ATA5760 ATMEL Corporation, ATA5760 Datasheet - Page 4

• Manufacturer Part Number: ATA5760
• Description: Ata5760 Uhf Ask/fsk Receiver
• Manufacturer: ATMEL Corporation

3. RF Front End
4
ATA5760/ATA5761
The RF front end of the receiver is a low-IF heterodyne configuration that converts the input sig-
nal into an about 1 MHz IF signal with an image rejection of typical 30 dB. According to
2-1 on page 3
mixer, polyphase lowpass filter and an IF amplifier.
The PLL generates the carrier frequency for the mixer via a full integrated synthesizer with inte-
grated low noise LC-VCO (Voltage Controlled Oscillator) and PLL-loop filter. The XTO (crystal
oscillator) generates the reference frequency f
the mixer drive frequency f
circuit (f
and compared with f
loop filter and thereby generates the control voltage for the VCO. If f
calculated using the following formula:
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
series capacitor C
somewhat inductive impedance at steady state oscillation and some PCB parasitics a lower
value of C
The value of C
f
for this capacitor) and hereby of f
and local oscillator accuracy, the accuracy of the crystal and the XTO must be considered.
If a crystal with ±30 ppm adjustment tolerance at 25°C, ±50 ppm over temperature –40°C to
+105°C, ±10 ppm of total aging and a CM (motional capacitance) of 7 fF is used, an additional
XTO pulling of ±30 ppm has to be added.
The resulting total LO tolerance of ±120 ppm agrees with the receiving bandwidth specification
of the 600 kHz version of ATA5760/ATA5761 if the T5750 has also a total LO tolerance of
±120 ppm.
For the ATA5760N3 crystals with ±55 ppm total tolerance are needed for receiver and transmit-
ter to cope with the reduced IF-bandwidth.
Figure 3-1.
XTO
XTO
= f
(the best way is to use a crystal with known load resonance frequency to find the right value
LO
LO
/128
L
= f
is normally necessary.
VCO
L
XTO Peripherals
the front end consists of an LNA (Low Noise Amplifier), LO (Local Oscillator), I/Q
should be optimized for the individual board layout to achieve the exact value of
/2). f
L
. The value of that capacitor is recommended by the crystal supplier. Due to a
XTO
VCO
. The output of the phase frequency detector is fed into an integrated
is divided by a factor of 256 and feeds into a phase frequency detector
VCO
. The I/Q signals for the mixer are generated with a divide by two
LO
. When designing the system in terms of receiving bandwidth
Figure
TEST 3
TEST 2
DVCC
XTAL
NC
3-1, the crystal should be connected to GND with a
XTO
V
S
. The integrated LC-VCO generates two times
C
L
LO
is determined, f
4896C–RKE–04/06
XTO
can be
Figure