TH72002

315MHz

ASK Transmitter

Page 5 of 21

REVISION 011 - JUNE 13, 2017

3901072002

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 capacitance

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