TH72001

315MHz

FSK Transmitter

3901072001

Page 5 of 20

Data Sheet

Rev. 010

Jun/12

2.4

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

applied 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

thermometer-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 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

desired 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.

2.5

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.

2.6

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.

&

&

&

PSEL

&

&

RPS

OUT