3

T5744

4521B–RKE–01/03

Figure 3. Block Diagram

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 3, the front end consists of an LNA

(Low-Noise Amplifier), LO (Local Oscillator), a mixer and RF amplifier.

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

(crystal oscillator) generates the reference frequency f

Oscillator) generates the drive voltage frequency f

the voltage at Pin LF. f

f

tor 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 f

equal to f

f

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 a 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 f

hereby of f

of the crystal and the XTO must be considered.

ASK-

Demodulator

and data filter

RSSI IF Amp

IF Amp

4. Order

LPF

3 MHz

LPF

3 MHz

Dem_out

RSSI

Standby logic

VCO

XTO

64

f

CDEM

AVCC

RSSI

AGND

DGND

MIXVCC

LNAGND

LNA_IN

DATA

ENABLE

TEST

MODE

LFGND

LFVCC

XTO

LF

DVCC

LNA

Data interface

Test

BR_0

BR_1