ATA5811 datasheet(21/90 Pages) ATMEL

21 / 90 page

ATA5811/ATA5812 [Preliminary]

4689B–RKE–04/04

Figure 17. Equivalent Circuit of the Switch

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 C

8 and C9 has an impedance of about 50 Ω locking from the trans-

mission line into the loop antenna with pin RF_OUT, L

2, C10, C8 and C9 connected

(using a C

9 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 10 on page 20 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 transmission 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 50 Ω transformation). To

keep the 50

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

7 has to be

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.

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 11 on page 22. This together with the losses of

the inductor L2 with 120 nH and QL2 = 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 compared to the optimum matched

loop antenna without L2 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

Ω. Since the impedance level at pin RX_TX1 in RX mode is about 50 Ω 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 C9 has to be taken

into account when choosing the values of C11 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.

1.6 nH

RX_TX1

2.5 pF

Ω

ATA5811 Datasheet(PDF) 21 Page - ATMEL Corporation

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