0
1
2
3
4
5
6
7
0
0.25
0.5
0.75
1
1.25
1.5
1.75
2
2.25
2.5
2.75
3
3.25
3.5
3.75
4
4.25
Input RF Carrier Level in V [V]
PP
RSSI Levels and Oscillator Status Register value (0x0F)
SLOS787H – MAY 2012 – REVISED APRIL 2014
6.5.1
Received Signal Strength Indicator (RSSI)
The TRF7964A incorporates in total three independent RSSI building blocks: Internal Main RSSI, Internal
Auxiliary RSSI, and External RSSI. The internal RSSI blocks are measuring the amplitude of the
subcarrier signal; the External RSSI block measures the amplitude of the RF carrier signal at the receiver
input.
6.5.1.1
Internal RSSI – Main and Auxiliary Receivers
Each receiver path has its own RSSI block to measure the envelope of the demodulated RF signal
(subcarrier). Internal Main RSSI and Internal Auxiliary RSSI are identical however connected to different
RF input pins. The Internal RSSI is intended for diagnostic purposes to set the correct RX path conditions.
The Internal RSSI values can be used to adjust the RX gain settings or decide which RX path (Main or
Auxiliary) provides the greater amplitude and hence to decide if the MUX may need to be reprogrammed
to swap the RX input signal. The measuring system latches the peak value, so the RSSI level can be read
after the end of each receive packet. The RSSI register values are reset with every transmission (TX) by
the reader. This ensures an updated RSSI measurement for each new tag response.
The Internal RSSI has 7 steps (3 bit) with a typical increment of approximately 4 dB. The operating range
is between 600 mV
PP
and 4.2 V
PP
with a typical step size of approximately 600 mV. Both Internal Main
and Internal Auxiliary RSSI values are stored in the RSSI Levels and Oscillator Status register (0x0F). The
nominal relationship between the input RF peak level and the RSSI value is shown in
.
Figure 6-5. Digital Internal RSSI (Main and Auxiliary) Value vs RF Input Level in V
PP
(V)
This RSSI measurement is done during the communication to the Tag; this means the TX must be on. Bit
1 in the Chip Status Control register (0x00) defines if Internal RSSI or the External RSSI value is stored in
the RSSI Levels and Oscillator Status register (0x0F). Direct command 0x18 is used to trigger an Internal
RSSI measurement.
6.5.1.2
External RSSI
The External RSSI is mainly used for test and diagnostic to sense the amplitude of any 13.56-MHz signal
at the receivers RX_IN1 input. The External RSSI measurement is typically done in active mode when the
receiver is on but transmitter output is off. The level of the RF signal received at the antenna is measured
and stored in the RSSI Levels and Oscillator Status register 0x0F. The relationship between the voltage at
the RX_IN1 input and the 3-bit code is shown in
Copyright © 2012–2014, Texas Instruments Incorporated
Detailed Description
23
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