TH7108 データシートの表示（PDF） - Unspecified

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TH7108

868/915MHz FSK/ASK/FM Transmitter

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TH7108

868/915MHz FSK/ASK/FM Transmitter

Theory of Opera-

tion

FSK Modulation

A Colpitts crystal oscillator (XOSC) is used

as the reference oscillator of a phase-

locked loop (PLL) synthesizer. FSK modu-

lation is achieved by pulling the crystal

(XTAL) through the data. So a CMOS-

compatible data stream applied at input

DATA digitally modulates the XOSC. Two

external pulling capacitors Cx1 and Cx2

allow the FSK deviation and center fre-

quency to be adjusted independently. At

DATA = LOW Cx2 is connected in parallel

to Cx1 leading to the low-frequency compo-

nent of the FSK spectrum (fmin); while at

DATA = HIGH Cx2 is deactivated and the

XOSC is set to its high frequency, leading

to fmax.

An external reference signal can be directly

AC-coupled to pin RO1. Then the TH7108

is used without an XTAL. The reference

signal has to contain the FSK (or FM) and

sets the carrier frequency.

ASK Modulation

The TH7108 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.

Frequency Modulation (FM)

For FM operation an external varactor is

required. It simply acts as a pulling ca-

pacitor connected in series to the crystal.

Then the analog modulation signal, applied

through a series resistor, directly modu-

lates the XOSC.

General

Other parts of the TH7108 transmitter are

the fully integrated voltage-controlled os-

cillator (VCO), the divide-by-32 divider

(div32), the phase-frequency detector

(PFD) and the charge pump. An external

loop filter between pins LF1 and LF2 de-

termines 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 = –12 to +3

dBm either by changing the value of re-

sistor Rps or by varying the voltage Vps at

pin PS. The open-collector differential

output (OUT1, OUT2) can be used to either

directly drive a loop antenna or to be con-

verted to a single-ended impedance by

means of a balanced-to-unbalanced (ba-

lun) transformer. For maximum available

output power, the differential output should

be matched to a load of approx. 0.5 to 1.5

kΩ .

Bandgap biasing ensures stable operation

of the IC at a power supply range of 2.1 to

5.5 V. The mode control logic allows four

different modes of operation as listed in the

following table. The mode control pins

ENCK and ENTX are internally terminated

to pull-down and pull-up resistors, respec-

tively. This guarantees that the whole cir-

cuit is shut down if these pins are left

floating.

ENCK

0

0

1

1

ENTX Mode

Description

0 all OFF whole circuit in

standby

1 TX TX functionality only,

only no clock available

0 clock TX in standby and

only clock available

1 all ON TX functional and

clock available

The clock output CKOUT can be used to

drive a µC. This output can be activated by

the ENCK pin as required for any specific

application. Clock frequency is 1/8 of the

reference crystal frequency.

Rev. 2.0

Preliminary Datasheet

2

February 2000

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