EVB72016 Datasheet, PDF(4/12 Page) Melexis Microelectronic Systems – 433MHz FSK/ASK Transmitter Evaluation Board Description

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EVB72016 Datasheet, PDF (4/12 Pages) Melexis Microelectronic Systems – 433MHz FSK/ASK Transmitter Evaluation Board Description

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EVB72016

433MHz FSK/ASK Transmitter

Evaluation Board Description

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 devia-

tion Îf and the center frequency fc to be ad-

justed 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 car-

rier frequency and may also contain the FSK (or

FM) modulation.

Fig. 2: Crystal pulling circuitry

VCC

ROI

XTAL

FSKSW

CX2

CX1

VEE

FSKDTA

Description

fmin= fc - Îf (FSK switch is closed)

fmax= fc + Îf (FSK switch is open)

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 calibra-

tion tolerance. The only way to pull the oscilla-

tion frequency is to vary the effective load ca-

XTAL

pacitance CLeff seen by the crystal.

Figure 3 shows the oscillation frequency of a

crystal as a function of the effective load ca- fmax

pacitance. This capacitance changes in accor-

dance with the logic level of FSKDTA around

the specified load capacitance. The figure illus- fc

trates the relationship between the external

pulling capacitors and the frequency deviation.

It can also be seen that the pulling sensitivity fmin

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 capaci-

tance could be chosen in order to reduce the

frequency drift caused by the tolerances of the

CX1 CRO CL

CX1+CRO

(CX1+CX2) CRO

CX1+CX2+CRO

chip and the external pulling capacitors.

Fig. 3: Crystal pulling characteristic

CL eff

For ASK applications CX2 can be omitted. Then CX1 has to be adjusted for center frequency.

3901272016 01

Rev. 004

Page 4 of 12

EVB Description

June/07

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