U3742BM Datasheet, PDF(7/32 Page) ATMEL Corporation

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U3742BM Datasheet, PDF (7/32 Pages) ATMEL Corporation – UHF ASK/FSK RECEIVER

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Analog Signal

Processing

IF Amplifier

RSSI Amplifier

Pin RSSI

U3742BM

Please note that for all coupling conditions (see Figure 5 on page 6 and Figure 6 on

page 6), the bond wire inductivity of the LNA ground is compensated. C3 forms a series

resonance circuit together with the bond wire. L = 25 nH is a feed inductor to establish a

DC path. Its value is not critical but must be large enough not to detune the series reso-

nance circuit. For cost reduction, this inductor can be easily printed on the PCB. This

configuration improves the sensitivity of the receiver by about 1 dB to 2 dB.

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 on page 6 to

determine the center frequency.

The receiver U3742BM - M3 employs an IF bandwidth of BIF = 600 kHz and can be used

together with the U2741B in FSK and ASK mode.

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 RSense. RSense 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.

The output voltage of the RSSI amplifier (VRSSI) is available at pin RSSI. Using the RSSI

output signal, the signal strength of different transmitters can be distinguished. The

usable input-power range PRef is -100 dBm to -55 dBm. The temperature coefficient TC

of VRSSI is typically -2.2 mV/K. Due to TC and gain tolerance, it is not possible to find out

the absolute level of each transmitter, but the level differences can be used to distin-

guish several transmitters. As illustrated in Figure 8 on page 8, the RSSI output voltage

is not constant over the temperature range. Figure 7 on page 8 illustrates an application

that realizes a temperature compensation of VRSSI.

4735AâRKEâ11/03

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