ATA5823-PLQW 80 Atmel, ATA5823-PLQW 80 Datasheet - Page 25

• Manufacturer Part Number: ATA5823-PLQW 80
• Manufacturer: Atmel

Specifications of ATA5823-PLQW 80

• Operating Temperature (min): -40C
• Operating Temperature (max): 105C
• Operating Temperature Classification: Industrial
• Product Depth (mm): 7mm
• Product Height (mm): 0.9mm
• Product Length (mm): 7mm
• Operating Supply Voltage (typ): 2.5/3.3V
• Operating Supply Voltage (max): 3.6V
• Lead Free Status / Rohs Status: Compliant

7.17
8. RF Transceiver in Full-duplex Mode
4829D–RKE–06/06
Matching Network in RX Mode
In RX mode the RF_OUT pin has a high impedance of about 7 k in parallel with 1.0 pF at
433.92 MHz as can be seen in
inductor L
optimum load impedance in TX mode for the power amplifier at RF_OUT is 890 the loss asso-
c i a t e d w i t h t h e i n d u c t o r L
10
and RF_OUT. The switch represents, in this mode at 433.92 MHz, about an inductor of 1.6 nH in
series with the parallel connection of 2.5 pF and 2.0 k . Since the impedance level at pin
RX_TX1 in RX mode is about 50 there is only a negligible damping of the received signal by
about 0.1 dB. When matching the LNA to the loop antenna the transmission line and the 7.6 pF
part of C
impedance seen from the loop antenna into the transmission line with the 7.6 pF capacitor con-
nected is 50 .
Since the loop antenna in RX mode is loaded by the LNA input impedance the loaded Q of the
loop antenna is lowered by about a factor of 2 in RX mode hence the antenna bandwidth is
higher than in TX mode.
.
Table 7-11.
Note that if matching to 50 , like in
Q > 70 should be used for L
dominant. The RX and TX losses will be in the range of 1.0 dB there.
The full-duplex mode of the ATA5823/ATA5824 is intended to be used for the purpose of secu-
rity against a so called relay attack in passive entry systems. A property of such a passive entry
system is that the user has not to push a key fob button like in a keyless entry system. If the user
approaches to the door of the car it will wake up the key (in most cases with a low frequency
125 kHz signal) and the communication between the key and the car starts without interaction of
the user and afterwards the door opens.
Due to this new feature of the system there is a new possibility of entering a car without permis-
sion. One can trigger this communication, take the 125 kHz signal from the car, remodulate it on
another carrier and transmit it over a much longer distance than intended by the system. Than
receive this signal and remodulate it onto the 125 kHz carrier and retransmit this signal close to
the user having the key fob with permission for the car. Such a system is called an RF-Relay and
therefore this kind of attack is called relay attack. The high frequency signals of the
ATA5823/ATA5824 could be treated the same way if only a half-duplex mode is used within a
passive entry system. If using half-duplex RF transceivers, the attacker can switch the direction
of the relay with a transmit power detector.
log(1 + 890/3700) = 0.95 dB compared to the optimum matched loop antenna without L
9
433.92
Frequency
868.3
2
315
has to be taken into account when choosing the values of C
with 120 nH and Q
Impedance RF_OUT Pin in RX mode
MHz
MHz
MHz
2
L2
to minimize its contribution to RX losses which will otherwise be
= 25 gives about 3.7 k loss impedance at RF_OUT. Since the
Table 7-11 on page
2
Figure 4-1 on page
a n d t h e R F _ O U T p i n c a n b e e s t i m a t e d t o b e
Z(RF_OUT)RX
36
19
2.8
j 502
j 366
j 141
25. This together with the losses of the
7, a high Q wire wound inductor with a
ATA5823/ATA5824
7 k
7 k
7 k
11
R
and L
P
//C
1.0 pF
1.0 pF
1.3 pF
P
1
so that the
25
2