Radio broadcast .. (1922-30)

Problems of Receiver Design How the Super-heterodyne Combines Sound Elements of Design — Theory of Construction of the "Super" WHAT MAKES THE WHEELS GO 'ROUND: X BY WALTER VAN B. ROBERTS IN THIS interesting article, part of Mr. Roberts' series of clear explanatory articles about radio and all its works, the author has not tried to cover the entire field of receiver design, but he has explained some technical points about the super-heterodyne receiver so well that no reader who prides himself on his theoretical knowledge should miss it. This is the tenth article by Mr. Roberts which explains radio theory and practise in his own clear fashion. This first appeared in our magazine for March, 1924. — THE EDITOR. IF ALL transmitting stations used the same wavelength (and took turns working) reception would be a much simpler problem. For the receiving set might have as many tuned circuits as the designer had any use for. The owner of the set would not need to change any of the adjustments so there would be no disadvantage, from the operating point of view, in having a very complicated circuit. The idea of the super-heterodyne is simply this: build the very best possible receiving set to work on a fixed frequency (what ever frequency it is easiest to work with), then build a frequency changing device that receives signals on a tuned loop or other antenna and changes their frequency to that for which the receiving set is designed. When tuning-in a station with this combination only two adjustments are necessary: the loop should be tuned to the incoming radio waves, and the frequency changing device should be set to change the frequency to that at which the receiving set is designed to work. Yet in both sensitivity and selectivity this combination has all the advantage given by the large number of tuned circuits and many stages of amplification that can be used in the fixed frequency receiving set. It is possible that other circuits may be invented in the future that are cheaper or easier to build than a super-heterodyne, but it seems impossible that any could be made to work better because the super-heterodyne can be made ideal from the operating point of view. By taking the trouble to cut condenser plates very accurately to special shapes, the two condensers that have to be changed in tuningin different stations could be worked simultaneously by a single knob, which leaves nothing to be imagined in the way of simplicity of tuning. 78. BAND FILTERS IF THE fixed frequency used is fairly low, the fixed frequency receiver can be made to respond equally well to a band of frequencies sufficiently wide for high quality, and yet not respond at all to frequencies lying only slightly outside of this band. This is ideal selectivity and is achieved by the use of a band filter in the fixed frequency receiver. A band filter u £ FREQUENCY CHANGING MECHANISM BAND FILTER SENSITIVE RECEIVING SET FOR FIXED FREQUENCV FIG. 65 is a complicated combination of inductances and capacities that allows free passage of frequencies lying inside a given band, but stops all frequencies lying outside that band. Band filters can not be made to select a narrow band from among very high frequencies and so can only be used after the frequency changer has changed the incoming radio signals down to the lower frequency at which the receiver is designed to work. Fig. 65