Coherent processing tunnel diode ultra wideband receiver

Abstract

A four-terminal network in tandem with a tunnel diode (TD) threshold receiver currently used in radar or communications improves its sensitivity. Previous inventors have shown that the temperature and sensitivity properties of a conventional TD threshold device used for detecting very short duration bursts of microwave energy would be enhanced by appropriately biasing the TD by a current derived from the thermal noise; the current sets the TD operating point. The magnitude of the current is determined by a constant false alarm rate (CFAR) feedback loop. The subject invention recognizes that a TD changes states (i.e., a detection event) when the area under the current vs. time curve or the charge passing through the device exceeds a prescribed number of picocoulombs. To maximize the charge and improve detection, a form of superheterodyne conversion is introduced to convert the oscillatory short pulse microwave signal received by an antenna to a monopolar baseband signal. This is done by mixing the incoming signal with a CW carrier whose frequency is precisely chosen so that the resulting beat frequency is one-half of an rf cycle for the given duration of the microwave burst. This maximizes the charge available to trigger the TD. Proper gain and filtering is provided in the adjunct four-terminal network to establish minimum noise figure and appropriate gain to drive the TD held in a CFAR loop. It is shown both mathematically and experimentally that the subject invention can increase the receiver sensitivity by a factor of 20 dB or greater over the prior art. Proper microwave gain is provided to establish the noise figure prior to mixing in the added four-terminal network and a bandpass filter is employed to discard frequency components and the gain of a wideband IF (e.g., 0-500 MGz) is selected to appropriately drive the TD.