www.austriamicrosystems.com/HF_RFID_Reader/AS3910

Revision 2.3

15 - 47

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

In relative mode low reference is 1/6 of VSP and high reference is 5/6 of VSP, so the input range is from 1/6VSP to 5/6VSP. Relative mode is only

used in phase measurement in which phase detector output is proportional to power supply. In all other cases absolute mode is used.

The A/D converter input can also be accessed externally. When the direct command AD Convert is sent, an A/D conversion of voltage present on

pin AD_IN is performed in absolute mode, result is stored in A/D Converter Output Register. AD_IN pin should be left non-connected in case A/

D conversion is not needed in application.

8.5 Phase and Amplitude Detector

8.5.1 Phase Detector

The phase detector is observing phase difference between the transmitter output signals (RFO1 and RFO2) and the pad signals RFI1 and RFI2,

which are proportional to the signal on the antenna LC tank. These signals are first passed by digitizing comparators. Digitized signals are

processed by a phase detector. Filter characteristics of the phase antenna are adapted to one of the two possible operation modes. For antenna

tuning check, a strong low power filter is used to get average phase difference, for PM demodulation a low pass filter having 1MHz corner

frequency is used to pass the subcarrier frequency.

LC tank tuning) results in VSP/2 output voltage. In case the antenna LC tank is detuned, phase shift changes which results in different phase

detector output voltage. In case of command Check Antenna Resonance phase detector output is applied to A/ D converter in relative mode.

Output of phase detector is also observed by comparator with reference signal VSP/2. Output of this comparator is used in execution of direct

command Calibrate Antenna.

848 kHz subcarrier signal. The output is then fed to Receiver.

8.5.2 Amplitude Detector

Signals from pads RFI1 and RFI2 are used as inputs to the self mixing stage. Output of this stage is DC voltage proportional to amplitude of

signal on RFI1 and RFI2 pads. This signal is fed to A/D converter when amplitude of signal on RFI inputs has to be measured (direct commands

Measure RF and Calibrate Modulation Depth).

8.6 External Field Detector

The External Field Detector is used in NFC mode to detect the presence of an external RF field. It is composed of two sub-blocks, Target

Activation Detector and a RF Collision Avoidance Detector. Input to both blocks is the signal from the RFI1 pad. The thresholds of the two blocks

can be independently set by writing the NFCIP Field Detection Threshold Register (#14). The outputs of both detectors are fed to a logic or gate,

output of which goes to the Control logic. A low to high transition of this logic or gate output triggers an interrupt (Interrupt due to nfc event)

8.6.1 Target Activation Detector

This block is turned on in NFC target mode to detect the presence of an interrogator field. It is enabled by setting the Operation Control Register

bit nfc_t. It is a low power block with an adjustable threshold in the range from 145mVpp and 590mVpp. This block generates an interrupt when

an external field is detected and also when it disappears. With such implementation it can also be used to detect the moment when the external

field disappears. This is useful to detect the moment when external NFC device (it can either an interrogator or a target) has stopped emitting an

RF field since a response can only be sent afterwards. Actual state of the Target Activation Detector can be checked by reading the bit rfp in the

Receiver State Display Register (#17). When this bit is set to logic one there is a signal higher then the threshold present on the input of Target

Activation Detector.

8.6.2 RF Collision Avoidance Detector

This block is activated during the RF Collision Avoidance sequence which is executed before every request or response in NFC active

communication (Initial or Response RF Collision Avoidance). In case during the RF Collision Avoidance sequence the presence of an external

field is detected, request/response is not sent, an interrupt is generated to inform the external controller about collision. During RF Collision

Avoidance, the Target Activation Detector is disabled in order to have the correct threshold when detection is made. The threshold of the RF

Collision Avoidance Detector can be adjusted in the range from 50 to 1080mVpp.

8.7 Quartz Crystal Oscillator

The quartz crystal oscillator can operate with 13.56MHz and 27.12MHz crystals. The oscillator is based on an inverter stage supplied by

controlled current source. A feedback loop is controlling the bias current in order to regulate amplitude on XTI pin to 1Vpp. This feedback

assures reliable operation even in case of low quality crystals with Rs up to 50

Ω. In order to enable a fast reader start-up an interrupt is sent

when oscillator amplitude exceeds 750mVpp.