The Square Kilometre Array (SKA) is a multi-billion dollar international project to create a receiving surface of a million square metres, one hundred times larger than the biggest receiving surface now in existence. The SKA core array will have to be located in a remote area. Therefore countries interested in hosting the SKA core array were requested to perform Radio Frequency Interference measurements at their site of choice. The systems that are to be used in the measurements must conform to a document called the “RFI Measurement Protocol for Candidates SKA Sites”, the SKA Memo 37.
The RFI protocol divides measurements into two parts, Mode 1 and Mode 2. Mode 1 is defined for the observation of strong RFI and is relevant for SKA receiver linearity analysis. Mode 2 is defined for the observation of weak interferences, which potentially threatens to obscure weak signals of interest.
In Mode 1, the RFI protocol specifies a dwell time of 2 μs duration over a large 1 MHz bandwidth in the 960 -1400 MHz band (L-band). The reason for this short dwell time is to capture and characterize pulsed interference from radars and Distance Measuring Equipment (DME) in this band. This kind of interference is expected, potentially with very high peak power and short dwell time. Executing these measurements with the spectrum analyzer is impractical because of the very long measurement times. It was therefore proposed to build a dedicated FFT spectrometer from standard components, state of the art FPGA board with a high speed 14 bit ADC.
The proposed system was designed by Dr. Adrian Tiplady and is called the RFI measurement system 4. This thesis investigates whether system 4 measures impulsive RFI as required by the protocol. This was done by simulating the receiver of system 4 and feeding the receiver with simulated DME signals. The simulated receiver findings were then compared to the real receiver findings when similar DME-like signals were injected into the real receiver.
The practical system was found to be able to withstand high power signals from DME systems without suffering from compression and inter-modulation distortion. The maximum signal that the receiver can handle under automatic gain control is 9 dBm, equivalent to 8 mW into 50 Ohms. However, the automatic gain mode was found not to be desirable for RFI measurements because of the unknown gain in the AGC. The simulated receiver was found to measure RFI as expected by the protocol without any compression and intermodulation distortion. Manual mode is recommended for making level measurements in absolute terms.