Highly Compact Time-Domain Borehole Radar With Time-Varying Gain Equivalent Sampling

In this article, a compact time-domain (TD) microwave system for borehole radar (BHR) is developed based on cost-effective microwave hardware. The time-varying gain equivalent sampling (TVGES) technique is proposed in the receiver, which consequently achieves an equivalent sampling rate of up to 33.3 GSa/s and a bandwidth of 618 MHz, enabling undistorted sampling of ultrawideband (UWB) pulse signals. Additionally, other components including pulse generator, antennas, and control circuits are purposely designed, and finally the complete system is integrated into a 120-cm-long polyvinyl chloride (PVC) pipe with a small diameter of 5.2 cm. The prototype can be operated with a portable android device (PAD) using a customized Ethernet cable, through Wi-Fi, or just working offline. Due to the proposed TVGES technique, the system is able to suppress disturbing early-arriving waveforms while effectively enhancing the reflections from targets. The performance of the developed system is evaluated for various measurement scenarios, especially by comparing with a measurement setup without TVGES. It shows that the TVGES-based system has a dynamic range of 61 dB and a signal-to-noise ratio (SNR) of up to 40 dB, and it can improve the SNR of late-arriving reflections by about 13 dB. To verify the proposed TD microwave BHR system, preliminary experiments are conducted, which show that the system can successfully detect a small target that is placed 0.4 m away from the BHR.