Design of Undersampled Digitally Heterodyned SFGPR with Variable Sampling Frequency

In this paper the design of an innovative architecture of an undersampled digitally heterodyned stepped frequency GPR is presented. The whole set of parameters characterising the performances of the SFGPR is optimised by means of a \brute force" approach by using the software tools previously developed in a high level language. The results of the simulations performed as well as the key aspects of the design are presented. It is well known that the capability of electromagnetic waves to propagate beyond the physical dis- continuities of propagation media makes it possible to exploit them to investigate internal features of dielectric bodies. From this property, an endless number of practical applications have been arisen, ranging from medical prospecting to detection of mines, nondestructive testing of industrial items and GPR applications. In (1) a new architecture of an Undersampled Digitally Heterodyned SFGPR with variable sampling frequency was presented. The key aspects and the advantages of the new architecture were presented and discussed: signal generation by means of DAC; undersampling of the echoes by means of a large bandwidth ADC with a planned step by step varying sampling frequency; digital quadrature demodulation of the undersampled echoes. In this paper a preliminary design of a SFPGR based on the previously proposed architecture is shown. The whole set of parameters characterising the performances of a GPR (step frequency, flrst frequency to generate, number of steps, :::) is optimised by means of a \brute force" approach by using the software tools previously developed in a high level language (LabView TM ). The results of the simulations of the whole post-processing chain, from the undersampling of the step frequencies to their quadrature demodulation and processing to synthesize simulated target, are shown to flx the performances achievable with the chosen set of parameters. 2. PROPOSED ARCHITECTURE The choice of a stepped frequency GPR (SFGPR) has been adopted for this design because it has several advantages with respect to the traditional impulsive GPR systems spacing from the larger dynamic range to greater signal to noise ratio attainable due to the narrow band of the receiver. The choice of an heterodyne architecture for the proposed design allows to reduce both its weight and power consumption. An heterodyne GPR detects targets by performing a downconversion of the received signal to a constant intermediate frequency (IF). Because this IF is greater than 0Hz, this architecture al- leviates the problems related to ∞icker noise and DC values drift with the temperature typical of homodyne receivers. The problems related to the IQ demodulation (errors due to phase and ampli- tude gain imbalances of the quadrature mixer) can be solved if it is performed in the digital domain instead of the analog one by acquiring directly the IF signal. Undersampling is the optimum choice that allow to solve both the problems: the downconversion of the signal is performed automatically (as result of the sampling process) and the signal is sampled optimally without the need to over- sample it (in fact the frequencies of the tones transmitted are in the order of 100MHz{400MHz that would require a sampling frequency of 250MHz{1GHz, well above the few kHz needed to ac- quire the transmitted bandwidth according to the Nyquist criteria for bandlimited signals). Once acquired the echoes are digitally downconverted (quadrature downconversion) to determine their phase.