ata5724p3c-tkqy ATMEL Corporation, ata5724p3c-tkqy Datasheet - Page 5

• Manufacturer Part Number: ata5724p3c-tkqy
• Description: Uhf Ask/fsk Receiver
• Manufacturer: ATMEL Corporation

3. RF Front-end
9248A–RKE–09/11
The RF front-end of the receiver is a low-IF heterodyne configuration that converts the input
signal into about 1MHz IF signal with a typical image rejection of 30dB. According to Figure
Figure 1-2 on page 3
lator), I/Q mixer, polyphase low-pass filter and an IF amplifier.
The PLL generates the drive frequency f
with integrated low noise LC-VCO (Voltage Controlled Oscillator) and PLL-loop filter. The XTO
(crystal oscillator) generates the reference frequency f
sions) or f
the mixer drive frequency f
or four circuit (f
sions). f
is compared with f
loop filter and thereby generates the control voltage for the VCO. If f
be calculated using the following formula:
f
bands.
The XTO is a two-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 pins XTAL1 and XTAL2. According to
to GND with two capacitors C
these capacitors are recommended by the crystal supplier. Due to an inductive impedance at
steady state oscillation and some PCB parasitics, a lower value of C
necessary.
The value of C
of f
quency to find the right value for this capacitor.) When designing the system in terms of
receiving bandwidth and local oscillator accuracy, the accuracy of the crystal and the XTO
must be considered.
Figure 3-1.
The nominal frequency f
using the following formula (low-side injection):
f
REF
LO
XTO
= f
= f
RF
LO
and hence of f
VCO
– f
/128 for 868MHz band, f
REF
IF
is divided by a factor of 128 or 64 and feeds into a phase frequency detector and
= f
Lx
XTO Peripherals
LO
XTO
should be optimized for the individual board layout to achieve the exact value
Atmel ATA5723C/ATA5724C/ATA5728C
= f
REF
/3 (315MHz version). The integrated LC-VCO generates two or four times
the front-end consists of an LNA (Low Noise Amplifier), LO (Local Oscil-
VCO
. The output of the phase frequency detector is fed into an integrated
LO
LO
. (The best way is to use a crystal with known load resonance fre-
/2 for 868MHz version, f
VCO
is determined by the RF input frequency f
. The I/Q signals for the mixer are generated with a divide by two
L1
and C
REF
TEST3
TEST2
XTAL2
XTAL1
DVCC
= f
L2
LO
LO
from XTAL1 and XTAL2 respectively. The value of
/64 for 433MHz bands, f
for the mixer using a fully integrated synthesizer
V
S
LO
Figure
= f
REF
C
C
VCO
L2
L1
3-1, the crystal should be connected
= f
/4 for 433MHz and 315MHz ver-
XTO
/2 (868MHz and 433MHz ver-
RF
LO
REF
and the IF frequency f
is determined, f
L1
= f
and C
LO
/64 for 315MHz
L2
is normally
XTO
can
IF
5