TH72002

315MHz

ASK Transmitter

3901072002

Page 4 of 20

Data Sheet

Rev. 010

Jun/12

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

frequency 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

calibration tolerance. The only way to pull the

oscillation frequency is to vary the effective load

Figure 2 shows the oscillation frequency of a

crystal as a function of the effective load

capacitance. This figure also illustrates the

relationship

between

the

external

pulling

capacitor and the center frequency.

It can be seen that the pulling sensitivity

increases with the reduction of CL. For high-

accuracy ASK applications, a higher load

capacitance 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