ISL3873A Datasheet, PDF (24/42 Pages) Intersil Corporation – Wireless LAN Integrated Medium Access Controller with Baseband Processor

ISL3873A
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 input 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 some
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.
Notes: 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 value used to represent the pad is programmable
(CR17), but is recommended to be set to 30dB.
ifCompDet is a signal from the HFA3783 chip. A '1' indicates
its inputs are near saturation and it needs the RF chip to
switch from high gain to low gain.
RX_IF_Det is the input to the ISL3873A chip which is
connected to ifCompDet on the HFA3783.
RX_RF_AGC is the output of the ISL3873A chip and '1' is
high gain, '0' is low gain.
Demodulator Description
The receiver portion of the baseband processor, performs A/D
conversion and demodulation of the spread spectrum signal.
It correlates the PN spread symbols, then demodulates the
DBPSK, DQPSK, or CCK symbols. The demodulator includes
a frequency tracking loop that tracks and removes the carrier
frequency offset. In addition, it tracks the symbol timing, and
differentially decodes and descrambles the data. The data is
output through the RX Port to the external processor.
The PRISM baseband processor in the ISL3873A uses
coherent demodulation. The ISL3873A is designed to
achieve rapid settling of the carrier tracking loop during
acquisition. Rapid phase fluctuations are handled with a
relatively wide loop bandwidth which is then stepped down
as the packet progresses. Coherent processing improves
the BER performance margin as opposed to differentially
coherent processing for the CCK data rates.
The baseband processor uses time invariant correlation to
strip the Barker code spreading and phase processing to
demodulate the resulting signals in the header and
DBPSK/DQPSK demodulation modes. These operations are
illustrated in Figure 18 which is an overall block diagram of
the receiver processor.
In processing the DBPSK header, input samples from the I and
Q A/D converters are correlated to remove the spreading
sequence. The peak position of the correlation pulse is used to
determine the symbol timing. The sample stream is decimated
to the symbol rate and corrected for frequency offset prior to
PSK demodulation. Phase errors from the demodulator are fed
to the NCO through a lead/lag filter to maintain phase lock. The
carrier is de-rotated by the carrier tracking loop. The
demodulated data is differentially decoded and descrambled
before being sent to the header detection section.
In the 1Mbps DBPSK mode, data demodulation is performed
the same as in header processing. In the 2Mbps DQPSK
mode, the demodulator demodulates two bits per symbol
and differentially decodes these bit pairs. The bits are then
serialized and descrambled prior to being sent to the output.
In the CCK modes, the receiver removes carrier frequency
offsets and uses a bank of correlators to detect the
modulation. A biggest picker finds the largest correlation in
the I and Q Channels and determines the sign of those
correlations. For this to happen, the demodulator must know
the starting phase which is determined by referencing the
data to the last bit of the header. Each symbol demodulated
determines 1 or 2 nibbles of data. This is then serialized and
descrambled before being passed to the output.
Carrier tracking is via a lead/lag filter using a digital Costas
phase detector. Chip tracking in the CCK modes is chip
decision directed or slaved to the carrier tracking
depending on whether or not the locked oscillator design is
utilized in the radio.
Acquisition Description
A projected worst case time line for the acquisition of a
signal with a short preamble and header is shown. The
synchronization part of the preamble is 56 symbols long
followed by a 16-bit SFD. The receiver must monitor the
antenna to determine if a signal is present. The timeline is
broken into 10µs blocks (dwells) for the scanning process.
This length of time is necessary to allow enough integration
of the signal to make a good acquisition decision. This worst
case time line example assumes that the signal arrives part
way into the first dwell such as to just barely catch detection.
The signal and the scanning process are asynchronous and
the signal could start anywhere. In this timeline, it is
assumed that the signal is present in the first 10µs dwell, but
was missed due to power amplifier ramp up.
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