ATA8743-PXQW【PDF 8/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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9152BINDCO02/10

ATA8743

3.1

Description of RF Transmitter

The integrated PLL transmitter is particularly suited to simple, low-cost applications. The VCO is

locked to 64 ?f

XTAL

hence a 13.5672 MHz crystal is needed for a 868.3 MHz transmitter and a

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 e 4 ms must be used until the CLK is used for the

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

out

/(I

S,PA

) of 24% for the power amplifier at

868.3 MHz when an optimized load impedance of Z

Load

= (166 + j226)?is used at 3V supply

voltage.

3.2

ASK Transmission

The RF TX block is activated by ENABLE = H. PA_ENABLE must remain L for t e 4 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.

3.3

FSK Transmission

The RF TX is activated by ENABLE = H. PA_ENABLE must remain L for t e 4 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. 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 ?5% when the fol-

lowing tolerances are considered.

Figure 3-1. Tolerances of the Frequency Modulation

TAL

Crystal equivalent circuit

wi h

Stray1

Stray2

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