TH72011

433MHz

FSK Transmitter

3901072011

Page 4 of 20

Data Sheet

Rev. 011

Jun/12

f

eff

CL

eff

CL

R1

C1

C0

L1

XTAL

CL

CX1 CRO

CX1+CRO

(CX1+CX2) CRO

CX1+CX2+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

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

adjusted 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

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

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

CX2

VCC

XTAL

CX1

ROI

FSKSW

VEE