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EVB72015-315-ASK-C Datasheet(PDF) 5 Page - Melexis Microelectronic Systems |
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EVB72015-315-ASK-C Datasheet(HTML) 5 Page - Melexis Microelectronic Systems |
5 / 14 page EVB72015 433MHz FSK/ASK Transmitter Evaluation Board Description 39012 72015 01 Page 5 of 14 EVB Description Rev. 007 June/07 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 |
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