232 BELL SYSTEM TECHNICAL JOURNAL 



The Heterodyne Method 



With the advance in carrier communication, greater refinement in 

 measurement became necessary, calling for still higher selectivity in 

 the analyzer. The best means for accomplishing this appeared to be to 

 heterodyne the wave under investigation in such a manner as to move 

 it to a lower position on the frequency scale. Then with a fixed tuned 

 circuit which would pass only the low frequency current corresponding 

 to the desired component, much greater selectivity might be obtained 

 because of the relatively greater spacing. 



To heterodyne the desired component there is required a separate 

 oscillator and a modulator in which the current to be measured and 

 the separately generated current are combined to produce a current of 

 lower frequency. This in effect translates the current under investiga- 

 tion from a high frequency to one of much lower frequency; retaining, 

 however, the relative amplitudes of the components. Since the amount 

 of this translation is determined by the frequency of the local oscillator, 

 a particular component can always be given a certain predetermined 

 value by adjusting the oscillator. This permits the use of a fixed 

 tuned circuit which is highly selective to the difference frequency in 

 the modulator output. By choosing a low value for this frequency it 

 is possible to make the percentage difference between this and interfer- 

 ing frequencies much larger than between the corresponding high 

 frequencies. In the present analyzer 800 cycles per second has been 

 chosen as a suitable value. If, then, the current to be measured had a 

 frequency of 20,000 cycles per second, the local oscillator would be set 

 at 20,800 cycles. It could of course also be set at 19,200 cycles if 

 desired and produce the same difference frequency. If there were also 

 present another current of say 20,500 cycles, the interval in the original 

 wave would only be 2.5 per cent; after heterodyning, however, it would 

 appear as a 300 cycle current, if heterodyned by the 20,800 cycle 

 current. The interval thus becomes nearly 40 per cent of the fre- 

 quency for which the tuned circuit is adjusted. If these currents are 

 heterodyned directly in a simple modulator, there is also the possibility 

 of modulation between components in the original complex wave. 

 This would result, in the case chosen, in a current having a frequency 

 of 500 cycles, but the percentage difference between the 800 and either 

 the 500 or the 300 cycle currents is many times greater than that be- 

 tween the original high frequency currents so that the fixed tuned 

 circuit would have a very high discrimination to the interfering current. 



In some cases the intermodulation between components of the origi- 

 nal wave may coincide with the component being measured, or may 



