Radio age (Jan-Dec 1925)

78 RADIO AGE for August, 1925 The Magazine of the Hour Here You Are I A Real Receiver A New 8-Tube Super-Heterodyne By JOHN B. RATHBUN SO much has been printed in RADIO AGE upon the elementary principles of the super-heterodyne that it seems hardly necessary to enter again into the theory in much detail. Briefly, the super-heterodyne is a special form of radio frequency circuit in which radio frequency amplification takes place at a much longer wavelength than that of the incoming radio waves, thus reducing the losses in the tubes and R. F. transformers and adding to the efficiency of the set. Broadcasting wavelengths ranging from 200 to 600 meters are converted into wavelengths approximating 10,000 meters before the waves enter the radio frequency stages. After amplification, the waves then are rectified by the usual detector tube producing audible signals which can be further amplified by one or more audio stages. A typical eight tube super-heterodyne of the type to be described consists of the following principal unit divisions of tubes: (1) The first detector tube. (2) Three radio frequency amplifying tubes. (3) One oscillator tube used as a frequency changer of the heterodyne type. (4) One second detector for rectifying the output of the radio frequency stages and thus producing audible signals. (5) Two audio frequency stages for increasing the volume of the audio component so that a loud speaker can be used. The Detector Tube TN a certain respect, the first detector -* tube (1) can be considered as a radio frequency amplifying stage, and in some circuits is used exclusively for this purpose without the conventional grid condenser and leak, but when iron core radio frequency transformers are used the detector tube is of advantage in providing an audio component in the Blueprints for less of the incoming radio frequency. AC, . u „ ■*. TL.^.4 /^.'r,, ^ r. The oscillator condenser forms one of ouper 1 hat Uives the Distance and Tone first stages of the circuit. It should be noted that the R. F. tubes and transformers work normally at a frequency which is not very much greater than the higher audio or voice frequencies, and therefore a certain amount of audio amplification is also possible in the radio stages, when iron core transformers are used at this point. With air core transformers the audio component receives little if any amplification in the radio stages, and hence under these conditions the rectification of the first tube (1) is not of importance and can be considered and used as a strictly radio frequency stage. The advantages and disadvantages of either system are still a matter of some dispute. We now come to the oscillator tube (3) by which the wavelength or frequency of the incoming waves is converted into the desired value for use in the radio frequency stages. Really this tube is an independent unit as far as the rest of the circuit is concerned, for it does not enter directly into the amplification or rectification of the waves. It simply produces a series of independent, continuous oscillations, which are combined with the incoming radio waves to form a third series of oscillations having a greater wavelength or lower frequency than either of the original series. This method of changing frequencies is known as heterodyning. The third wave is amplified by the succeeding radio stages. The frequency of the oscillations set up by the oscillator tube is determined by an inductance coil and a variable condenser in such a way that a constant frequency is maintained in the R. F. stages, regard le two tuning controls of the circuit. The R. F. Transformers ALL of ■** former the radio frequency transrmers operate at a constant fixed wavelength or frequency at which maximum amplification takes place, regardless of the frequency of the broadcasting station which may then be tuned in. This optimum frequency is determined by the windings of the transformers and more particularly by the filter condensers placed across the first or last R. F. transformers. The transformers are therefore sharply tuned to a single definite wavelength or frequency to which the heterodyned waves must be adjusted by means of the oscillator variable, condenser. As the transformers will not respond to any other frequency than that determined by the filter, the oscillator condenser becomes an effective tuning control, which in connection with the aerial or loop condenser insures a high degree of selectivity. All the radio frequency transformers, whether of the air core or iron core type, are of the "long wave" design especially designed for the super-heterodyne circuit. In the circuit illustrated, the transformers are designed for operation on approximately 45 kilocycles (45,000 cycles frequency) which corresponds to a wavelength of 6,000 meters. This is not a fixed standard frequency which must be maintained strictly in all cases, but has been found in practice as the most effective compromise for the circuit at hand. Actually, the waveband may extend from 1,200 to 25,000 meters in many circuits, but at either extreme certain objectionable features appear which make the 6,000 meter type the logical compromise for iron core transformers. (Turn the page) the New Super-Heterodyne on Pages Following gggggggggooeeaogBe^ Like a Woman s Heart— Always room for one more subscriber in the RADIO AGE family. Each month prizes are to be given for those who send in the largest B number of paid up yearly subscriptions to this magazine. Details of the contest are to be found on page 18. 0 Summer-time is the time to go after your prospects ; neither you nor S the other fellow is too busy. And with this DeLuxe edition as a fore runner of the good things you may expect form RADIO AGE in the 0 future, you should have no difficulty in bringing more radio recruits 8 into the ranks. jbaeoaaooeeoggoeseaBoefflegaggoeagaoaooflgggflgoesoggasegegBi.'