ATA8742-PXQW【PDF 9/238 页】MCU W/TRANSMITTER ASK/FSK 24QFN

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Sheet目录52

5页 6页 7页 8页 [9页]10页 11页 12页 13页

9151AINDCO07/09

ATA8742

Using C

= 9.2 pF ?%, C

= 6.8 pF ?%, a switch port with C

Switch

= 3 pF ?0%, stray capaci-

tances on each side of the crystal of C

Stray1

= C

Stray2

= 1 pF ?0%, a parallel capacitance of the

crystal of C

= 3.2 pF ?0% and a crystal with C

= 13 fF ?0%, results in a typical FSK devia-

tion of ?1 kHz typical with worst case tolerances of ?6.3 kHz to ?8.8 kHz.

3.4

CLK Output

An output CLK signal is provided for the integrated AVR. The delivered signal is CMOS compat-

ible if the load capacitance is lower than 10 pF.

3.4.1

Clock Pulse Take-over

The clock of the crystal oscillator can be used for clocking the microcontroller. Atmel

?/DIV>

s AVR

microcontroller starts with an integrated RC-oscillator to switch on the RF TX with ENABLE = H,

and after 1 ms assumes the clock signal of the transmission IC, so that the message can be sent

with crystal accuracy.

3.4.2

Output Matching and Power Setting

The output power is set by the load impedance of the antenna. The maximum output power is

achieved with a load impedance of Z

Load,opt

= (166 + j223)? There must be a low resistive path

to V

to deliver the DC current.

The delivered current pulse of the power amplifier is 9 mA and the maximum output power is

delivered to a resistive load of 465?if the 1.0 pF output capacitance of the power amplifier is

compensated by the load impedance.

An optimum load impedance of:

Load

= 465﹟| j/(2 ??1.0 pF) = (166 + j223)?is achieved for the maximum output power of

7.5 dBm.

The load impedance is defined as the impedance seen from the RF TXs ANT1, ANT2 into the

matching network. This large signal load impedance should not be confused with the small sig-

nal input impedance delivered as input characteristic of RF amplifiers and measured from the

application into the IC instead of from the IC into the application for a power amplifier.

Less output power is achieved by lowering the real parallel part of 465?where the parallel imag-

inary part should be kept constant.

Output power measurement can be done using the circuit shown in Figure 8-4 on page 16. Note

that the component values must be changed to compensate the individual board parasitics until

the RF TX has the right load impedance Z

Load,opt

= (166 + j223)? In addition, the damping of the

cable used to measure the output power must be calibrated out.

4. Microcontroller Block

More detailed information about the microcontroller block can be found in the appendix.

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