ATA5423-PLSW datasheet(24/100 Pages) ATMEL

24 / 100 page

4841D–WIRE–10/07

ATA5423/ATA5425/ATA5428/ATA5429

3.15

Matching Network in TX Mode

In TX mode the 20 mm long and 0.4 mm wide transmission line which is much shorter than

λ/4 is

approximately switched in parallel to the capacitor C

9 to GND. The antenna connection between

8 and C9 has an impedance of about 50Ω locking from the transmission line into the loop

antenna with pin RF_OUT, L

2, C10, C8 and C9 connected (using a C9 without the added 7.6 pF

as discussed later). The transmission line can be approximated with a 16 nH inductor in series

with a 1.5

Ω resistor, the closed switch can be approximated according to Table 3-10 on page 23

with the series connection of 1.6 nH and 5

Ω 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

9 which is then higher, as needed for 50Ω transforma-

tion). To keep the 50

Ω impedance in RX mode at the end of this transmission line, C

7 also has 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.

3.16

Matching Network in RX Mode

In RX mode the RF_OUT pin has a high impedance of about 7 k

Ω in parallel with 1.0 pF at

433.92 MHz as can be seen in Table 3-11. This, together with the losses of the inductor L

2 with

120 nH and Q

L2 = 25, gives about 3.7 kΩ loss impedance at RF_OUT. Since the optimum load

impedance in TX mode for the power amplifier at RF_OUT is 890

Ω the loss associated with the

inductor L

2 and the RF_OUT pin can be estimated to be 10 × log(1 + 890/3700) = 0.95 dB com-

pared to the optimum matched loop antenna without L

2 and RF_OUT. The switch represents, in

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

Ω. Since the impedance level at pin RX_TX1 in RX mode is about 50Ω

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

9 have to be taken into account

when choosing the values of C

11 and L1 so that the impedance seen from the loop antenna into

the transmission line with the 7.6 pF capacitor connected is 50

Ω. 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.

Note that if matching to 50

Ω, like in Figure 2.2 on page 8, a high Q wire-wound inductor with a

Q > 70 should be used for L

2 to minimize its contribution to RX losses that will otherwise be

dominant. The RX and TX losses will be in the range of 1.0 dB there.

Table 3-11.

Impedance RF_OUT Pin in RX Mode

Frequency

Z(RF_OUT)RX

RP//CP

315 MHz

Ω – j 502Ω

Ω//1.0 pF

345 MHz

Ω – j 480Ω

Ω//1.0 pF

433.92 MHz

Ω – j 366Ω

Ω//1.0 pF

868.3 MHz

2.8

Ω – j 141Ω

Ω//1.3 pF

915 MHz

2.6

Ω – j 135Ω

Ω//1.3 pF

ATA5423-PLSW Datasheet(PDF) 24 Page - ATMEL Corporation

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