U2741B datasheet(4/9 Pages) ATMEL | UHF ASK/FSK TRANSMITTER

4 / 9 page

U2741B

4733A–RKE–11/03

General Description

The fully integrated VCO and the “single-ended open-collector” output allow particularly

simple, low-cost RF miniature transmitters to be assembled. The single-ended output

enables a considerably simplified adaptation of both a magnetic loop antenna of any

form or a

l/4 antenna. This is because the load impedance must not be balanced as

would be the case with a differential output.

The XTO's frequency can be selected at either 13.56 MHz (USA 9.844 MHz) or

6.78 MHz (USA 4.9219 MHz). Thus, it is possible to use not only exceptionally small

SMD crystals at 13.56 MHz but also very low-cost 6.78 MHz crystals in a wired metal

package (e.g., in the HC49S housing). The frequency is selected by connecting pin 16

(DIVC) to either GND or V

At high frequencies, crystals have a very fast start-up time (< 1.5 ms at 13.56 MHz,

< 3 ms at 6.78 MHz), whereby a wait time of 5 to 10 ms is required until the transmitter

IC is locked. This means that the processor does not need to poll a lock detect output.

Functional

Description

The IC can be switched on at both the FSK and the ASK input. The IC's ChipSelect is

performed by the logical OR operation of ASK and FSK input. In the case of V

FSK,

ASK £ 0.3 V, the power-down supply current is ISoff < 0.35 µA. The ASK input activates

the power amplifier and the PLL. The FSK input only activates the PLL and, if capacitor

3 is installed, pulls the crystal to the lower frequency, whereby the transmitter is FSK

modulated. After switching on at FSK, the VCO locks onto the 32 or 64 times higher fre-

quency of the crystal oscillator.

FSK Transmission

The U2741B is switched on by V

FSK = VS. 5 ms later, VS is applied to VASK. The output

can then be modulated by means of pin FSK. This is done by connecting capacitor C

3 in

parallel to the load capacitor C

ASK Transmission

The U2741B is activated by V

FSK = VS. VASK remains 0 V for 5 ms, then the output power

can be modulated by means of pin ASK. In this case, V

FSK remains = VS during the mes-

sage, the capacitor C

3 is not mounted.

Take-over of the Clock

Pulse in the

Microcontroller

The clock of the crystal oscillator can be used for clocking the microcontroller. The

ATAR090 and ATAR890 have the special feature of starting with an integrated RC oscil-

lator to switch on the U2741B with VFSK = VS. 5 ms later, the 3.39-MHz clock frequency

is present, so that the message can be sent with crystal accuracy.

Application Circuit

The following component values are recommendations for a typical application. C

5, C6,

and C

7 are block capacitors. The values of these capacitors depend on the board layout.

5 =1nF, C6 = 1 nF, and C7 = 22 nF are typically used here. For C5, the impedance

between f = 100 MHz and f = 1 GHz should be as low as possible.

3 is not needed in ASK transmitter applications. In the case of FSK, C3 is selected in

such a way that the desired transmission frequency deviation is achieved (typical

±30 kHz). The capacitance here depends upon the crystal's load capacity (C

4) recom-

mended by the manufacturer of the crystal. C

2 = 3.9 nF, C1 = 15 nF, and R4 = 220 W.

Loop1 and CLoop2 are selected so that the antenna oscillates in resonance and the adap-

tation to the appropriate impedance transformation is possible.

Feed i s an induc t o r for the antenna' s DC cur r ent supply. A typical value is

Feed = 220 nH. LFeed can be either printed on the PC board or be a discrete component.

U2741B Datasheet(PDF) 4 Page - ATMEL Corporation

U2741B Datasheet(HTML) 4 Page - ATMEL Corporation

Html Pages

1 2 3 4 5 6 7 8 9

Datasheet Download

Related Electronics Part Number

Link URL