AT86RF211DAI-R datasheet(10/48 Pages) ATMEL

10 / 48 page

AT86RF211

1942C–WIRE–06/02

Overview and Choice of

Intermediate Frequencies

For selectivity and flexibility purpose, a classical and robust 2 IF superheterodyne archi-

tecture has been selected for the AT86RF211. In order to minimize the external

components cost, the most popular IF values have been chosen. The impedances of the

input/output of the mixing stages have been internally matched to the most usual

ceramic filter impedances.

Two typical IF values are suggested:

•

10.7 MHz is the most popular option.

•

21.4 MHz: the image frequency is far enough from the carrier frequency to enable

the use of a front-end ceramic filter instead of a SAW filter. It is also noticeable that

21.4 MHz quartz filters usually have more abrupt slopes than 10.7 MHz ceramic

filters.

Rx - Tx Switch

A SPST switch is integrated. In the transmission mode, it protects the LNA input from

the large voltage swings of the PA output (up to several volts peak-to-peak), which is

switched to a high impedance state. It is automatically turned ON or OFF by the RX/TX

control bit. The insertion loss is about 2 dB and the reverse isolation about 30 dB in a

300

Ω environment.

Image Rejection and RF Filter

The immunity of the AT86RF211 can be improved with an external band-pass filter.

For example, when using a SAW Filter, this device must be matched with the LNA input

and the switch output. The following scheme gives the typical implementation for an

868 MHz application with a 50

Ω/50Ω SAW filter.

Figure 9. Typical 50

Ω SAW Filter Implementation in the 868 MHz Band

See Table 2 for precise matching information.

The SAW filter can be replaced by a TEM ceramic, helicoidal or a ceramic coax

λ/4 res-

onator designed as a narrow band-pass filter. For instance, with an IF selected at

10.7 MHz, a -3 dB bandwidth of 5 MHz, with an insertion loss of 1 dB and an image

rejection of 12 dB can be achieved with the following:

RXIN

SPST Switch

Ω

12 nH

2.2 nH

SWOUT (pin 48)

(pin 45)

SAW

These inductors can be printed

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