U3742BM Datasheet, PDF(4/32 Page) ATMEL Corporation

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U3742BM Datasheet, PDF (4/32 Pages) ATMEL Corporation – UHF ASK/FSK RECEIVER

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Pin Description (Continued)

Symbol Function

Selecting 433.92 MHz/315 MHz

MODE Low: 4.90625 MHz (USA)

High: 6.76438 (Europe)

RSSI

Output of the RSSI amplifier

TEST Test pin, during operation at GND

Enables the polling mode

ENABLE Low: polling mode off (sleep mode)

High: polling mode on (active mode)

DATA

Data output/configuration input

RF Front End

The RF front end of the receiver is a heterodyne configuration that converts the input

signal into a 1 MHz IF signal. According to Figure 2 on page 2, the front end consists of

an LNA (low noise amplifier), LO (local oscillator), a mixer and an RF amplifier.

The LO generates the carrier frequency for the mixer via a PLL synthesizer. The XTO

(crystal oscillator) generates the reference frequency fXTO. The VCO (voltage-controlled

oscillator) generates the drive voltage frequency fLO for the mixer. fLO is dependent on

the voltage at pin LF. fLO is divided by a factor of 64. The divided frequency is compared

to fXTO by the phase frequency detector. The current output of the phase frequency

detector is connected to a passive loop filter and thereby generates the control voltage

VLF for the VCO. By means of that configuration, VLF is controlled in a way that fLO/64 is

equal to fXTO. If fLO is determined, fXTO can be calculated using the following formula:

fXTO = fLO/64

The XTO is a one-pin oscillator that operates at the series resonance of the quartz crys-

tal. According to Figure 4, the crystal should be connected to GND via the capacitor CL.

The value of that capacitor is recommended by the crystal supplier. The value of CL

should be optimized for the individual board layout to achieve the exact value of fXTO and

hereby of fLO. When designing the system in terms of receiving bandwidth, the accuracy

of the crystal and the XTO must be considered.

Figure 4. PLL Peripherals

DVCC

XTO

LFGND

R1 = 820 â¦

C9 = 4.7 nF

C10 = 1 nF

LFVCC

C10

4 U3742BM

4735AâRKEâ11/03

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