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Wake-up Receiver Hardware

Wireless Sensor Node with WuRx

alt: "General structure of a WuRx-capable WSN", x:1.5, label: "fig:schema_wsn"

  • The figure above shows the general structure of a wireless sensor node.
  • This structure shows the general building block. More details are needed to describe the structure completely.
  • The following sections describe the shown building blocks.


  • Antenna is used for the wake-up receiver and the wireless transceiver.
  • PCB antenna or whip antenna can be used.
  • Using an antenna with a length of λ/2 results in better receiving performance of the whole system.

RF Switch

  • The RF switch is needed to connect either the WuRx or the transceiver to the antenna.
  • If the RF switch is used in the design, two separate antennas are needed to connect the WuRx and the wireless transceiver.

WuRx Receiver

  • The WuRx receiver is of course the main feature of WSN.
  • If the WuRx is enabled it is listening constantly the channel for wake-up packages (WuPt).
  • The WuRx sends a digital signal to the microcontroller when a WuPt was received.
  • If a addressed WuRx is build in, the WuRx only accepts packages with the correct address. The address and other settings. needs to be configured by the microcontroller.

Wireless Transceiver

  • A separate IC or and SOC with integrated transceiver and microcontroller can be used.
  • The tasks of the transceiver are to send the WuPt to wake up a WSN and to enable the bidirectional communication with the other WSN.
  • The frequency range of the wireless transceiver defines the working frequency of the WSN and the WuRx.


  • The task of the microcontroller is to setup the WuRx, to setup the transceiver, fulfill the communication protocol and to readout connected sensors.
  • A ultra-low-power capable microcontroller should be used, to reduce the current consumption of whole system.
  • A SOC with the wireless transceiver integrated can be used instead.


  • Sensors can be added to fulfill the reception needs of the WSN.
  • Sensors can be:
    • Internal sensors of the microcontroller (e. g. temperature or battery voltage)
    • Analog sensors: using the ADC of the microcontroller
    • Digital sensors: using the serial interfaces of the microcontroller (e.g. SPI or I²C)

Power Source
e. g. USB for testing, batteries, DC-DC converter to ensure stable supply voltage

WuRx Building Blocks

alt: "WuRx Building Blocks", x:1.5, label:"fig:schema_wurx"

  • The WuRx can be divided into multiple building blocks. The previous Figure shows the WuRx in a detailed view.
  • The following sections describe the building blocks of the WuRx.

Antenna or RF Input

  • Transmission line from the antenna or the RF switch.
  • The line impedance of the signal should be 50 Ω

RF Bandpass Filter

  • A RF bandpass filter is used to let only signals of the correct frequency band through.
  • Typically a SAW filter with an input and output line impedance of 50 Ω is used.
  • The bandwidth of such a filter is very high and lies in the range of several megahertz. That means signals of the whole frequency band are analysed - channel frequency cannot be separated.

Low Noise Amplifier

  • A LNA can be introduced to boost the RF signal level.
  • The current consumption of a LNA is rather high in the range of 1 mA.
  • The LNA needs to be duty-cycled to ensure a lower current consumption.

Envelope Detector

  • The envelope detector is used to convert the RF amplitude modulated signal or RF OOK signal to a low frequency signal.
  • Typically schottky diodes are used to perform the envelop detection.
  • The WuPt is a OOK modulated signal, because only this kind of signals can be passively detected by an envelope detector.
  • Because the impedance of the detector diodes is not equal to 50 Ω a impedance matching circuit needs to be added in front of the diode circuit.
  • All signals after the envelope detector are low frequency signals. The maximum signal frequency is equal to the modulation frequency (typically 1...500 kHz)
  • Detailed description of the envelope detector and diode selection, see:
    • Fromm, R., Schott, L. and Derbel, F. (2020). An Efficient Low-Power Wake-Up Receiver Architecture for Power Saving for Transmitter and Receiver Communications
    • Sections 4.1 and 4.2

LF Amplifier

  • Signals from the diodes are very low. A voltage conversion of about 80 mV/µV can be estimated. Resulting in a voltage of only 800 µV at -50 dBm RF power.
  • The LF signal needs to be boosted in order to be processed by the LF WuRx.

LF Wake-Up Receiver

  • The LF Wake-Up Receiver converts the LF signal into a digital bit stream.
  • The bit stream is matched with the address of the WuRx.
  • On a successful match the wake signal is set.

WuRx Booster Node BJT


alt: "WuRX-Booster BJT Node 1 on Launchpad", w: 33

alt: "WuRx-Booster BJT Node 1 - MSP430 Pinout", x:1

Building Block Component
Power Supply USB or battery, no DC-DC converter
Antenna SMA antenna connectors, e. g. half-wave antenna (see
RF Switch HMC221B
Wireless Transceiver SPIRIT1, SPS-GRFC-868 module
MCU MSP430G2553 on MSP-EXP430G2ET
Sensors LSM9DS1, Si7021
RF Bandpass Filter B39871B3725U410
Low Noise Amplifier no
Envelope Detector Greinacher Voltage Double, SMS7630-006LF
LF Amplifier Single stage BJT amplifier, BFP 405
LF Wake-Up Receiver AS3933

Battery and Reverse Current Protection

alt: "Schematic of battery and reverse current protection", w:33

  • Note the polarity of the used Varta CR1/2 AA, see
  • A reverse polarity protection is introduced by the MOSFET Q1.
  • MOSFET Q1 does not stop current from flowing into the battery.
  • Never connect battery and USB power supply at the same time!

Antenna Switch

alt: "Schematic of antenna switch", w:45

  • The RF switch HMC221B connects either the SPIRIT1 or the WuRx to the antenna.
  • The HMC211B need two inverted digital signals A, B to switch properly.
  • The CMOS inverter U3 generates the inverted signal.
  • The antenna switch is controlled by P2.2.
  • P2.2 = H → SPIRIT1 path connected to antenna
  • P2.2 = L → WuRx path connected to antenna

SPIRIT1 wireless transceiver

alt: "Schematic of SPIRIT1", w:60

  • For the wireless transceiver the per-made module SPSGRFC-868 is used.
  • Contains SPIRIT1 transceiver with all complementary components (filter capacitor, RF matching circuit).
  • The SPSGRFC-868 is matched for the 868 MHz frequency band.
  • SPIRIT1 is connected to the SPIA (USCIA) of the MSP430G2553.
  • SPIRIT1 has four GPIO (general-purpose input/output).
  • GPIO0 is used as an interrupt signal from SPIRIT1 to MSP430G2553.
    (signalling successful packet transmission or reception)
  • The CS pin P2.1 is low active.

AS3933 Low-frequency Wake-up Receiver

alt: "Schematic of AS3933", w:60

  • AS3933 receives the demodulated WuPt and performs the pattern matching.
  • AS3933 must be configured over SPI.
  • No external clock is used, instead the internal RC oscillator.
  • The RC oscillator is calibrated over SPI.
  • The WAKE pin signals a pattern match, connected to P2.4
  • CL_DAT and DAT are additional output signals, not connected to MSP430 pins.
  • The CS pin P2.5 is high active!


alt: "Schematic of sensors", w:60

  • Two external pull-up resistors are added for the I²C signal lines.
  • Si7021 is used as a temperature and humidity sensor.
  • The HTU21D might be populated instead! (chip shortage!)
  • The LSM9DS1 as accelerometer, gyroscope and magnetometer is not populated (chip shortage!)