Comparing baseband and intermediate frequency FMCW radar receivers

Meyer, Bernard (2015-12)

Thesis (MSc)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Frequency modulated continuous wave (FMCW) radars have a large range of applications in the civilian and military sectors, including aircraft navigation (radio altimeters), subsurface examination, weather monitoring, and many more remote sensing applications. In general receivers have limited dynamic range. Therefore, FMCW radar receivers can saturate due to large reflections from close range targets or poor isolation between the radar system’s transmit and receive paths. It is hypothesised that certain radar receiver topologies are more suited to coping with saturation. Therefore, the intermediate frequency (IF) and the commonly-used baseband (or zero-IF) FMCW radar receiver topologies are compared in terms of component nonlinearity. Receiver saturation generates unwanted signals that can result in the detection of false targets. The study proposes that an IF receiver, under specific operating conditions, can suppress unwanted harmonics and intermodulation products (IMPs) much more effectively than an equivalent baseband receiver. A detailed IF receiver design is presented that includes a discussion on image rejection, signal quantization, anti-aliasing, and sensitivity frequency control. It is found that the IF receiver is immune to the detection of false targets caused by poor isolation between the demodulator’s local oscillator and radio frequency ports. A noise analysis shows that the IF receiver’s noise performance is potentially greater than that of an equivalent baseband receiver due to pink noise that has a 1/f power spectral density, which decreases with an increase in operating frequency. The non-linear behaviour of the IF receiver is investigated by analysing the circuit components that have the highest risk of saturation in the receiver chain. The magnitude of harmonics and IMPs are determined using an algebraic approximation of a non-linear model presented in the study. The model is used to predict under which conditions false targets will be detected. It is found that the zero-IF receiver is susceptible to distortions that are suppressible by an IF receiver. Though the application has limitations with regards to the received signal characteristics, the IF receiver has better or equivalent performance when compared to a baseband receiver.

AFRIKAANSE OPSOMMING: Frekwensie gemoduleerde kontinue golf (FMCW) radars het verskye toepassings in die privaat en militêre sektore, insluitend vliegtuig navigasie stelsels (radio altimeters), grond penetrasie radars, weer monitors, en nog vele afstandswaarnemingstoepassings. In die algemeen het ontvangers ’n beperkte dinamiese bereik. Dus kan FMCW radar ontvangers versadig as gevolg van groot weerkaatsings vanaf naby geleë teikens of swak isolasie tussen die radarstelsel se transmissie en ontvang paaie. Dit word gepostuleer dat sekere radar ontvanger topologieë meer geskik is vir die hantering van ontvanger versadiging. Daarom word die intermediêre frekwensie (IF) en die meer algemene basisband FMCW radar ontvanger topologieë vergelyk in terme van komponent nie-lineariteit. Ontvanger versadiging genereer ongewenste seine wat as vals teikens gesien kan word. Die studie wys dat ’n IF ontvanger ongewenste harmonieke en intermodulasie produkte (IMPs) kan onderdruk wat ’n soortgelyke basisband ontvanger nie kan nie. ’n Gedetailleerde IF ontvanger ontwerp word voorgelê met ’n bespreking oor beeld verwerping, sein kwantisering,anti-aliasering, en frekwensie sensitiwiteit beheer. Met die gebruik van ’n IF ontvanger is dit bevind dat vals teikens weens swak isolasie tussen die demodulator se plaaslike ossillator en radio frekwensie poorte nie veroorsaak kan word nie. Die ontvanger ruis analiese toon dat die IF ontvanger beter ruis gedrag het as ’n ekwivalente basisband ontvanger, omdat pienk ruis met ’n 1/f drywingsdigtheidspektrum verminder met 'n toename in frekwensie. Die nie-lineêre gedrag van die IF ontvanger word ondersoek deur die komponente met die hoogste risiko van versadiging in die ontvanger-ketting te ontleed. Die grootte van harmonieke en IMPs word bepaal met behulp van ’n algebraïese uitbreiding wat ’n nie-lineêre model beskryf. Hierdie model is toegepas om te voorspel onder watter voorwaardes vals teikens bespeur sal word. Dit is bevind dat die basisband ontvanger vatbaar is vir distorsies wat die IF ontvanger kan onderdruk. Alhoewel die toepassing beperkings het met betrekking tot die ontvangssein se eienskappe, is die IF ontvanger beter of gelykstaande aan ’n soortgelyke basisband ontvanger.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/97858
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