AS3909 datasheet(17/77 Pages) AMSCO

17 / 77 page

ams Datasheet, Confidential: 2013-Oct [3-02]

AS3909/AS3910 – 17

Appl icatio n I n f o rm a t i o n

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.

Setting Gain Reduction in Receiver Configuration Register

(#06)

By setting bits rg2 to rg0 in Receiver Configuration Register (#06)

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.

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 than 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.

AM and PM Demodulation

In addition to usual AM demodulation, the AS3909/10 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.

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