XE1202AI027TRLF Datasheet XE1202AI027TRLF | P10-P12

XE1202A TrueRF™

4.1.3 RSSI

When enabled, this function provides an RSSI (Received Signal Strength Indication) based on the signal at the

output of the base-band filter. To enable the RSSI function, the bit “RTParam_RSSI” should be set to “1” (see the

Configuration register section below). When enabled, the status of the RSSI is a 2-bit word stored in register

“DataOut_RSSI”, which can be read via the serial control interface. The contents of the register are defined in

Table 6 below, where V

RFFIL

is the differential amplitude of the equivalent input RF signal when the receiver is

operated in A-mode. The thresholds VTHRi are the thresholds at the output of the base-band filter divided by the

gain between the input of the receiver and this output.

DataOut_RSSI Description

0 0

RFFIL

≤ VTHR1

0 1

VTHR1 < V

RFFIL

≤ VTHR2

1 0

VTHR2 < V

RFFIL

≤ VTHR3

1 1

VTHR3 < V

RFFIL

Table 6: RSSI status description

Two ranges, each of three VTHRi thresholds are defined and selected via the setting of the register

“RTParam_RSSR”, to provide an overall RSSI range of typically 25 dB.

The timing diagram of an RSSI measurement is illustrated by Figure 3 below. When the RSSI function has been

activated the signal strength is periodically measured and the result is stored in the register “DataOut_RSSI” at

each rising edge of DATAIN. TS_RS is the wake-up time required after the function has been enabled to ensure

that a valid reading of RSSI is obtained. For a proper operation, the pulse length on DATAIN has to be higher than

8µs.

xxx

val1 val2 val3 val4 0

val1 val4xxx

TS_RS

datain

RSSI_out

(internal signal)

/en

RTParam_RSSI

DtaOut_RSSI

TS_RSM

Figure 3: RSSI measurement timing diagram

For applications where a valid RSSI reading is required within as short a time frame as possible, enabling the RSSI

during receiver mode 010 instead of 100 (see the definition of TS_RS in Table 4) allows a valid RSSI within 1 ms of

valid data being received.

4.1.4 Frequency Error Indicator - FEI

When enabled, this function provides an indication of the frequency error of the local oscillator compared with the

received carrier frequency. For guaranteed operation of the FEI function, the following two conditions should be

satisfied.

XE1202A TrueRF™

The modulation index, b, of the modulated carrier should meet the following condition:

≥=

∆⋅

where ∆f is the frequency deviation and BR is the bit rate.

The bandwidth of the baseband filter (BBW) must be greater than the sum of the frequency offset and the received

signal bandwith, as defined below:

BBW > f

OFFSET

+ BW

SIGNAL

where BBW is the baseband filter bandwidth defined by the RTParam_BW parameter (see the Configuration

Registers section below), f

OFFSET

is the difference between the carrier frequency and the LO frequency, and

SIGNAL

is equal to

⎟

⎠

⎞

⎜

⎝

⎛

∆+ f

The FEI function has two modes of operation, defined by the value set in the register “RTParam_Fsel” (see the

Configuration register section below).

4.1.4.1 “RTParam_Fsel” = 1

With the “RTParam_FEI” bit set to “1” and the “RTParam_Fsel” bit set to “1”, the FEI uses frequency correlation to

provide a 2-bit status word, which is stored in the register “DataOut_FEI”. The contents of this register are defined

below in Table 7. The status of this register is provided in the following table, where f

is the internal local oscillator

frequency, and f

is the carrier frequency of the received signal.

DataOut_FEI Meaning

0 0

⏐f

-f

⏐ ≤ f

ERR

0 1

1 0

-f

) > f

ERR

1 1

-f

) < -f

ERR

Table 7: FEI status description

The value f

ERR

= FERR * BR, where BR is the bit rate and FERR is a ratio given in the electrical specifications. As

an example, for a bit rate of 4.8kbits/s and with FERR = 0.5, f

ERR

is 2.4 kHz.

The FEI-Correlator function works properly only if the input signal is the preamble sequence defined under the

Receiver section above, and if the frequency error to be detected is lower than 20 kHz.

The time diagram of an FEI measurement is similar to that of an RSSI measurement, and is illustrated in Figure 4

below. When the FEI is enabled, the frequency error is periodically measured and the result is stored in the register

“DataOut_FEI” at each rising edge of DATAIN. TS_FE is the wake-up time required after the function has been

enabled to obtain a valid result. For a proper operation, the pulse length on DATAIN has to be higher than 8µs.

XE1202A TrueRF™

4.1.4.2 “RTParam_Fsel” = 0

With the “RTParam_FEI” bit set to “1” and the RTParam_Fsel” bit set to “0”, the FEI function uses over sampling of

the signal at the output of the demodulator. When activated by the rising edge of DATAIN, this function provides an

8-bit word equivalent to the duty cycle of the demodulated preamble, stored in register “DataOut_FEI”.

Each sample is used to control an up/down counter, when the sample is "1" the content of the counter is

incremented, when the sample is "0" the content of the counter is decremented. As a consequence, the final 8-bit

value of the counter stored in “DataOut_FEI” gives an indication of the duty cycle of the demodulated signal. The

range of stored values is from -128 and +127. The further from 0 the value of DataOut_FEI, the higher the error on

the LO frequency. If the stored value in “DataOut_FEI” is typically zero, then the duty cycle of the preamble is about

50 %, and the LO frequency is nominally correct.

Since this FEI uses the signal before the bit synchronizer, its value can vary from one measurement to another,

due to the presence of jitter and glitches in the signal. If possible, it is advised to make 4-5 measurements and take

the average value.

The timing diagram for this FEI measurement is illustrated in Figure 5 below. The FEI function is activated at the

rising edge of the /EN signal when the RTParam_FEI bit is set to “1”. Then, the internal FEI counter is activated at

the rising edge of DATAIN. After a period TS_FE equal to the duration of 4 bits (see Electrical Specifications), the

counter is stopped and the contents are stored in the register DataOut_FEI. For a proper operation, the pulse

length on DATAIN has to be higher than 8µs.

The maximum delay between the rising edge of DATAIN and the first clock on the internal FEI counter is 1/(16*BR),

where BR is the bit rate.

XX VAL1 VAL2 VAL3 VAL4 XX

XX VAL2 VAL4

RTParam_FEI

fei_out

datain

DataOut_FEI

TS_FE

XX VAL1 VAL2 VAL3 VAL4 XX

XX VAL2 VAL4

RTParam_FEI

fei_out

datain

DataOut_FEI

TS_FE

Figure 4: FEI measurement timing diagram when “RTParam_Fsel” = 1

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XE1202AI027TRLF

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RF Transceiver XE1202AI027TRLF

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