We design a time domain microwave system dedicated to medical imaging. The measurement accuracy of the system, that is, signal-to-noise ratio, due to voltage noise and timing noise, is evaluated. Particularly, the effect of coupling media on the measurement accuracy is investigated both numerically and experimentally. The results suggest that the use of suitable coupling media betters the measurement accuracy in the frequency range of interest. A signal-to-noise ratio higher than 30？dB is achievable in the range of 500？MHz to 3？GHz when the effective sampling rate is 50？Gsa/s. It is also indicated that the effect of the timing jitter on the strongest received signal is comparable to that of the voltage noise. 1. Introduction As a potential technology for object detection and identification, ultrawideband (UWB) microwave imaging for medical applications has been a subject of extensive research in the past few years [1–6]. Microwave imaging is carried out by sending microwave signals into an object under examination and receiving reflected or scattered fields. The received signals are processed to produce an image of the object. The object response under the microwave excitation is dependent on the illuminating frequency; therefore more information about the object’s property can be collected with a UWB illumination in comparison with a monofrequency case. Moreover, the conflict between spatial resolution and penetration depth, which is the technical challenge for monofrequency microwave imaging, can be resolved by using UWB technology. Most UWB microwave imaging systems are based on commercial instruments, for example, a vector network analyzer (VNA) [1–4], a sampling or a high-speed real-time oscilloscope [5, 6]. While these instruments provide ready solutions for experimental purposes, their high cost and massive size make these solutions not applicable for practical use. Therefore, it is desirable to have a custom-designed UWB microwave imaging system with compact size, low cost, and high speed. In comparison with a stepped-frequency technique [7], pulsed time domain measurement technology is preferable for UWB system design due to a more simple system architecture and a higher measurement speed. The generation of UWB pulses with duration less than hundred picoseconds (the power spectrum covers the band of interest for biomedical applications) has become possible due to the fast development of advanced solid-state technology. Despite the advantages, time domain systems have worse signal-to-noise ratios (SNRs) in comparison with frequency domain
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