ATA8742-PXQW【PDF 8/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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9151AINDCO07/09

ATA8742

3.1

Description of RF Transmitter

The integrated PLL transmitter is particularly suitable for simple, low-cost RF applications. The

VCO is locked to 32 f

XTAL

hence a 13.56 MHz crystal is needed for a 433.92 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 needs < 1 ms until the PLL is locked and the

CLK output is stable. There is a wait time of e 1 ms 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 simple matching to any kind of antenna or to 50? This results

in a high power efficiency of ? P

out

/(I

S,PA

) of 36% for the power amplifier when an optimized

load impedance of Z

Load

= (166 + j223)?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 e1 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 e1 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 ?5% when the fol-

lowing tolerances are considered.

Figure 3-1. Tolerances of Frequency Modulation

TAL

Crystal equivalent circuit

wi h

Stray1

Stray2

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