Wake-up Receiver Hardware
Wireless Sensor Node with WuRx
- 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.
- 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.
- 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.
- 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)
e. g. USB for testing, batteries, DC-DC converter to ensure stable supply voltage
WuRx Building Blocks
- 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.
- 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
- 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
|Power Supply||USB or battery, no DC-DC converter|
|Antenna||SMA antenna connectors, e. g. half-wave antenna (see mouser.de)|
|Wireless Transceiver||SPIRIT1, SPS-GRFC-868 module|
|MCU||MSP430G2553 on MSP-EXP430G2ET|
|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
- 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!
- 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
- 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
- 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!
- 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!)