Detecting and identifying spurious signals generated in radar receivers

HF radar receivers require the suppression of inherent spurious signals (spurii) far below the IEC specified level. Measurements of the appropriate sensitivity are often thwarted by spurii originating in test equipment, making it difficult to identify those spurii which originate in the receiver. A technique is proposed which allows spurii generated from harmonics of the tuned local oscillator to be differentiated from those originating from sources external to the receiver. The method involves stepping the tuned frequency of the receiver through its range with the receiver input terminated in a dummy load and examining the receiver output. The tuning frequency steps of the receiver are made much smaller than the output bandwidth so that spurious signals generated due to harmonics of the local oscillator are not missed in the scan of the receiver´s RF range. The method requires the search to be automated with the receiver local oscillator and the spectrum analyser at the receiver output under computer control. Spurii detected by the spectrum analyser may be analysed immediately by the computer. Signals which are detected on successive scans of the baseband may have their origins identified, at least in part, by noting their shift in frequency across the output band from one scan to the next. A shift of N times the tuned frequency increment indicates that the N^th harmonic of the tuning local oscillator is responsible. A shift equal to the increment indicates that the fundamental of the LO is involved or alternatively that the spur source is external to the receiver