An Impulse Interference Blanker for Communications Receivers

Abstract

The necessity for adequate and reliable communications for military operations has always existed. Today, with the trend toward everincreasing specializations of military forces and the continued growth of complex systems of radar, fighter control, etc. , this requirement has become more urgent. Since items of communication apparatus are often in close physical proximity to facilitate coordination, the equipment has almost invariably been subjected to detrimental effects from mutual interference. When communications receivers are placed close to radars, impulse interference may occur, which is at times severe enough to cause complete loss of intelligibility. Such interference may be introduced into a receiver through various channels: a. Shock excitation of the tuned circuits of the receiver by the large modulatorgenerated d-c pulses. b. Direct pick-up of the radiated r-f radar pulses. c. Feed-back through common power supplies. d. Non-linear detection of the r-f pulse. As a remedial measure, the ground communications equipment has often been located as far as one-half mile from the radar sites. Such physical separation naturally increases the problems of remoting, maintenance, real estate procurement, additional buildings, heating, etc. Where airborne equipment is involved, such remoting possibilities obviously cannot be plausibly considered. Therefore, as the most effective method of reducing the effects of pulse interference to a minimum or negligible value, the impulse interference blanker for communications receivers was developed. Study and investigation by engineers of the Interference Analysis and Control Section of the General Engineering Laboratory, RADC, has shown that (a) above is the most common type of interference. However, the impulse interference blanker as developed will eliminate impulse interference no matter how it is introduced into the receiver. The blanker facilitates close-in operation of communications receivers with high-power pulsed equipments of present and projected design. It is so synchronized with the interfering pulse that it generates a blanking gate which is applied to the local oscillator of the affected receiver. Such a measure greatly attenuates the interfering impulse.