AS3909 データシートの表示（PDF） - austriamicrosystems AG
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AS3909 Datasheet PDF : 77 Pages
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Application Information
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.
ams Datasheet, Confidential: 2013-Oct [3-02]
AS3909/AS3910 – 17
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