T5761 ATMEL [ATMEL Corporation], T5761 Datasheet - Page 3

• Manufacturer Part Number: T5761
• Description: UHF ASK/FSK Receiver
• Manufacturer: ATMEL [ATMEL Corporation]

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 f
erates two times the mixer drive frequency f
signals for the mixer are generated with a divide by two
circuit ( f
and feed into a phase frequency detector and compared
Rev. A2, 19-Oct-00
LO
LNAGND
LNAREF
LNA_IN
= f
CDEM
AGND
DGND
DVCC
AVCC
SENS
VCO
/2 ). f
XTO
VCO
LNA
. The integrated LC-VCO gen-
is divided by a factor of 256
Rssi
and data filter
f0=950 kHz/
fg=2.2MHz
Poly–LPF
fg=7MHz
demodulator
FSK/ASK–
4. Order
Preliminary Information
LPF
1 MHz
RSSI IF
Amp.
IF
Amp.
Limiter out
VCO
Figure 3. Block diagram
. The I/Q
f
:2
Sensitivity–
reduction
Dem_out
with f
feed into an integrated loopfilter and thereby generates
the control voltage for the VCO. If f
f
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 C
mended by the crystal supplier. Due to a somewhat
inductive impedance at steady state oscillation and some
PCB parasitics a lower value of C
XTO
LC–VCO
f
Standby logic
Loop–
filter
:256
can be calculated using the following formula:
XTO
f
Polling circuit
control logic
FE
XTO
. The output of the phase frequency detector is
and
= f
interface
Data –
LO
XTO
CLK
L
/ 128
. The value of that capacitor is recom-
T5760 /
IC_ACTIVE
DATA
POLLING/_ON
DATA_CLK
XTAL
L
is normally necessary.
LO
T5761
is determined,
3 (32)