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AS3910

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

8.3.1 Gain Reduction, AGC and Squelch

The total gain of receiver chain is 160. In certain conditions it is desirable to reduce this gain. There are several features implemented in the

Receiver to reduce this gain.

conditions receiver chain is in saturation and demodulation can be influenced by system noise and saturation of last gain stage. 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 three times larger then 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 then three times larger than its

window.

AGC operation is controlled by the Receiver Configuration Register (#06) bits agc_en and agc_m. Agc_en bit enables AGC operation, agc_m

defines AGC operating mode. The AGC action is started 20µs after rising edge of signal rx_on. In case agc_m bit is 0 it will operate during a

complete receive period, in case it is 1 it will operate on first 8 subcarrier pulses. The AGC is reducing gain to -21dB in 7 steps of 3dB. When

signal rx_on is low AGC is in reset.

takes place when the data sent by the reader is being processed and an answer prepared. It can also be used in noisy environment.

Transponder processing noise (or environment noise) may be misinterpreted as start of transponder response which results in reader decoding

error. These problems are avoided by reducing receiver gain so that there are no transitions of output when noise is present. This is done by

sending direct command Squelch.

During execution of the direct command Squelch the digital output of receiver (output of window comparator mentioned above) is observed. In

case there are more then 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 (21dB) is reached. This

setting is cleared with direct command Clear Squelch.

receiver gain can also be reduced in 7 steps of 3dB.

Actual gain reduction is combination of all three gain reduction features mentioned above (AGC, Squelch and setting gain reduction in Receiver

Configuration Register). Actual gain reduction state can also be observed by reading the Receiver State Display Register (#17) bits gr_2 to gr_0.

8.3.2 RSSI

The receiver also comprises of an RSSI block (Received Signal Strength Indicator) which measures the strength of the modulated signal that is

superimposed on the 13.56MHz carrier. RSSI is a peak hold system which is started 20μs and 16 transitions of demodulated signal after rising

edge of rx_on. It stays active while signal rx_on is high; while rx_on is low it is frozen. Result of RSSI measurements is 4 bit value which can be

observed by reading Receiver State Display Register (#17) bits rssi_3 to rssi_0. The RSSI range calculated back on RFI1 input is 280μVrms to

8.8mVrms, one LSB represents step of 2.15dB.

Since the RSSI measurement is a peak hold then the RSSI result will not follow any variations in the signal strength (the highest value will be

kept). In such a case it is possible to reset RSSI bits of Receiver State Display Register and restart the measurement by sending direct command

Clear RSSI.

8.3.3 AM and PM Demodulation

In addition to usual AM demodulation, the AS3910 also includes the possibility of PM demodulation. Readers which have only AM demodulation

may have so called communication holes in operating volume. Communication holes are areas where transponder is not seen, they depend on

transponder characteristics such as Q factor and resonant frequency variation. Usually both AM and PM modulation are present, in so called

communication holes receiver input signal is only PM modulated. Choice between AM and PM demodulation is done by setting the bit pmd in the

Configuration Register 5 (#05); default setting is AM. As mentioned above an RSSI measurement is continually done while transponder

message is being processed. By comparing RSSI value in AM and PM mode the external controller can opt for the demodulation mode in which

there is more signal. PM demodulation is done by processing phase signal coming from the Phase Detector.

8.4 A/D Converter

The AS3910 contains an 8 bit successive approximation A/D converter. Input to A/D converter can be multiplexed from different sources to be

used in several direct commands and adjustment procedures. The result of last A/D conversion is stored in a register which can be read through

the SPI interface (address #0D). Typical conversion time is 12μs.

The A/D converter has two operating modes, absolute and relative.

In absolute mode the low reference is 0V and the high reference is 2V. This means that A/D converter input range is from 0 to 2V, 00 code means

input is 0V or lower, FF means that input is 2V or higher, LSB is 7.8125mV.