ISL5416KI Intersil, ISL5416KI Datasheet - Page 16

• Manufacturer Part Number: ISL5416KI
• Description: Up/Down Conv Mixer 1.8V 256-Pin BGA
• Manufacturer: Intersil

Specifications of ISL5416KI

• Package: 256BGA
• Operating Supply Voltage: 1.8 V

AGC
The automatic gain control (AGC) section adds gain to
maintain the output signal level at a programmed level. The
AGC moderates signal level variation at the output of the
part and reduces the number of bits that must be carried in
any post processing. In the ISL5416, the AGC follows the
channel filtering. The gains through the NCO, mixer, and FIR
filter sections are fixed gains and do not induce AM distortion
before the large interfering signals can be filtered out. If large
interfering signals are not removed by the filtering prior to the
AGC, the gain adjustments by the AGC can AM modulate
the large signals and cause AM sidebands to fall inside the
frequency band of interest.
A block diagram of the AGC is included in figure 6. The AGC
consists of a forward gain path and a loop filter path. In the
forward gain path, the I/Q samples are scaled by the AGC
forward gain value provided by the loop filter. The forward
path gain is divided between a barrel shifter and a multiplier.
The overall forward path gain range is 0 to 96.33 dB. The
barrel shifter provides 0 to 90 dB of gain in steps of 6 dB.
The multiplier provides linear gain between 1.0 and 2.0.
Saturation is provided if there is overflow. The AGC only
adds gain. The loop filter path computes the gain error, filters
it, compares it to gain limits, and provides it to the forward
gain path, to the uP interface, and to the output section. In
the loop filter path the gain error is computed by first
computing the magnitude of the forward path output. The
magnitude is then subtracted from a programmable
threshold or set point. The resulting error value is then
scaled by a programmable loop gain and integrated and
provided to the forward path. Programmable limits on the
forward gain allow the user to restrict the gain to a smaller
range than the 96 dB provided.
The forward gain control word and programmable gain limits
are floating-point numbers consisting of a four-bit exponent
that controls the barrel shifter and a mantissa portion that
controls the multiplier. The barrel shifter gain is 2
multiplier gain is 16 bits, but the two MSBs are fixed at “01”
and are not included in the gain control word. The mantissa
MSB is therefore weighted as 0.5 and the mantissa gain is
1.0 + MANT. The total AGC gain in dB is then:
The AGC range is then 0 to 96.33 dB for the EXP range of 0
to 15 and MANT range from 0 to 1. Plots of AGC gain versus
the control word are provided in figures 5A and 5B.
The AGC gain word is available through the uP interface and
as a real time output. The gain word is inversely proportional
to the received signal strength in the channel. Signal
strength in dB can be easily estimated by complementing the
gain word and adding an offset equal to the fixed receive
path gain in dB.
20*log
10
( 2
EXP
* (1.0 + MANT)).
16
EXP
. The
ISL5416
The AGC includes a set of counters to synchronize the AGC
to system timing. The counters can be aligned to the
SYNCInX signals if enabled in IWA *000h. One counter is
programmed to count modulo N clocks where N is the length
of the time slot. This counter can be restarted with SYNCInX
to align/re-align it with the slots. A second counter counts out
a delay from the SYNCInX or counter-generated sync. This
delays the AGC timing from the SYNCInX signal to
compensate for filter group delay or other system delays. A
third counter counts out an interval. The interval can be used
to divide the slot into fast and slow update periods (timed
mode) or into measurement and update periods (sampled
mode). The counters can also be disabled and the AGC
allowed to free run (continuos mode).
A programmable data delay can be inserted in the forward
data path. The loop filter uses the samples into the delay for
computing the new forward gain. The forward gain is then
applied to the samples coming out of the delay. The gain
applied to the output can be continuously updated or can be
updated under the control of the counters. When updated
continuously, the delay causes the forward gain to be based
on samples before and after the delayed sample. This
moderates large signal variations and minimizes the amount
of time that the forward path may be in saturation or be at a
small level.
The sampled mode is used for burst type signals where the
gain adjustment is made during the first part of the burst and
then held for the duration of the burst. The programmable
delay can be set so that the first samples of the burst are
exiting the delay when the gain is updated. In this mode, the
gain may have large instantaneous changes, so proper
timing alignment is very important.
In the timed mode, loop filter continuously updates the
forward gain but uses one set of loop gains during part of the
burst and another set for the rest of the burst. This allows the
time slot to be divided into adapt/hold or fast/slow intervals.
The maximum throughput of the AGC depends on the mode.
In the continuous (counters disabled) and timed modes
without delay, the minimum spacing between samples into
the AGC is 2 clocks. When the delay is enabled, this
increases to 4. In the sampled mode, the delay is always
enabled and the minimum spacing is 4. The minimum
spacing is 1 when the AGC is bypassed.
The AGC loop feedback path includes a magnitude
computation, an error detector, error scaling (loop gain), and
a loop filter. The magnitude computation in the loop filter is a
multi-pass operation with one pass computed per clock
cycle. The accuracy of the computation depends on the
number of passes. The minimum number of clocks between
samples into the AGC is 2. There is a gain in the magnitude
computation that must be taken into account when
programming the AGC set point. This gain also depends on