TH7108 datasheet(2/10 Pages) ETC

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TH71082

868/915MHz

ASK Transmitter / LO Source

TH71082 Data Sheet

Page 2 of 10

Dec. 2001

3901071082

Rev. 004

General Description

The TH71082 ASK transmitter IC is designed for

applications in the European 868MHz industrial-

scientific-medical (ISM) band, according to the EN

300 220 telecommunications standard. It can also

used

for

any

other

system

with

carrier

frequencies ranging from 800 MHz to 930 MHz

(e.g. for applications in the US 915MHz ISM band).

The transmitter's carrier frequency fc is determined

by the frequency of the reference crystal fref that is

used. The integrated PLL synthesizer ensures that

each RF value, ranging from 800 MHz to 930 MHz,

can be achieved by using a crystal with reference

frequency according to: fref = fc/N, where N = 32 is

the PLL feedback divider ratio.

Theory of Operation

General

As depicted in Fig.1, the TH71082 transmitter

consists of a fully integrated voltage-controlled

oscillator (VCO), a divide-by-32 divider (div32), a

phase-frequency detector (PFD) and a charge

pump. An external loop filter at pin LF determines

the dynamic behaviour of the PLL and suppresses

reference spurious signals.

The VCO’s output signal feeds the power amplifier

(PA). RF signal power Po can be adjusted in six

steps from Po = –15 dBm to -1 dBm, either by

changing the value of resistor R1 or by varying the

voltage VPS at pin PS/DATA. The open-collector

output (OUT) can be used to either drive a loop

antenna or to feed a single-ended load impedance.

This could be, for example

λ /4 monopol antenna

or a 50

Ω output port. In any case, an impedance

matching network should be added in order to

achieve maximum available RF power.

Bandgap biasing ensures stable operation of the IC

at a power supply range of 2.2 V to 5.5 V.

Clock Output

The TH71082 feature a clock output (CKOUT) that

can be

used to drive a µC. The frequency at

CKOUT is fref/8. The clock output is slew-rate limited

in order to keep spurious signal emission as low as

possible. The voltage swing at CKOUT depends on

the capacitive loading at this pin.

It is approximate 2 Vpp at Cload = 5 pF

ASK Modulation

The TH71082 can be ASK-modulated by applying

data directly at pin PS. This turns the PA on and off

and therefore leads to an ASK signal at the output.

LO Source

Many applications require a stable RF source. For

this purpose, the TH71082 can be used without

modulation

easy-to-use,

PLL-stabilized,

continuous wave (CW) generator.

Frequency Modulation (FM)

For FM operation an external varactor is required. It

simply acts as a pulling capacitor connected in

series to the crystal. Then the analog modulation

signal, applied through a series resistor, directly

modulates the XOSC.

Mode Control Logic

The mode control logic allows two different modes

of operation as listed in the following table. The

mode control pin ENTX is pulled-down internally.

This guarantees that the whole circuit is shut down

if this pin is left floating.

ENTX

Mode

Description

TX disabled whole circuit in standby

TX enable

TX active

Table 1: Modes of operation

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