ATA5423 ATMEL Corporation, ATA5423 Datasheet - Page 24

• Manufacturer Part Number: ATA5423
• Manufacturer: ATMEL Corporation

3.15
3.16
24
Matching Network in TX Mode
Matching Network in RX Mode
ATA5423/25/28/29 [Preliminary]
In TX mode the 20 mm long and 0.4 mm wide transmission line which is much shorter than /4 is
approximately switched in parallel to the capacitor C
C
antenna with pin RF_OUT, L
as discussed later). The transmission line can be approximated with a 16 nH inductor in series
with a 1.5
with the series connection of 1.6 nH and 5
impedance circuit with little RF power going into it looking from the loop antenna into the trans-
mission line a capacitor of about 7.6 pF to GND is needed at the beginning of the transmission
line (this capacitor is later absorbed into C
tion). To keep the 50
to be about 7.6 pF. This reduces the TX power by about 0.5 dB at 433.92 MHz compared to the
case the where the LNA path is completely disconnected.
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
120 nH and Q
impedance in TX mode for the power amplifier at RF_OUT is 890
inductor L
pared to the optimum matched loop antenna without L
this mode at 433.92 MHz, approximately an inductor of 1.6 nH in series with the parallel connec-
tion of 2.5 pF and 2.0 k . Since the impedance level at pin RX_TX1 in RX mode is about 50
this only negligibly dampens 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
when choosing the values of C
the transmission line with the 7.6 pF capacitor connected 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 3-11.
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.
8
and C
433.92
9
Frequency
868.3
2
315
345
915
has an impedance of about 50
and the RF_OUT pin can be estimated to be 10
resistor, the closed switch can be approximated according to
L2
Impedance RF_OUT Pin in RX Mode
MHz
MHz
MHz
MHz
MHz
= 25, gives about 3.7 k loss impedance at RF_OUT. Since the optimum load
impedance in RX mode at the end of this transmission line, C
2
2
, C
to minimize its contribution to RX losses that will otherwise be
11
Table
10
and L
, C
Figure 2.2 on page
3-11. This, together with the losses of the inductor L
8
1
and C
Z(RF_OUT)RX
(2.8 – j141)
so that the impedance seen from the loop antenna into
(36 – j502)
(19 – j366)
9
which is then higher, as needed for 50
TBD
TBD
9
locking from the transmission line into the loop
connected (using a C
in this mode. To have a parallel resonant high
9
to GND. The antenna connection between
2
and RF_OUT. The switch represents, in
8, a high Q wire–wound inductor with a
log(1 + 890/3700) = 0.95 dB com-
9
have to be taken into account
the loss associated with the
9
without the added 7.6 pF
Table 3-10 on page 23
7 k //1.0 pF
7 k //1.0 pF
7 k //1.3 pF
R
TBD
TBD
P
//C
P
4841A–RKE–02/05
transforma-
7
also has
2
with