ISL3874 Datasheet, PDF (25/33 Pages) Intersil Corporation – Wireless LAN Integrated Medium Access Controller with Baseband Processor with Mini-PCI

ISL3874
result of RSSI against a threshold. The threshold may be set to
an absolute power value, or it may be set to be N dB above the
measured noise floor. See CR35. The ISL3874 measures and
stores the RSSI level when it detects no presence of BPSK or
QPSK signals. The average value of a 256 value buffer is taken
to be the noise floor. Thus, the value of the noise floor will adapt
to the environment. A separate noise floor value is maintained
for each antenna. An initial value of the noise floor is
established within 50µs of the chip being active and is refined
as time goes on. Deasserting RX_PE does not corrupt the
learned values. If the absolute power metric is chosen, this
threshold is normally set to between -70dBm and -80dBm.
If desired, ED may be used in the acquisition process as well
as CCA. ED may be used to mask (squelch) weak signals
and prevent radio reception of signals too weak to support
the high data rates, signals from adjacent cells, networks, or
buildings.
The Configuration registers effecting the CCA algorithm
operation are summarized below (more programming details
on these registers can be found under the Control Registers
section of this document).
CR9(6:5) allow CCA to be programmed to be a function of ED
only, the logical operation of (CS1 OR SQ1), the logical function
of (ED AND (CS1 OR SQ1)), or (ED OR (CS1 OR SQ1)).
CR9(7) lets the user select from sampled CCA mode, which
means CCA will not glitch, is updated once per symbol and is
valid for reading at 15.8µs or 18.7µs. In non-sampled mode,
CCA may change at any time, potentially several times per slot,
as ED and CS1 operate asynchronously to slot times.
In a typical system CCA will be monitored to determine when
the channel is clear. Once the channel is detected busy,
CCA should be checked periodically to determine if the
channel becomes clear. Once MD_RDY goes active, CCA
should be ignored for the remainder of the message. Failure
to monitor CCA until MD_RDY goes active (or use of a time-
out circuit) could result in a stalled system as it is possible for
the channel to be busy and then become clear without an
MD_RDY occurring.
AGC Description
The AGC system consists of the 3 chips handling the receive
signal, the RF to IF downconverter HFA3683, the IF to
baseband converter HFA3783, and the baseband processor
(BBP) section of the ISL3874. The AGC loop (Figure 11) is
digitally controlled by the BBP. Basically it operates as follows:
Initially, the receiver is set for high gain. The percent of time
that the A/D converters in the baseband processor are
saturated is monitored along with signal amplitude and the
gain is adjusted down until the amplitude is what will
optimize the demodulator’s performance. If the amount of
saturation is great, the initial gain adjust steps are large. If
the signal overload is small, they are less. When the gain is
about right and the A/Ds’ outputs are within the lock window
(CR19), the BBP declares AGC lock and stops adjusting for
the duration of the packet. If the signal level then varies
more than a preset amount (CR20, CR29), the AGC is
declared unlocked and the gain again allowed to readjust.
The BBP looks for the locked state following an unlocked
state (CS1) as one indication that a received signal is on the
antenna. This starts the receive process of looking for PN
correlation (SQ1). Once PN correlation and AGC lock are
found, the processor begins acquisition.
For large signals, the power level in the RF stage output is
also monitored and if it is large, the LNA stage gain is
dropped. This removes 30dB of gain from the receive chain
which is compensated for by replacing 30dB of gain in the IF
AGC stage. There is some hysteresis in this operation and
once the AGC locks, it is locked as well. This improves the
receiver dynamic range.
RX_RF_AGC Pad Operation
30dB Pad Engaging (RF Chip Low Gain)
If the AGC is not locked onto a packet, a ‘1’ on the
ifCompDet (see notes below) state will engage in the 30dB
attenuation pad. This causes the AGC to go out of lock and
also forces the attenuation accumulator to be set to the
programmed value of CR27. The AGC then attempts to lock
on the signal.
If the AGC is locked on a packet, ifCompDet is ignored.
30dB Pad Releasing (RF Chip High Gain)
If the AGC is not locked onto a packet and the attenuation
accumulator sum falls below the programmable threshold
(CR27), the pad will release. This is for the case where a
noise spike kicked in the 30dB pad and the pad should
release when the noise spike ends. Since the noise floor is
different for different environments, it is possible that in many
cases CR27’s programmed value will be below the noise floor
and the pad will not be removed except by RXPE going low.
There is a recommended value to program CR27 (24dB), but
that depends on what environment the radio is in.
During a packet (after AGC lock), the 30dB pad is held
constant and the CR27 threshold is ignored.
RXPE low forces the pad to release whether in the middle of
a packet or not. At the end of a packet, RXPE always goes
low, forcing the pad to release.
The following notes apply:
• The attenuation accumulator is basically about equal to
the current RSSI value.
• The accumulator output, after going through the
interpolator lookup table, feeds the AGC D/A.
• The pad value is programmable (CR17), but is
recommended to be set to 30dB.
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