplitude
of the first (strongest) alias as a function of the signal
frequency or bandwidth from DC. For example, with a 5^th
order CIC and f_S/R = 0.125 (signal frequency is 1/8 the CIC
output rate) Figure 19A shows a first alias level of about -87
dB. Figure 19A is also listed in table form in Table 84.
The CIC filter order is programmable from 0 to 5. The CIC
may be bypassed by setting the CIC IWA = *001h bit 15.
A barrel shifter precedes the CIC filter to compensate for the
large gain range of the CIC. As the barrel shifter only adjusts
in 6 dB steps the total CIC/barrel shifter gain ranges from 0.5
to 1.0.
The barrel shifter is also used to convert floating point input
data to fixed point for processing. The exponent bits from the
input and/or range control are added to the shift code
programmed by the user to expand the input range. The shift
code that the user programs must take the expected
exponent range into account i.e. the computed shift control
must be reduced by the maximum exponent value. Also note
that since the exponent shifting reduces the effective size of
the integrators, the maximum decimation factor is reduced
(See Tables 2-4).
The integrator bit widths are 69, 62, 53, 44, and 34 for the 1^st
through 5^th stages, respectively, while the comb bit widths
are all 24. The integrators are sized for decimation factors of
up to 512 with 5 stages, 2048 with 4 stages, 32768 with 3
stages, and 65536 with 1 or 2 stages. Higher decimations in
the CIC should be avoided as they will cause integrator
overflow. In the ISL5416, the integrators are slightly oversized
to reduce the quantization noise at each stage.
A CIC filter has a gain of R^N, where R is the decimation factor
and N is the number of stages. Because the CIC filter gain
can become very large with decimation, an attenuator is
provided ahead of the CIC to prevent overflow. The 24 bits of
mixer output are placed on the low 24 bits of a 69 bit bus
(width of the first CIC integrator) for a gain of 2^-45. A 48 bit
barrel shifter then provides a gain of 2⁰ to 2⁴⁷ inclusive before
passing the data onto the CIC. The overall gain in the pre-CIC
attenuator can therefore be programmed to be any one of 48
values from 2^-45 to 4, inclusive (see IWA = *005h, bits 25:20).
This shift factor is adjusted to keep the total barrel shifter and
CIC filter between 0.5 and 1.0. The equation which should be
used to compute the necessary shift factor is:
BASE SHIFT = MAX(0, 45 - CEIL( LOG2( R^N ) ) - MAXEXP)
MAXEXP = sum of the maximum exponent range from a
floating point input and the range control.
CIC barrel shifts of greater than 45 will cause MSB bits to be
lost. Most of the floating point modes on the ISL5416 make
use of the CIC barrel shifter for gain. This limits the
maximum usable decimation. See floating point input mode
section for details.
If the CIC is bypassed, BASE SHIFT = 45 - MAXEXP.
MAXEXP = sum of the maximum exponent range from a
floating point input and the range control.
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