TH72011 datasheet(5/21 Pages) MELEXIS

5 / 21 page

TH72011

433MHz

FSK Transmitter

Page 5 of 21

REVISION 012 - JUNE 14, 2017

3901072011

f min

fmax

eff

XTAL

CX1 CRO

CX1+CRO

(CX1+CX2) CRO

CX1+CX2+CRO

2.2. FSK Modulation

FSK modulation can be achieved by pulling the

crystal oscillator frequency. A CMOS-compatible

data stream applied at the pin FSKDTA digitally

modulates the XOSC via an integrated NMOS

switch. Two external pulling capacitors CX1 and CX2

allow the FSK deviation f and the center frequency

fc to be adjusted independently. At FSKDTA = 0, CX2

is connected in parallel to CX1 leading to the low-

frequency component of the FSK spectrum (fmin);

while at FSKDTA = 1, CX2 is deactivated and the

XOSC is set to its high frequency fmax.

An external reference signal can be directly AC-

coupled to the reference oscillator input pin ROI.

Then the transmitter is used without a crystal. Now

the reference signal sets the carrier frequency and

may also contain the FSK (or FM) modulation.

Fig. 2: Crystal pulling

circuitry

2.3. Crystal Pulling

A crystal is tuned by the manufacturer to the

required oscillation frequency f0 at a given load

capacitance CL and within the specified calibration

tolerance. The only way to pull the oscillation

frequency is to vary the effective load capacitance

CLeff seen by the crystal.

Figure 3 shows the oscillation frequency of a crystal

as a function of the effective load capacitance. This

capacitance changes in accordance with the logic

level of FSKDTA around the specified load

capacitance. The figure illustrates the relationship

between the external pulling capacitors and the

frequency deviation.

It can also be seen that the pulling sensitivity

increases with the reduction of CL. Therefore,

applications with a high frequency deviation require

a low load capacitance. For narrow band FSK

applications, a higher load capacitance could be

chosen in order to reduce the frequency drift

caused by the tolerances of the chip and the

external pulling capacitors.

Fig. 3: Crystal pulling characteristic

FSKDTA

Description

fmin= fc - f (FSK switch is closed)

fmax= fc + f (FSK switch is open)

CX2

VCC

XTAL

CX1

ROI

FSKSW

VEE

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