U2745B Datasheet, PDF (4/9 Pages) ATMEL Corporation – UHF ASK TRANSMITTER

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 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.
Functional
Description
ASK Transmission
Take-over of the Clock
Pulse in the
Microcontroller
The U2745B is activated by EN = V_S. V_ASK must remain 0 V for 5 ms, then the output
power can be modulated by means of pin ASK. V_EN remains = V_S during the transmis-
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_EN = V_S. 5 ms later, the 3.39-MHz clock frequency is
present, so that the message can be sent with crystal accuracy.
Application Circuit
Output Power
Measurement
The following component values are recommendations for a typical application. C₄, C₅,
and C₆ are block capacitors. The values of these capacitors depend on the board layout.
C₄ = 1 nF, C₅ = 1 nF, and C₆ = 22 nF are typically used here. For C₅, the impedance
between f = 100 MHz and f = 1 GHz should be as low as possible.
C_Loop1 and C_Loop2 are selected so that the antenna oscillates in resonance and the adap-
tation to the appropriate impedance transformation is possible.
L_Feed is an inductor for the antenna's DC current supply. A typical value is
L_Feed = 220 nH. L_Feed can be either printed on the PC board or be a discrete component.
The following output network (see Figure 4 on page 5) can be used for output power
evaluation, the exact values of L₁₀ and C₁₀ are dependent on the layout.
L₁₀ and C₁₀ form the transformation network to adopt the output impedance of the IC to
50 W. The following table shows the values for an output power of 2 mW and an
R_PWRSET = 1.2 kW.
Table 1. Transformation Network
f/MHz
315
433.92
C10/pF
2.7
1.8
L10/nH
56
33
Z_{Load_opt}/W
260 + j330
185 + j268
4 U2745B
4734A–RKE–11/03