ATA5811_06 ATMEL [ATMEL Corporation], ATA5811_06 Datasheet - Page 22

• Manufacturer Part Number: ATA5811_06
• Description: UHF ASK/FSK TRANSCEIVER
• Manufacturer: ATMEL [ATMEL Corporation]

5.15
5.16
22
Matching Network in TX Mode
Matching Network in RX Mode
ATA5811/ATA5812
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 resistor, the closed switch can be approximated according to
with the series connection of 1.6 nH and 5 in this mode. To have a parallel resonant high
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 impedance in RX mode at the end of this transmission line C
also 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
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 this only negligiblably 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
seen from the loop antenna into 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 5-11.
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.
9
8
has to be taken into account when choosing the values of C
and C
log(1 + 890/3700) = 0.95 dB compared to the optimum matched loop antenna without L
433.92
9
Frequency
868.3
2
315
has an impedance of about 50
with 120 nH and Q
Impedance RF_OUT Pin in RX Mode
MHz
MHz
MHz
2
2
, C
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 5-11 on page
10
2
, C
Figure 3-1 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
8
and C
Z(RF_OUT)RX
2.8 – j 141
36 – j 502
19 – j 366
9
which is then higher as needed for 50 transforma-
9
locking from the transmission line into the loop
connected (using a C
9
to GND. The antenna connection between
22. This together with the losses of the
7, a high Q wire wound inductor with a
11
and L
9
without the added 7.6 pF
1
Table 5-10 on page 21
so that the impedance
7 k
7 k
7 k
R
P
//C
1.0 pF
1.0 pF
1.3 pF
P
4689F–RKE–08/06
7
has to be
2