T5743 Просмотр технического описания (PDF) - Atmel Corporation

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T5743

UHF ASK/FSK Receiver

Atmel Corporation 

T5743

Analog Signal Processing

IF Amplifier

RSSI Amplifier

The signals coming from the RF front-end are filtered by the fully integrated 4th-order IF

filter. The IF center frequency is fIF = 1 MHz for applications where fRF = 315 MHz or

fRF = 433.92 MHz is used. For other RF input frequencies refer to Table 1 to determine

the center frequency.

The T5743 is available with two different IF bandwidths. T5743P3, the version with

BIF = 300 kHz, is well suited for ASK systems where Atmel’s PLL transmitter U2741B is

used. The receiver T5743P6 employs an IF bandwidth of BIF = 600 kHz. Both versions

can be used together with the U2741B in ASK and FSK mode. If used in ASK applica-

tions, it allows higher tolerances for the receiver and PLL transmitter crystals. SAW

transmitters exhibit much higher transmit freqeuncy tolerances compared to PLL trans-

mitters. Generally, it is necessary to use BIF = 600 kHz together with such transmitters.

The subsequent RSSI amplifier enhances the output signal of the IF amplifier before it is

fed into the demodulator. The dynamic range of this amplifier is DRRSSI = 60 dB. If the

RSSI amplifier is operated within its linear range, the best S/N ratio is maintained in ASK

mode. If the dynamic range is exceeded by the transmitter signal, the S/N ratio is

defined by the ratio of the maximum RSSI output voltage and the RSSI output voltage

due to a disturber. The dynamic range of the RSSI amplifier is exceeded if the RF input

signal is about 60 dB higher compared to the RF input signal at full sensitivity.

In FSK mode the S/N ratio is not affected by the dynamic range of the RSSI amplifier.

The output voltage of the RSSI amplifier is internally compared to a threshold voltage

VTh_red. VTh_red is determined by the value of the external resistor RSens. RSens is con-

nected between Pin SENS and GND or VS. The output of the comparator is fed into the

digital control logic. By this means it is possible to operate the receiver at a lower

sensitivity.

If RSens is connected to GND, the receiver operates at full sensitivity.

If RSens is connected to VS, the receiver operates at a lower sensitivity. The reduced sen-

sitivity is defined by the value of RSens, the maximum sensitivity by the signal-to-noise

ratio of the LNA input. The reduced sensitivity depends on the signal strength at the out-

put of the RSSI amplifier.

Since different RF input networks may exhibit slightly different values for the LNA gain,

the sensitivity values given in the electrical characteristics refer to a specific input

matching. This matching is illustrated in Figure 6 and exhibits the best possible

sensitivity.

RSens can be connected to VS or GND via a microcontroller. The receiver can be

switched from full sensitivity to reduced sensitivity or vice versa at any time. In polling

mode, the receiver will not wake up if the RF input signal does not exceed the selected

sensitivity. If the receiver is already active, the data stream at Pin DATA will disappear

when the input signal is lower than defined by the reduced sensitivity. Instead of the

data stream, the pattern according to Figure 7 is issued at Pin DATA to indicate that the

receiver is still active (see also figure 34).

Figure 7. Steady L State Limited DATA Output Pattern

DATA

t DATA_min

t DATA_L_max

7

4569A–RKE–12/02

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