Radio doings (Dec 1930-Jun1932)

Page Twenty-eight RADIO DOINGS January, 1931 THE JXJPEC-HETECCDyNE By K. G. ORMISTON The word "Super-heterodyne" is before the publie today in an ever increasing degree. The advertisements of many of the leading radio manufacturers blazon this intriguing word across pages of advertising as though to hypnotize the beholder. And its elegance spellbinds, because even though you may be but casually interested in radio, there is a vague idea in the mind that this word signifies a circuit which represents the acme of perfection in receiving circuits. It is timely therefore, that we stop and analyze the '"super," review its advantages and disadvantages, and gain a mental picture of what the name means when expressed in terms of actual radio performance. The super-heterodyne system of reception is briefly as follows: The received signal frequency is combined with a second frequency, which is generated within the receiver. The resultant frequency, which is still an inaudible radio frequency, is then amplified through an RF amplifier, the tuning of which is fixed and constant. A second detector and audio channel completes the circuit. This method of reception was patented by Armstrong in 1920. The greai. advantage of Armstrong's invention was in the fact that the frequency which resulted from the heterodyning or combining of the signal and locally generated frequency, could be made low enough to be amplified with satisfactory efficiency by the tubes of that day. Tubes with an appreciable degree of inter-electrode capacity do not amplify the higher frequencies as well as the low. As applied to modern practice, the super-heterodyne has three important advantages. First, an RF amplifier may have a vastly greater gain at a frequency such as 175 kilo cycles than at broadcast frequencies. Second, an RF amplifier which functions always at but one frequency with accurately tuned circuits may have greater efficiency than one which must be constantly variable over the broadcast band. Third, the receiver may have a greater and more constant selectivity than is possible with other types because of the highly selective circuits which may be used with the amplifier (due to their being fixed and not constantly variable) and because energy of this frequency is obtained by adjustment of the local oscillator as well as tuning to the received signal. In terms of performance, these features result in a high sensitivity and selectivity. These two vital qualities have ever been the strong points of the "super," but have been usually acquired only in exchange for certain disadvantages. Chief among the disadvantages is the presence of harmonics, "image frequency" interference, and "repeats" of the locals, due to the many possible combinations of frequencies between the received signals, the locally generated frequency and the harmonics of both. The second important disadvantage is the ability of the local oscillator to reduce heterodyne interference in the neighboring receivers. The Radio Corporation of America has passed through many years of development work in super-heterodyne receivers, and only now, in conjunction with General Electric and Westinghouse, have they produced a series of supers which are devoid of harmonics, do not radiate, possess real 10 kilocycle selectivity and a sensitivity of a fraction of a microvolt per meter. In order to accomplish the satisfactory elimination of the disadvantages of the super-heterodyne they have used a tuned link circuit and a signal frequency amplifier ahead of the first detector or mixing tube. And we do not believe that it can be accomplished in any other way. It is vitally important that the mass of super-heterodyne receivers which will be sold this year be absolutely free from the radiation evil. If not, it is likely that we will see city ordinances passed prohibiting the sale or use of such sets. It is equally important that there be no harmonics cluttering up the dial, for the public will not stand for this type of interference. For the benefit of the technically minded, we are reproducing the circuit diagram of the new Stenode Radiostat receiver perfected by Dr. Robinson of London. This circuit is more or less upsetting modulation theories, and radio receiver design. Heretofore great care has been exercised in receiver design to provide for a band pass of 10 kilocycles width, in order that the sidebands may be passed intact, thereby preserving the tone quality of the set. Dr. Robinson, working on the basis that modulation constitutes a variation in amplitude of the carrier frequency, and that the side bands are a by-product and unnecessary in the receiver, has inserted a quartz crystal in the last radio frequency circuit of a conventional super-heterodyne. This crystal is arranged in a bridge circuit so that energy reaching the grid of the following tube can come through the crystal only. This crystal passes a frequency band not more than 50 cycles in width. And yet, this set possesses tone quality on a par with our best receivers! It was recently demonstrated to the engineers of the Federal Radio Commission, Bureau of Standards and others in this country. While it is a high priced laboratory experiment at present, it has great promise. Selectivity on the order of 50 cycles or less would not only solve the interference problem, make possible the operation of thousands of stations in the present band without interference, but also meet television's greatest obstacle. The name "Stenode Radiostat" means "narrow-path radio device," and it is surely well named. Robinson's Stenode Radiostat Circuit