TH72002

315MHz

ASK Transmitter

3901072002

Page 4 of 20

Data Sheet

Rev. 007

June/07

f

eff

CL

eff

CL

R1

C1

C0

L1

XTAL

CL=

CX1 CRO

CX1+CRO

2

Functional Description

2.1

Crystal Oscillator

A Colpitts crystal oscillator with integrated functional capacitors is used as the reference oscillator for the PLL

synthesizer. The equivalent input capacitance CRO offered by the crystal oscillator input pin ROI is about

18pF. The crystal oscillator is provided with an amplitude control loop in order to have a very stable fre-

quency over the specified supply voltage and temperature range in combination with a short start-up time.

2.2

ASK Modulation

The PLL transmitter can be ASK-modulated by

applying a data stream directly at the pin

ASKDTA.

This

turns

the

internal

current

sources of the power amplifier on and off and

therefore leads to an ASK signal at the output.

ASKDTA

Description

0

Power amplifier is turned off

1

Power amplifier is turned on (according

to the selected output power step)

2.3

Crystal Pulling

A crystal is tuned by the manufacturer to the

capacitance CL and within the specified calibra-

tion tolerance. The only way to pull the oscilla-

tion frequency is to vary the effective load ca-

Figure 2 shows the oscillation frequency of a

crystal as a function of the effective load ca-

pacitance. This figure also illustrates the rela-

tionship between the external pulling capacitor

and the center frequency.

It can be seen that the pulling sensitivity in-

creases with the reduction of CL. For high-

accuracy ASK applications, a higher load ca-

pacitance should be chosen in order to reduce

the frequency drift caused by the tolerances of

the chip and the external pulling capacitor.

Fig. 2:

Crystal pulling characteristic