U2741B_05 Datasheet, PDF (4/11 Pages) ATMEL Corporation – UHF ASK/FSK TRANSMITTER

3. 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 consid-
erably simplified adaptation of both a magnetic loop antenna of any form or a λ/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_S.
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.
4. 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, V_ASK ≤0.3V, the power-
down supply current is IS_off < 0.35 µA. The ASK input activates the power amplifier and the PLL.
The FSK input only activates the PLL and, if capacitor C₃ 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 frequency of the crystal oscillator.
4.1 FSK Transmission
The U2741B is switched on by V_FSK = V_S. 5 ms later, V_S is applied to V_ASK. The output can then
be modulated by means of pin FSK. This is done by connecting capacitor C₃ in parallel to the
load capacitor C₄.
4.2 ASK Transmission
The U2741B is activated by V_FSK = V_S. V_ASK remains 0V for 5 ms, then the output power can be
modulated by means of pin ASK. In this case, V_FSK remains = V_S during the message, the
capacitor C₃ is not mounted.
4.3 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 oscillator to switch on
the U2741B with V_FSK = V_S. 5 ms later, the 3.39-MHz clock frequency is present, so that the
message can be sent with crystal accuracy.
4 U2741B
4733B–RKE–09/05