Estimating the sensitivity and linearity requirements of an OTH radar receiving system

A desirable goal for an over-the-horizon radar (OTHR) receiver chain is to avoid degradation of available signal to noise ratio over unused channels within a nominated band of interest. If there is a design requirement for the receiver to perform instantaneously wideband, this can present a significant challenge as the dynamic range of signals within the high frequency (HF) band in the form of large broadcast signals, by way of contrast to the very low spectral densities characterizing cosmic noise, can easily exceed the linear range of components in a typical receiver chain. In narrowband systems, this has been handled in various ways by constraining the bandwidth impinging on sensitive downstream components, with a highly linear bandpass filter, or custom mixer and filter. Even in narrowband systems care must be taken to ensure that the receive chain internal noise and nonlinear products do not significantly impact at operational frequencies of interest. More recently, with the advent of direct digital receivers (DDRx) based around high speed high dynamic range analogue to digital converters (ADCs), a desire to operate OTHR with wide open front ends (WOFE) has emerged. In a WOFE DDRx there is no bandpass filtering within the operational range of the receiver. The principle is to enable multiple simultaneous functions using the large radar receive array, e.g. the usual narrowband radar with a complementary wideband frequency management support (FMS) system [1]. This goal demands simultaneous access across the full spectrum and therefore constrains the filtering solution and increases the technical demand on the receiver components. As discussed in Barnes et. al. [2] the general application of Commercial-Off-The-Shelf (COTS) DDRx to attempt WOFE at HF will generally result in some level of system imposed performance limiting. The use of customized, dynamically adaptive front-end gain stages, including the use of equalization, where the gain is a tailored function of frequency, can allow the receiver to approach a satisfactory WOFE solution in some applications, e.g. the spectral environment evaluation and recording [2] receiver.