ATA8742 ATMEL Corporation, ATA8742 Datasheet - Page 8

• Manufacturer Part Number: ATA8742
• Manufacturer: ATMEL Corporation

3.1
3.2
3.3
8
Description of RF Transmitter
ASK Transmission
FSK Transmission
ATA8742
The integrated PLL transmitter is particularly suitable for simple, low-cost RF applications. The
VCO is locked to 32 f
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 needs < 1 ms until the PLL is locked and the
CLK output is stable. There is a wait time of
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 simple matching to any kind of antenna or to 50 . This results
in a high power efficiency of = P
load impedance of Z
The RF TX block is activated by ENABLE = H. PA_ENABLE must remain L for t 1 ms, then the
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 1 ms, then the CLK
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. When the switch is closed, the output frequency is lower than
when the switch is open. After transmission, PA_ENABLE is switched to L and the microcon-
troller 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 Frequency Modulation
XTAL
V
Load
XTAL
S
= (166 + j223) is used at 3V supply voltage.
hence a 13.56 MHz crystal is needed for a 433.92 MHz transmitter. All
C
Stray1
out
/(I
Crystal equivalent circuit
S,PA
C
M
V
S
L
C
) of 36% for the power amplifier when an optimized
M
0
1 ms until the CLK is used for the microcontroller
R
S
C
Stray2
C
5
C
Switch
C
4
9151A–INDCO–07/09