ATA8743 ATMEL Corporation, ATA8743 Datasheet - Page 8

• Manufacturer Part Number: ATA8743
• Manufacturer: ATMEL Corporation

3.1
3.2
3.3
8
Description of RF Transmitter
ASK Transmission
FSK Transmission
ATA8743
The integrated PLL transmitter is particularly suited to simple, low-cost applications. The VCO is
locked to 64
14.2969 MHz crystal for a 915 MHz transmitter. All other PLL and VCO peripheral elements are
integrated.
The XTO is a series resonance oscillator so that only one capacitor together with a crystal con-
nected in series to GND are needed as external elements.
The crystal oscillator together with the PLL typically need < 1 ms until the PLL is locked and the
CLK output is stable. There is a wait time of
microcontroller and the PA is switched on.
The power amplifier is an open-collector output delivering a current pulse, which is nearly inde-
pendent from the load impedance. Thus, the delivered output power is controllable via the
connected load impedance.
This output configuration enables a simple matching to any kind of antenna or to 50 . This
results in a high power efficiency of = P
868.3 MHz when an optimized load impedance of Z
voltage.
The RF TX block is activated by ENABLE = H. PA_ENABLE must remain L for t
CLK signal is taken to clock the AVR and the output power can be modulated by means of pin
PA_ENABLE. After transmission, PA_ENABLE is switched to L and the microcontroller switches
back to internal clocking. The RF TX is switched back to standby mode with ENABLE = L.
The RF TX is activated by ENABLE = H. PA_ENABLE must remain L for t
signal is taken to clock the AVR and the power amplifier is switched on with PA_ENABLE = H.
The chip is then ready for FSK modulation. The AVR starts to switch on and off the capacitor
between the XTAL load capacitor and GND with an open-drain output port, thus changing the
reference frequency of the PLL. If the switch is closed, the output frequency is lower than if the
switch is open. After transmission PA_ENABLE is switched to L and the microcontroller switches
back to internal clocking. The RF TX is switched back to standby mode with ENABLE = L.
The accuracy of the frequency deviation with XTAL pulling method is about ±25% when the fol-
lowing tolerances are considered.
Figure 3-1.
Tolerances of the Frequency Modulation
f
XTAL
XTAL
V
hence a 13.5672 MHz crystal is needed for a 868.3 MHz transmitter and a
S
C
Stray1
Crystal equivalent circuit
C
M
L
C
out
M
0
/(I
4 ms must be used until the CLK is used for the
S,PA
R
S
Load
V
S
= (166 + j226) is used at 3V supply
) of 24% for the power amplifier at
C
Stray2
C
5
C
Switch
C
4
4 ms, then the CLK
9152A–INDCO–07/09
4 ms, then the