TH72002

315MHz

ASK Transmitter

Page 6 of 21

REVISION 011 - JUNE 13, 2017

3901072002

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 3. 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, ASK signal

and the output of the low voltage detector are

gating this current source.

Fig. 3: 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.

ASKDTA

&

&

&

PSEL

&

&

RPS

OUT