Radio broadcast .. (1922-30)

Problems of Receiver Design 743 shows schematically the operation of the super-heterodyne system. 79. HOW THE FREQUENCY-CHANGER WORKS FIGURE 66 shows a typical frequency changer. Let es sin st be the voltage of signal frequency picked up by the tuned loop. Let eh sin ht be the voltage of the heterodyne oscillator's frequency picked up by the small coil coupled loosely to the heterodyne oscillator. The grid potential of the tube is the sum of these two and the C battery voltage, so the plate current will be i,, = K [B + n (C + es sin st + eh sin ht)|+ (small terms that we need not consider here.) = K [(B + M C) + /* (es sin st + eh sin ht)]2 = K (B + jLtC)2 which is direct current -f-2k/x (B + /xC) (es sinst -f eh sin ht) which are amplified currents of the signal and heterodyne frequencies. + k/z (es sin'2st -f eh2 sin -ht) which reduces to direct currents and frequencies twice the signal frequency and twice the heterodyne frequency. +2kju2es sin st Ch sin ht which is the only term we have any use for, because it splits up into two parts, one of them — Kju2eseh cos (s + h ) t which is the sum of the signal and heterodyne frequencies, and of no interest to us, but the other is k n-esQh cos (s — h) t which is the new frequency that we are going to use. The tuned circuit that connects to the fixed frequency receiver picks up only this frequency. It is obvious that this new frequency can be adjusted to any desired value by simply adjusting the frequency of the heterodyne oscillator. For instance, suppose the fixed frequency receiver is built to work at 100,000 cycles and the radio waves are coming in at a frequency of 1,000,000. If we adjust the heterodyne to oscillate at 900,000 cycles the new frequency will be the difference of the two, or 100,000, which is just right to be picked up and received by the fixed frequency set. On the other hand if the heterodyne oscillator is adjusted to 1,100,000 the difference will again be 100,000 so that there are evidently always two possible settings for the heterodyne condenser either of which produces the proper frequency for the fixed or intermediate frequency receiver. Sometimes there is less interference experienced when using one of these settings than the other but usually it makes no difference. From the coefficient of the new frequency term it is evident that its strength depends upon the amount of voltage picked up from the heterodyne oscillator as well as the signal itself. Hence this should be made large by making the coupling to the heterodyne oscillator sufficiently close. The C battery voltage should be greater than the heterodyne voltage in order to keep the grid at all times negative. The best C and B battery voltages and best coupling can be most simply found by experiment. 80. PROBLEMS OF "SUPER" DESIGN IN THE actual construction of a superheterodyne, we are caught between two fires. On the one hand, if we build the fixed, or intermediate-frequency receiver to operate at a fairly high frequency, say one or two hundred kilocycles, we will have difficulty in making it sensitive and selective enough. On the other hand, if we use a very low intermediate frequency, say 30 kilocycles, we are TO HETERODYNE OSCILLATOR -L. TO FIXED FREQUENCf RECEIVING b£T THESE TWO TUNED CIRCUITS TAKE THE PLACE OF THE BAND FILTER WELL ENOUGH FOR MOST PURPOSES FIG. 66 likely to run into two troubles. The first is that the quality tends to be bad on account of the selectivity being too great, and the other is that unpleasant complications occur in operating the set, due to the signal and heterodyne frequencies being so nearly equal. As the heterodyne condenser is varied there may be a click when the heterodyne frequency passes the value for which the signal circuit is tuned. Also, the same setting of the heterodyne condenser will often bring in two different stations at once whose frequencies are really different by twice the intermediate frequency used, and when the latter is very low, these two frequencies are too close together for the signal circuit or loop to select one to the exclusion of the other. In view of these considerations three courses seem to be open: (i) to effect the best compromise between the advantages and disadvantages of high and low intermediatefrequency amplification, the choice depending upon what is desired of the set and the location where it is to work, (2) to use the best intermediate frequency for amplification and quality and use a frequency changing device employing special circuits so arranged that only