u2745b ATMEL Corporation, u2745b Datasheet - Page 4

• Manufacturer Part Number: u2745b
• Description: Uhf Ask Transmitter
• Manufacturer: ATMEL Corporation

General Description
Functional
Description
ASK Transmission
Take-over of the Clock
Pulse in the
Microcontroller
Application Circuit
Output Power
Measurement
4
U2745B
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 to be either 13.56 MHz or 9.844 MHz (USA). At
these frequencies, crystals have a very fast start-up time (< 1.5 ms), whereby a wait
time of 5 to 10 ms is required until the transmitter IC is locked. This means that the pro-
cessor does not need to poll a lock detect output.
The U2745B is activated by EN = V
power can be modulated by means of pin ASK. V
sion of the message. The ASK input activates the power amplifier and the PLL.
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 U2745B with V
present, so that the message can be sent with crystal accuracy.
The following component values are recommendations for a typical application. C
and C
C
between f = 100 MHz and f = 1 GHz should be as low as possible.
C
tation to the appropriate impedance transformation is possible.
L
L
The following output network (see Figure 4 on page 5) can be used for output power
evaluation, the exact values of L
L
50 W . The following table shows the values for an output power of 2 mW and an
R
Table 1. Transformation Network
Feed
10
F e e d
4
Loop1
PWRSET
= 1 nF, C
and C
= 220 nH. L
6
433.92
f/MHz
and C
is an inductor for the antenna's DC current supply. A typical value is
are block capacitors. The values of these capacitors depend on the board layout.
315
= 1.2 k W .
10
form the transformation network to adopt the output impedance of the IC to
Loop2
5
= 1 nF, and C
Feed
are selected so that the antenna oscillates in resonance and the adap-
can be either printed on the PC board or be a discrete component.
C10/pF
2.7
1.8
6
= 22 nF are typically used here. For C
10
and C
EN
S
. V
= V
ASK
10
S
are dependent on the layout.
. 5 ms later, the 3.39-MHz clock frequency is
must remain 0 V for 5 ms, then the output
L10/nH
56
33
EN
remains = V
S
during the transmis-
Z
260 + j330
185 + j268
5
, the impedance
Load_opt
4734A–RKE–11/03
/ W
4
, C
5
,