The Multi-copter was built as a metrological device with dual-embedded an- tennas feeding into two receiver systems. Because of the obvious weight re- strictions a light design was preferred. For propagation studies, a known signal is transmitted at a known position and then received at a second known po- sition. The propagation characteristics are then inferred from the change in signal between the transmitter and receiver. The transmitted signal consists of a single-frequency carrier, sweeping-frequency carrier or a time-domain pulse. A single carrier frequency simplifies the receiver that is needed in the Multi- copter as well as post-processing of data. However, this limits the investigation to that single frequency for the entire flight. It is possible to sweep the fixed carrier during the flight. This will, however, reduce the spatial resolution of each frequency and could introduce measurement inaccuracies if the transmit- ter and receiver are not properly synchronised. Nonetheless, it is possible to achieve continuous frequency data while preserving the spatial resolution by using a pulse generator and a transient real-time analyser as the receiver on the vehicle. However, this dramatically increases the complexity of the on-board receiver.
For most of our propagation measurements, the single-carrier frequency scheme was used. This preserved simplicity and eliminated extra unknowns when de-embedding the vehicle from the measurements. Sec. 5.3.1 discusses the narrowband receiver used for our propagation studies. Additionally, a miniaturised lightweight real-time transient analyser is also investigated and developed in Sec. 5.3.2.
5.3.1
Narrowband receiver
The nature of narrowband propagation measurements involves a fixed carrier being transmitted at a single frequency. Therefore, a sensor that can receive this carrier and measure its signal strength would be sufficient. It was decided to use a transceiver module, RFM22B seen in Fig. 5.3a, which incorporates a digital receive-signal-strength-indicator (RSSI) with a 0 to -120 dBm range. Along with an 8-bit analog-to-digital converter (ADC), the module can deliver up to 0.5 dB resolution with a linear trend. The primary interface to this module is through a serial peripheral interface (SPI). To keep the interface to the flight controller as well as the receiver simple it was decided to use a
CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 70
(a) RFM22B transceiver module (b) Raspberry-Pi SBC
Figure 5.3: Photos of the RFM22B transceiver module. In (b), the transceiver is shown on its daughter board attached to the Raspberry Pi.
low-cost single-board computer (SBC) called Raspberry Pi. This computer has on-board digital input and output pins which support SPI. Fig. 5.3b shows the SBC with the receiver connected on to its digital output pins. The connector board between the SBC and receiver was designed in-house and can be found in Appendix E.1.
One of the main advantages of this vehicle is the speed at which it can collect data. This is because the SBC continuously interrogates the availability of new data using multiple threads. As soon as new data is available it is stored on an SD card, connected to the SBC. The software uses individual threads for the flight controller, receiver and data storage. A diagram illustrating the operation of the software can be seen in Fig. 5.4. The software starts by creating the two main threads, a receiver thread and flight controller thread. The receiver thread and flight controller thread continuously poll their devices for new data. As soon as new data becomes available, flags are set. The storage thread, also the main thread, will keep a record of the latest available data and monitor the flags. As soon as both the receiver and flight controller indicate the availability of new data, the storage thread will store the data on the SD card. The multi-threading makes it possible for the receiver thread to average the signal data while other data is being collected. However, care was needed to prevent race conditions. Therefore mutexes3 were used throughout
the data accessing portions of the code. The script that was used for the data collection during measurements can be found in Appendix E.4
To ensure accurate measurements, it was necessary to calibrate the receiver. This was done using a signal generator, checked against a calibrated spectrum analyser, for each of the calibration frequencies. With the power level known
3
Mutual-exclusion mechanism ensuring that no two concurrent processes access a critical section at the same time. The critical section here represents a section of code modifying or using variables.
CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 71
Start Create Receiver
d Check if data is available
Store data Create data file with headers
Yes No
Initialise Radios Update receive level
Initialise Flight Controller Update receive level 1us Sleep
1us Sleep
Figure 5.4: Software diagram of the logging script running on the Raspberry- Pi. This python script is initialised before each flight and is only terminated after the flight has ended.
