TH72035

868/915MHz

FSK/ASK Transmitter

Page 5 of 21

REVISION 012 - JUNE 14, 2017

3901072035

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 deviation f and the center frequency

is connected in parallel to CX1 leading to the low-

while at FSKDTA = 1, CX2 is deactivated and the

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

FSKDTA

Description

0

1

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 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

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

CX2

VCC

XTAL

CX1

ROI

FSKSW

VEE