Investigation of the roles of filters for a harmonic FMCW radar

In this paper, the importance of RF bandpass filters in a harmonic FMCW radar system is investigated. The harmonic radar system is designed using the National Instruments (NI) AWR Design Environment, and simulation is performed with the Visual System Simulator (VSS). The roles of band-pass filters are analyzed. System level simulation indicates that to detect the weak harmonic signal from a non-linear target, a band-pass filter on the transmitter path is necessary to block the harmonics of the carrier signal, a band-pass filter on the local oscillator path is important to block the fundamental frequency leakage, and another band-pass filter before the LNA on the receiver chain is indispensable to reject the fundamental frequency captured by the receive antenna. Moreover, the group delays of the filters need to be properly controlled for reliable range detection.

I. Introduction

The concept of harmonic radar has been investigated for decades. By detecting unintended harmonic returns caused by the inherent nonlinear properties of circuit elements in many RF and microwave devices, harmonic radars can pinpoint those devices without any cooperation from the targets. Because most naturally occurring objects do not exhibit significant nonlinear characteristics [1]–[3], this provides a useful electronic solution for counter intelligence and surveillance. However, there are still two major challenges for practical use of this technology.