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AS3911
Datasheet - Application Information
Squelch. This feature is designed for operation of receiver in noisy environment. The noise can come from tags in which processing of data
sent by the reader is going on and an answer is being prepared. Noise can also be generated by noisy environment. This noise may be
misinterpreted as start of transponder response which results in decoding error.
During execution of the Squelch procedure the output of Digitizing comparator is observed. In case there are more than two transitions on this
output in 50µs time period, gain is reduced for 3dB and output is observed during next 50µs. This procedure is repeated until number of
transitions in 50µs is lower or equal to 2 or until maximum gain reduction is reached. This setting is cleared by sending direct command
Reset
Rx Gain
.
There are two possibilities of performing squelch: automatic mode and using direct command
Squelch
.
Automatic mode is started in case bit sqm_dyn in the
Receiver Configuration Register 2
is set. It is activated automatically 18.88µs
after end of Tx and is terminated with Mask Receive timer expire. This mode is primarily intended to suppress noise generated by tag
processing during the time when the tag response is not expected (covered by Mask Receive timer).
Command
Squelch
is accepted in case it is sent when signal rx_on is low. It can be used in case the time window in which noise is present
is known by the controller.
AGC. AGC (automatic gain control) is used to reduce gain to keep receiver chain out of saturation. In case gain is properly adjusted the
demodulation process is also less influenced by system noise.
AGC action starts when signal
rx_on is asserted high and is reset when it is reset to low. At low to high transitions of the rx_on the state of the
receiver gain is stored in the
Gain Reduction State Register
, therefore reading this register later gives the information of the gain setting
used during last reception.
When AGC is switched on receiver gain is reduced so that the input to digitizer stage is not saturated. The AGC system comprises a window
comparator which has its window 3.5 times larger than window of digitalization window comparator. When the AGC function is enabled gain is
reduced until there are no transitions on its output. Such procedure assures that the input to digitalization window comparator is less than 3.5
times larger than its window.
AGC operation is controlled by the control bits
agc_en, agc_m and agc_fast in the
Receiver Configuration Register 2
. Bit agc_en enables
the AGC operation; bit
agc_m defines the AGC mode while bit agc_alg define the AGC algorithm.
Two AGC modes are available, AGC can operate during complete Rx process (as long as signal
rx_on is high) or it can be enabled only during
first eight sub-carrier pulses.
Two AGC algorithms are available; AGC can either start by presetting of code 4h (max digitizer window, max gain) or by resetting the code to 0h
(min digitizer window, max gain). Algorithm with preset code is faster, therefore it is recommended for protocols with short SOF (like ISO14443A
fc/128).
Default AGC settings are: AGC is enabled, AGC operates during complete Rx process, algorithm with preset is used.
RSSI. The receiver also performs the RSSI (Received Signal Strength Indicator) measurement of both channels. RSSI measurement is started
after rising edge of
rx_on. It stays active while signal rx_on is high; while rx_on is low it is frozen. It is a peak hold system; the value can only
increase from initial zero value. Every time the AGC reduces the gain the RSSI measurement is reset and starts from zero. Result of RSSI
measurements is 4-bit value which can be observed by reading the
RSSI Display Register
. The LSB step is 2.8dB, the maximum code is Dh
(13d).
Since the RSSI measurement is of peak hold type the RSSI measurement result does not follow any variations in the signal strength (the highest
value will be kept). In order to follow RSSI variation it is possible to reset RSSI bits and restart the measurement by sending direct command
Clear RSSI
.
8.3.5 Receiver in NFCIP-1 Active Communication Mode
There are several features built in receiver to enable reliable reception of active NFCIP-1 communication. All these settings are automatically
preset by sending direct command
Analog Preset
after the NFCIP-1 mode has been configured. In addition to filtering options there are two
NFC specific configuration bits stored in the
Receiver Configuration Register 3
.
Bit
lim enables clipping circuits which are positioned after first and second gain stages. The intention of clipping circuits is to limit the signal level
for the following filtering stage (in case the NFC peer is close the input signal level can be quite high).
Bit
rg_nfc forces gain reduction of second and third filtering stage to -6dB while keeping the digitizer comparator window at maximum level.