Using Tuned-Radio-Frequency Techniques on FIM Receivers

Tuned-Radio-Frequency (TRF) or direct detection techniques are significant for Field-Intensity-Meter (FIM) receivers because TRF applications involve a square-law detector response as contrasted with the many " mix products" encountered in superheterodyne receivers. Renewed interest in TRF techniques was sparked over a decade ago when low-noise RF amplifiers in the microwave bands became commercially available. This renewed interes t was prompted by the possibility of overcoming the sensitivity limitation which, coupled with the dynamic range limitation, had caused overwhelming favor of the superheterodyne receiver. Dynamic range is actually less of a limitation with FIM receivers than with operational receivers because FIM's are used essentially as test equipment, and the dynamic range can be extended via use of calibrated attenuators. Of course, such dynamic range extension is premised upon high sensitivity. It is this area of increased sensitivity that has received our attention. Typical MDS (minimum discernible signal) for usual operating radars is in the area of -105 dBm to -110 dBm and this level of sensitivity is higher than that encountered in most of our operational FIM's. Sensitivities of this order are achieved via usual superheterodyne techniques, although we have observed MDS's exceeding -130 dBm when low-noise, low power amplifiers are used in the pre-mixer stages of superheterodyne receivers. On TRF receivers, where signals are detected directly and without superheterodyne action or RF amplification, we have found that advertised MDS's of - 55 dBm to -60 dBm are difficult to achieve even across a single band and that an MDS of - 50 dBm across L, S, C, and X-bands approaches optimum. There are ways to improve upon these sensitivities, however, and the experimental data following is an indication of the possibilities. The breadboard FIM considered is diagramed in Figure 1. The crystal detector utilized is a standard 1 N21 C in a crystal holder which is matched carefully to the video amplifier. The low-noise traveling-wave amplifiers are the Watkins-Johnson WJ-268 series which operate at low power over octave regions from L-band through X-band, and the high gain traveling-wave amplifiers are the Huggins series of medium power, high gain tubes. It is noted that the same crystal detector and video amplifier are used for all frequencies although four sets of the tandem TWT' s are used to cover the L-band to X-band microwave regions. Figures 2 through 5 illustrate MDS sensitivities at various frequencies, and Figures 6 through 9 illustrate the same signals at tangential sensitivity levels. Basic dynamic range of the detector is illustrated in Figures 10 and 11 although effective dynamic range can be extended by using the calibrated variable attenuator.