Browsing by Author "Stofberg, Anneke"

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    IQ reflected power canceller for an FMCW radar
    (Stellenbosch : Stellenbosch University, 2014-04) Stofberg, Anneke; De Swardt, J. B.; Van der Walt, P. W.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: Large close range environmental reflections or poor isolation between the transmit and receive paths of an FMCW radar can overload the receiver. The In phase and Quadrature phase (IQ) Reflected Power Canceller (RPC) provides a solution to the problem by cancelling any close range reflections. In this study a procedure to optimise the design of an RPC is developed and the performance limits of a practical RPC is investigated in depth. There are four focus areas in the evaluation and design of the IQ Reflected Power Canceller. First, an analysis was performed on a theoretical IQ Reflected Power Canceller, which provided insight into how the system functioned and made it possible to identify practical application issues that would arise during the design. The next focus area was the IQ Reflected Power Canceller’s dynamic range. Equations, based on the power and noise characteristics of each component in the canceller, were derived. From these equations, a system, with an optimised dynamic range, could be developed. Next, the IQ Reflected Power Canceller’s feedback loop stability was investigated. The canceller is an active negative feedback control system but, in order to obtain the negative feedback, the feedback signal has to be phase shifted by 180 degrees to the phase of the input signal. An analysis of the canceller’s RF phase contribution resulted in an equation that can be used to manage the nett RF phase in the feedback loop. The evaluation model of the IQ Reflected Power Canceller produced favourable results. The tests performed on the system included measuring the level of cancellation that can be achieved, whether the dynamic range corresponds to the predicted values and the amount of RF phase error that can be introduced in the feedback path while maintaining a stable system. The IQ Reflected Power Canceller was found to perform well in the evaluation. It provided a cancellation of more than 45 dB for close range reflections and the canceller remained stable across a wide range of RF centre frequencies (1 GHz). This means that the FMCW radar’s frequency modulation bandwidth will not be limited because of the IQ Reflected Power Canceller. The evaluation clearly showed that the modulator in the feedback loop is the critical element that determines the dynamic range of the radar with an RPC.
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    Phase tracking electronically variable attenuators with receiver protection
    (Stellenbosch : Stellenbosch University, 2018-12) Stofberg, Anneke; De Swardt, J. B.; Van der Walt, P. W.; Stellenbosch University. Faculty of Engineering. Dept. of Electrical and Electronic Engineering.
    ENGLISH ABSTRACT: This dissertation presents the development of a set of optimal phase tracking electronically variable attenuators. Secondly, a compact high power PIN diode limiter is developed and its minimum attainable resistance is extracted through high power measurements. Close range reflections cause the receiver of a multi-channel digital beamforming radar to saturate. Controlled attenuation over time, implemented with electronically variable attenuators, is used to prevent receiver saturation (sensitivity time control). An electronically variable attenuator is placed in front of the first low noise amplifier in each channel; its insertion loss directly adds to the receiver’s noise figure. A multi-channel digital beamforming radar receiver requires good phase tracking between its receiver channels to minimise direction of arrival estimation errors. The set of electronically variable attenuators used for sensitivity time control need to track in phase over the control range. In this dissertation, sensitivity analysis is used to identify a set of optimal phase tracking electronically variable attenuators. A root sum square error measure is derived from the multiple output sensitivities of an electronic network. The error measure gives the expected RMS phase error within a set of networks. Applying the error measure to several electronically variable attenuators over the control range, the cascaded parallel quarter-wave attenuator is identified as having optimal phase tracking within a set of attenuators over control range. The cascade parallel quarter-wave attenuator is developed further and optimised through the application of sensitivity analysis. The final attenuator has excellent attenuation flatness, attenuation range, phase tracking and a simple biasing scheme. A multi-channel digital beamforming radar receiver also has to be protected against large signals. These large potentially damaging signals are either due to the radar’s own transmitted signal, or from other radars transmitting large amounts of power in the same frequency band. A receiver protector (e.g. a limiter) typically supplies this function. In a multi-channel digital beamforming radar, a compact circuit based high power limiter has many advantages in terms of space and cost when it is compared to a waveguide limiter. The compact high power limiter developed in this dissertation consists of PIN diodes implemented on a multi-layer printed circuit board. The circuit is referred to as an active PIN-Schottky limiter. The maximum power handling capability of the active PIN-Schottky limiter is determined by the PIN diode at the limiter’s input. The minimum attainable resistance is not given by the manufacturers, so that the diode’s minimum attainable resistance can not be found from the datasheet information. It is difficult to estimate how much power is dissipated in the diode when a large signal is incident. Through a temperature controlled measurement, the PIN diode’s voltage decrease as a function of junction temperature increase is measured. By fitting the PIN diode’s measured and simulated junction temperature increase, it is possible to extract the resistance of the diode when large forward bias is applied. Once the resistance is known, the power dissipated in the PIN diode can be calculated for different operating conditions.