An Analysis of the Effects of Digital Phase Errors on the Performance of a FMCW-Doppler Radar
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An Analysis of the Effects of Digital Phase Errors on the Performance of a FMCW-Doppler Radar by Kurt Peek Picture: www.thalesgroup.com A thesis submitted in partial fulfillment Of the requirements for the degree of MASTER OF SCIENCE in APPLIED PHYSICS at The University of Twente Under the supervision of prof. dr. A.P. Mosk, prof. dr. W. Vos, and ir. R. Vinke September 2011 Abstract In modern frequency-modulated continuous-wave (FMCW) radars, the transmitter is increasingly being implemented using direct digital chirp synthesis (DDCS), which provides improvements in sweep linearity, stability, precision, agility, and versatility over analog techniques. Its main limitations are errors due to sampling of the modulating signal, phase truncation, and digital-to- analog converter (DAC) quantization, which produce spurious signals due to their deterministic nature. This thesis presents an analysis and simulation of the effect of two sources of digital phase errors – namely, the ‘staircase’ approximation of the linear frequency sweep and phase truncation – on the performance of a FMCW-Doppler transceiver which employs a DDCS in its transmitter. An upper bound for the amplitude of spurious targets resulting from these digital phase errors is established. Further, it is shown that provided the phase errors are periodic with the sweep period, the spurious targets are not offset from the target in Doppler. An algorithm for selecting the digital chirp parameters of a DDCS so as to ensure periodic and phase-continuous sweep transitions is devised. Finally, we investigate parallels of FMCW radar in the optical domain, and consider the fundamental question whether range resolution is fundamentally limited by the bandwidth of a transmitted signal, or its carrier frequency. 2 Table of Contents 1 Introduction .............................................................................................................................. 4 1.1 Linear frequency-modulated continuous-wave radar ......................................................... 5 1.2 Frequency sweep linearity.................................................................................................. 9 1.3 Digital chirp synthesis....................................................................................................... 11 1.4 This thesis ........................................................................................................................ 14 2 Theory of operation of a model FMCW-Doppler transceiver .................................................... 15 2.1 Description of the model system ...................................................................................... 15 2.2 FMCW Doppler signal processing ..................................................................................... 17 2.3 Direct digital synthesis ..................................................................................................... 33 2.4 Statement of the problem ................................................................................................ 41 3 An analysis of the effect of digital phase errors in the transmit signal on the beat signal spectrum ......................................................................................................................................... 42 3.1 Mathematical model of a FMCW transceiver .................................................................... 42 3.2 Model of the phase errors in the digital samples generated by a DDS ............................... 53 3.3 The effect of periodic digital phase errors on the FMCW transceiver ................................ 56 3.4 Form of the digital phase error ......................................................................................... 68 3.5 An upper bound for the effect of digital phase errors on the beat signal spectrum ........... 72 4 The effect of phase errors on Doppler processing .................................................................... 74 4.1 The effect of sinusoidal phase errors in the beat signal .................................................... 74 4.2 The effect of sinusoidal phase errors in the transmitted signal ......................................... 75 4.3 Verification by simulation ................................................................................................. 77 4.5 Conclusion ....................................................................................................................... 78 5 Implementing periodic and phase-continuous sweep transitions ............................................. 79 5.1 Introduction ..................................................................................................................... 79 5.2 Chirp DDS discretization constraints ................................................................................. 82 5.3 Selecting chirp parameters for CP frequency sweeping .................................................... 85 5.4 Algorithm for selecting DDCS parameters ......................................................................... 87 5.5 Concluding remarks.......................................................................................................... 90 6 Conclusions and discussion ...................................................................................................... 91 6.1 Summary of contributions to knowledge .......................................................................... 91 6.2 Implications for technological development ..................................................................... 92 6.3 Recommendations for further research ............................................................................ 93 Appendix: FMCW-Doppler simulation ............................................................................................ 100 3 1 Introduction “To see and not be seen” has been a cardinal principle of military commanders since the inception of warfare. Until the advent of World War II, the only means available to commanders from this point of view was espionage and intelligence gathering missions behind enemy lines. Just prior to World War II, the allies came up with a groundbreaking invention, the pulsed radar1 which radically changed the equation and for the first time one could see without being seen in true sense of the term. Figure 1 One of the 60 Chain Home radar sites employed by the Royal Air Force (RAF) during World War II. Three of the four transmitter aerials are visible to the left, while four receiver masts are grouped to the right. (Picture from http://spitfiresite.com/2010/04/early-radar-memories.html). The pulsed radar estimates target range by transmitting a sequence of pulses of radio frequency (RF) energy and measuring the time for echoes scattered off the target to return to the radar. One of its earliest embodiments, the Chain Home coastal surveillance system (Figure 1), could sight German fighter formations well before they reached the English coast and could therefore concentrate allied fighter groups where they were most needed. The German fighters were not even aware that they were detected. In effect, the pulsed radar acted as a force multiplier and helped the allies defeat the vastly superior Luftwaffe in the Battle of Britain. The allies pressed home their advantage of having radar by going on the win the Battle of the Atlantic against German U-boats, catching them unawares on the surface at nighttime when they were charging their batteries. The German reaction to these events was slow and by the time they came up with their own radars and radar emission detectors (now called intercept receivers) it was too little, too late to influence the outcome of the war in their favor (Willis and Griffiths 2007). After the war, understandably, radar engineers around the world concentrated on developing radar emission detectors (intercept receivers) during the Cold War that followed. This led to a “classic” situation in which intercept receivers had no difficulty detecting radar beams, and sometimes even their sidelobes, at long ranges. This is because the radar signals have to travel to the target and be partially reflected back to the radar, whereas the intercept receiver can intercept them after only a single trip. Since the ‘reflection’ of a radar signal generally involves the target intercepting only a small amount of the transmitted signal and scattering it over a wide volume, the effect is worse than 1 The word “RADAR” is an acronym for Radio Detection And Ranging, coined by the U.S. Navy in 1940. 4 just a doubling of the path length: it changes the fall-off in signal strength with range from a square- law process for an intercept receiver to one following a fourth-power law for a radar (Skolnik 2008). The intercept receiver does not have things all its own way, however, because the radar has control of its own transmissions, and knows exactly what the received signals should look like, whereas the intercept receiver has only an approximate idea of the radar’s characteristics, and must be general enough to be able to detect a wide variety of different radars. This means that whereas the radar receiver is optimally matched to receive its own transmissions, the ESM receiver is poorly matched to the radar (Fuller 1990). Low probability of intercept (LPI) radars – which as their name suggests, can be intercepted, but with a low probability – attempt to exploit this advantage by resorting to continuous-wave (CW) transmissions instead of pulsed