Rainbow Electronics ATA5812 - Manual (Page 6)

6
ATA5811/ATA5812 [Preliminary]
4689B–RKE–04/04
Typical Key Fob or Sensor Application with 1 Battery
Figure 4. Typical RKE Key Fob or Sensor Application, 433.92 MHz, 1 Battery
Figure 4 shows a typical 433.92 MHz RKE key fob or sensor application with one battery
The external components are 11 capacitors, 1 resistor, 2 inductors and a crystal. C
1
to
C
4
are 68 nF voltage supply blocking capacitors. C
5
is a 10 nF supply blocking capaci-
tor. C
6
is a 15 nF fixed capacitor used for the internal quasi peak detector and for the
highpass frequency of the data filter. C
7
to C
11
are RF matching capacitors in the range
of 1 pF to 33 pF. L1 is a matching inductor of about 5.6 nH to 56 nH. L
2
is a feed induc-
tor of about 120 nH. A load capacitor of 9 pF for the crystal is integrated. R
1
is typically
22 k
Ω
and sets the output power to about 5.5 dBm. The loop antenna’s quality factor is
somewhat reduced by this application due to the quality factor of L
2
and the RX/TX
switch. On the other hand, this lower quality factor is necessary to have a robust design
with a bandwidth that is broad enough for production tolerances. Due to the single-
ended and ground-referenced design, the loop antenna can be a free-form wire around
the application as it is usually employed in RKE uni-directional systems. The
ATA5811/ATA5812 provides sufficient isolation and robust pulling behavior of internal
circuits from the supply voltage as well as an integrated VCO inductor to allow this.
Since the efficiency of a loop antenna is proportional to the square of the surrounded
area it is beneficial to have a large loop around the application board with a lower quality
factor to relax the tolerance specification of the RF components and to get a high
antenna efficiency in spite of their lower quality factor.
AVCC
RX
_
T
X
1
RX
_
T
X
2
DV
CC
VS2 VS1
VSINT
VAUX
PWR
_
O
N
N_RESET
XT
AL
1
XTAL2
CLK
T
EST
1
DEM_OUT
T1 T2 T3 T4 T5
IRQ
RSSI
CDEM
CS
SCK
SDI_TMDI
SDO_TMDO
VSO
UT
NC
RX_
A
CT
IVE
RF_IN
NC
NC
R_PWR
RF_OUT
433_N868
PWR_H
NC
NC
NC
NC
NC
NC NC T
EST
2
NC
NC NC
VCC
VSS
AVCC
20 mm x 0.4 m
m
Loop antenna
Microcontroller
ATA5811/ATA5812
Litihum-
Cell
C
1
C
2
C
9
C
8
C
10
C
5
C
11 C
6
C
3
+
L
1
L
2
C
4
R
1
13.25311 MHz
C
7