T5760 Ver la hoja de datos (PDF) - Atmel Corporation

Número de pieza

componentes Descripción

Fabricante

T5760

UHF ASK/FSK Receiver

Atmel Corporation 

Block Diagram

CDEM

SENS

AVCC

AGND

DGND

DVCC

T5760 / T5761

FSK/ASK–

demodulator

and data filter

Dem_out

Data –

interface

Rssi

Limiter out

RSSI IF

Amp.

4. Order

f0=950 kHz/

1 MHz

Sensitivity–

reduction

Polling circuit

and

control logic

FE

CLK

LPF

fg=2.2MHz

Standby logic

DATA

POLLING/_ON

DATA_CLK

IC_ACTIVE

IF

Amp.

Loop–

filter

Poly–LPF

fg=7MHz

LC–VCO

XTO

XTAL

LNAREF

LNA_IN LNA

LNAGND

f

:2

f

:256

Figure 3. Block diagram

RF Front End

The RF front end of the receiver is a low-IF heterodyne

configuration that converts the input signal into a

950-kHz/ 1-MHz IF signal with an image rejection of typ-

ical 30dB. According to figure 3 the front end consists of

an LNA (low noise amplifier), LO (local oscillator), I/Q

mixer, polyphase lowpass filter and an IF amplifier.

The PLL generates the carrier frequency for the mixer via

a full integrated synthesizer with integrated low noise

LC-VCO (voltage controlled oscillator ) and PLL-loop-

filter. The XTO ( crystal oscillator ) generates the

reference frequency fXTO. The integrated LC-VCO gen-

erates two times the mixer drive frequency fVCO. The I/Q

signals for the mixer are generated with a divide by two

circuit ( fLO = fVCO/2 ). fVCO is divided by a factor of 256

and feed into a phase frequency detector and compared

with fXTO. The output of the phase frequency detector is

feed into an integrated loopfilter and thereby generates

the control voltage for the VCO. If fLO is determined,

fXTO can be calculated using the following formula:

fXTO = fLO / 128

The XTO is a one-pin oscillator that operates at the series

resonance of the quartz crystal with high current but low

voltage signal, so that there is only a small voltage at the

crystal oscillator frequency at Pin XTAL. According to

figure 4, the crystal should be connected to GND with a

series capacitor CL. The value of that capacitor is recom-

mended by the crystal supplier. Due to a somewhat

inductive impedance at steady state oscillation and some

PCB parasitics a lower value of CL is normally necessary.

Rev. A2, 19-Oct-00

3 (32)

Preliminary Information

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