TH72035

868/915MHz

FSK/ASK Transmitter

39010 72035

Page 5 of 20

Data Sheet

Rev. 011

Jan/12

2.4

ASK Modulation

The PLL transmitter can be ASK-modulated by

applying a data stream directly at the pin

ASKDTA.

This

turns

the

internal

current

sources of the power amplifier on and off and

therefore leads to an ASK signal at the output.

ASKDTA

Description

0

Power amplifier is turned off

1

Power amplifier is turned on (according

to the selected output power step)

2.5

Output Power Selection

The transmitter is provided with an output power selection feature. There are four predefined output power

steps and one off-step accessible via the power selection pin PSEL. A digital power step adjustment was

chosen because of its high accuracy and stability. The number of steps and the step sizes as well as the

corresponding power levels are selected to cover a wide spectrum of different applications.

The implementation of the output power control

logic is shown in figure 4. There are two

matched current sources with an amount of

about 8 µA. One current source is directly ap-

plied to the PSEL pin. The other current source

is used for the generation of reference voltages

with a resistor ladder. These reference voltages

are defining the thresholds between the power

steps. The four comparators deliver thermome-

ter-coded control signals depending on the

voltage level at the pin PSEL. In order to have a

certain amount of ripple tolerance in a noisy

environment the comparators are provided with

a little hysteresis of about 20 mV. With these

control signals, weighted current sources of the

power amplifier are switched on or off to set the

desired output power level (Digitally Controlled

Current Source). The LOCK, ASK signal and

the output of the low voltage detector are gating

this current source.

Fig. 4:

Block diagram of output power control circuitry

There are two ways to select the desired output power step. First by applying a DC voltage at the pin PSEL,

then this voltage directly selects the desired output power step. This kind of power selection can be used if

the transmission power must be changed during operation. For a fixed-power application a resistor can be

used which is connected from the PSEL pin to ground. The voltage drop across this resistor selects the de-

sired output power level. For fixed-power applications at the highest power step this resistor can be omitted.

The pin PSEL is in a high impedance state during the “TX standby” mode.

2.6

Lock Detection

The lock detection circuitry turns on the power amplifier only after PLL lock. This prevents from unwanted

emission of the transmitter if the PLL is unlocked.

2.7

Low Voltage Detection

The supply voltage is sensed by a low voltage detect circuitry. The power amplifier is turned off if the supply

voltage drops below a value of about 1.85 V. This is done in order to prevent unwanted emission of the

transmitter if the supply voltage is too low.

ASKDTA

&

&

&

PSEL

&

&

RPS

OUT