TRF7970A Datasheet, PDF (20/83 Pages) Texas Instruments – MULTI-PROTOCOL FULLY INTEGRATED 13.56-MHz RFID/NEAR FIELD COMMUNICATION

TRF7970A
SLOS743 – AUGUST 2011
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The primary function of the auxiliary receiver is to monitor the RX signal quality by measuring the RSSI of
the demodulated subcarrier signal (internal RSSI). After startup, RX_IN2 is multiplexed to the auxiliary
receiver. The auxiliary receiver has an RF envelope detection stage, first gain and filtering with AGC stage
and finally the auxiliary RSSI block.
The default MUX setting is RX_IN1 connected to the main receiver and RX_IN2 connected to the auxiliary
receiver. To determine the signal quality, the response from the tag is detected by the "main" (pin RX_IN1)
and "auxiliary" (pin RX_IN2) RSSI. Both values measured and stored in the RSSI level register (address
0x0F). The MCU can read the RSSI values from the TRF7970A RSSI register and make the decision if
swapping the input- signals is preferable or not. Setting B3 in Chip Status Control register (address 0x00)
to 1 connects RX_IN1 (pin 8) to the auxiliary received and RX_IN2 (pin 9) to the main receiver. This
mechanism needs to be used to avoid reading holes.
The main and auxiliary receiver input stages are RF envelope detectors. The RF amplitude at RX_IN1 and
RX_IN2 should be approximately 3 VPP for a V_IN supply level greater than 3.3 V. If the V_IN level is lower,
the RF input peak-to-peak voltage level should not exceed the V_IN level.
5.3.2 Receiver Gain and Filter Stages
The first gain and filtering stage has a nominal gain of 15 dB with an adjustable band-pass filter. The
band-pass filter has programmable 3d-B corner frequencies between 110 kHz to 450 kHz for the
high-pass filter and 570 kHz to 1500 kHz for the low-pass filter. After the band-pass filter, there is another
gain-and-filtering stage with a nominal gain of 8 dB and with frequency characteristics identical to the first
band-pass stage.
The internal filters are configured automatically depending on the selected ISO communication standard in
the ISO Control register (address 0x01). If required, additional fine tuning can be done by writing directly
to the RX special setting registers (address 0x0A).
The main receiver also has a second receiver gain and digitizer stage which is included in the AGC loop.
The AGC loop is activated by setting the bit B2 = 1 in the Chip Status Control register (0x00). When
activated, the AGC continuously monitors the input signal level. If the signal level is significantly higher
than an internal threshold level, gain reduction is activated.
By default, the AGC is frozen after the first 4 pulses of the subcarrier signal. This prevents the AGC from
interfering with the reception of the remaining data packet. In certain situations, this AGC freeze is not
optimal, so it can be removed by setting B0 = 1 in the RX special setting register (address 0x0A).
Table 5-4. RX Special Setting Register (0x0A)
Bit
Function
B7
Bandpass from 110 kHz to 570 kHz
B6
Bandpass from 200 kHz to 900 kHz
B5
Bandpass from 450 kHz to 1.5 MHz
B4
Bandpass from 100 kHz to 1.5 MHz
B3
00 = no gain reduction
01 = gain reduction for 5 dB
B2
10 = gain reduction for 10 dB
11 = gain reduction for 15 dB
B1
0 = 5 times minimum digitizing level
1 = 3 times minimum digitizing level
B0
0 = AGC freeze after 16 subcarrier edges
1 = AGC always on during receive
Comments
Appropriate for any 212-kHz subcarrier systems (for example, FeliCa)
Appropriate for 424-kHz subcarrier systems (for example, used in
ISO15693)
Appropriate for Manchester-coded 106-kbps 848-kHz subcarrier systems
(for example, used in ISO14443A)
Appropriate for highest bit rate (848 kbps) used in high-bit-rate ISO14443B.
Gain is reduced by 7 dB.
Sets the RX digital gain reduction (changing the window of the digitizing
comparator)
AGC activation level change. From 5 times the minimum RX digitizing level
to 3 times the minimum digitizing level. The minimum RX digitizing level can
be adjusted by B2 and B3 (gain reduction)
AGC action is not limited in time or to the start of receive. AGC action can
be done any time during receive process. The AGC can only increase and,
therefore, clips on the peak RX level during the enable period. AGC level is
reset automatically at the beginning of each receive start frame.
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Detailed System Description
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