u2741b ATMEL Corporation, u2741b Datasheet - Page 4

• Manufacturer Part Number: u2741b
• Description: Uhf Ask/fsk Transmitter U2741b
• Manufacturer: ATMEL Corporation

3. General Description
4. Functional Description
4.1
4.2
4.3
4
FSK Transmission
ASK Transmission
Take-over of the Clock Pulse in the Microcontroller
U2741B
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 /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.
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
down supply current is IS
The FSK input only activates the PLL and, if capacitor C
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.
The U2741B is switched on by V
be modulated by means of pin FSK. This is done by connecting capacitor C
load capacitor C
The U2741B is activated by V
modulated by means of pin ASK. In this case, V
capacitor C
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
message can be sent with crystal accuracy.
3
is not mounted.
4
.
FSK
= V
off
S
< 0.35 µA. The ASK input activates the power amplifier and the PLL.
. 5 ms later, the 3.39-MHz clock frequency is present, so that the
FSK
FSK
= V
= V
S
. V
S
ASK
. 5 ms later, V
remains 0V for 5 ms, then the output power can be
FSK
S
remains = V
is applied to V
3
is installed, pulls the crystal to the
FSK
S
during the message, the
ASK
, V
S
ASK
. The output can then
.
3
0.3V, the power-
in parallel to the
4733B–RKE–09/05