200 300 400 500 600 700 800 900 1000Frequency [MHz]
−100
−90
−80
−70
−60
−50
−40
−30
−20
−10
Receiver Measur
ed [dBm]
-100.5 dB
-90.5 dB
-80.5 dB
-70.5 dB
-60.5 dB
-50.5 dB
-40.5 dB
-100.5 dB
-90.5 dB
-80.5 dB
-70.5 dB
-60.5 dB
-50.5 dB
-40.5 dB
Receiver and pre-amp characterisation
Figure 5.5: The calibration file used during post-processing for a pre-amp receiver pair. This file was created using a known signal-generator as an input while logging the output of the receiver.
CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 72
Figure 5.6: System diagram of the real-time transient analyser.
to the point of the receiver connector, it was possible to generate a calibration file. Such a file contains the power level transmitted into the receiver versus the power level measured with the receiver. An example of such a calibration file can be seen plotted in Fig. 5.5. This data, using interpolation, is then used during post-processing to remove the effect of the receiver.
5.3.2
Wideband receiver
A real-time transient analyser has the capability of sampling short duration pulses. With the frequency content of the pulse at transmission known, it is possible to characterise a broad range of frequencies by comparing it to the spectral content of the received pulse. Relative measurements are also possible where the pulse is not exactly characterised. An example of such a measure- ment is sparking on power lines discussed in Kibet-Langat [6] and Groch [5] where the pattern of the broadband radiation pattern can be characterised. Such wideband emissions are cumbersome to measure using a narrowband re- ceiver.
In this investigation, a compact and lightweight direct sampling transient analyser is designed and tested. Such a receiver uses an analogue front-end to filter a specific Nyquist zone that is sampled by a high-speed sampling ADC. Data from the high-speed ADC is buffered using high-speed memory. This memory is in most cases limited and needs to be cleared before measuring the next pulse. A simplified diagram of the transient analyser can be seen in Fig 5.6.
It was decided to develop the transient analyser for measuring power-line sparking. An in-house 12 kV line with a spark-gap was used to generate a power-line sparking scenario. The sparking was then recorded using a full- size transient analyser developed in house called RaTTy [13]. Measurements revealed that a bandwidth of 500 MHz would be sufficient and that the emis- sions were powerful enough to be received without any amplification stages, see Appendix F.1. With this bandwidth, The HMCAD1511 1 GSPS 8-bit ADC with a full power bandwidth of 650 MHz from Hittite was chosen. To
CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 73
Figure 5.7: Front-end switchable filter-bank configuration.
Figure 5.8: Photo of the digital board containing the FPGA, ADC and clock generator.
cover the full power bandwidth, the front-end would consist of two switch-able filters with specifications illustrated in Fig. 5.7. The ADC will feed its data through 8 differential LVDS lines at 1 Gb/s into a Xilinx Spartan 6 FPGA. Each data channel will be de-serialised and then stored into a FIFO buffer. A state-machine on-board the FPGA will make the data available through a serial interface that will then be downloaded onto the Raspberry-Pi for stor- age. Schematics, PCB layouts and VHDL code can be found in Appendix F.2. The manufactured digital ADC/FPGA board can be seen in Fig. 5.8. An implementation of the switch-able filter bank can be seen in Fig. 5.9.
Similar to the narrowband receiver, calibration will be necessary to remove errors introduced between the ADC and front-end input. However, due to unforeseen manufacturing errors signal integrity policies on the main-digital board were severely compromised. As a result, the system could never be fully implemented on the full Multi-copter platform. Nevertheless, sufficient data was captured using the narrow band receiver in previous campaigns to enable off-site propagation studies. Test measurements on the compromised system
CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 74
Figure 5.9: Photo of the unshielded switch-able filter bank. Filters are imple- mented using lumped elements.
CHAPTER 5. MULTI-COPTER METROLOGY DEVELOPMENT 75