EVB72016

433MHz FSK/ASK Transmitter

Evaluation Board Description

3901272016 01

Page 4 of 12

EVB Description

Rev. 004

June/07

f

eff

CL

eff

CL

R1

C1

C0

L1

XTAL

CL

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 devia-

tion

justed independently. At FSKDTA = 0, CX2 is

connected in parallel to CX1 leading to the low-

frequency component of the FSK spectrum

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

FSKDTA

Description

0

1

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 3 shows the oscillation frequency of a

crystal as a function of the effective load ca-

pacitance. This capacitance changes in accor-

dance with the logic level of FSKDTA around

the specified load capacitance. The figure illus-

trates 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 capaci-

tance 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

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

CX2

VCC

XTAL

CX1

ROI

FSKSW

VEE