A New Type of Multiple Tuner 



By Thomas W. Benson 



THE multiple tuner has in actual 

 practice shown its superiority over 

 all other common methods of tuning and, 

 despite its selectivity, it is rarely found 

 in the amateur's station. 



A multiple tuner is one in which an 

 intermediate circuit is used to transfer or 

 carry the energy from the aerial circuit 



Marconi 



A Diagram of the Marconi Scheme for a Multiple 

 Tuner. 



to the detector and 'phone circuit. The 

 advantage of this arrangement lies in the 

 fact that as the intermediate circuit has 

 little or no damping it oscillates longer 

 even when energized by highly damped 

 waves in the aerial circuit. These os- 

 cillations, having a small decrement, can 

 be sharply tuned by the detector circuit, 

 resulting in great selectivity. 



The disadvantage, however, of the 

 multiple tuner has been in the room it 

 takes up, requiring as it does two sets of 

 variable, inductively coupled coils, be- 

 sides the various variable condensers. To 

 overcome this Prof. Cohen, of the Bu- 

 reau of Standards, has brought forward 

 a tuner possessing all the selectivity of 

 the Marconi type with only two in- 

 ductances, and he secures loud signals, 

 due to his extremely close coupling. 



In Fig. I are shown both the Marconi 

 and Cohen circuits, simplified, of course, 

 to enable them to be understood at a 

 glance. 



In the Marconi hookup the inter- 

 mediate circuit consists of inductances S 

 and P 1 which are of fixed value, and 



variable capacity VC which is used to 

 tune the circuit. 



In the Cohen circuit instead of induc- 

 tively coupling the intermediate circuit 

 he connects it direct, thus coming back 

 to the double slide tuner days. His ap- 

 paratus actually consists of nothing but 

 two tuners connected together with the 

 variables. The middle circuit in this 

 case, therefore, is formed by 2, F 1 , 7, 8, 

 F 2 , 3 and F 3 . This circuit is closely 

 coupled both to the aerial and detector 

 circuits and this explains the louder sig- 

 nals. 



The condensers are used to tune this 

 intermediate circuit, but it will be noticed 

 that F 3 has a switch to cut it out and by 

 following the next few paragraphs care- 

 fully it will be shown how it is possible 

 to design a multiple tuner that will not 

 need any condensers in the intermediate 

 circuit. 



Consider F 3 switched out of the cir- 

 cuit, add series capacity by turning F 1 



Cohen 



Wiring Plan of the Cohen Circuit for a Multiple 

 Tuner. 



and F 2 and as the capacity is added the 

 wave length, to which the circuit will 

 respond, will become shorter and shorter 

 until a point will be reached, provided 

 there is sufficient capacity, when the in- 

 ductances will be offset by the capacities 

 and a circuit will have, practically speak- 

 ing, no capacity or inductance. That is, 

 although both are present, they counter- 

 balance each other and neither has any 

 effect on current flowing in the circuit. 



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