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Show Device That Aids Superheterodynes Corrects Weakness of Bringing Bring-ing in Signals on Tvo Places on Dial. By F. I. ANDERSON in QST The hook-up herewith presented provides an automatic frequency changer foe. superheterodynes ; and at the same time does away with the troublesome double beat, that irritat-; irritat-; ing weakness of the super in bring-I bring-I ing In a .signal at two points on the oscillating dial. J The oscillator In this case is fixed, I and long-wave, its frequency being matched to tho t '. fi -mors chosen. I I 1 1 " Qwy" I l i ?i a i i 1 j y Li ox-1' Capacity-Impedance Bridge Is Shown in the Diagram. Referring to the diagram, tube P, I is the "first detector"; and tube A is a t.uned radio rreo,neney stage preceding, preced-ing, it. Tube C is the long wave oscillator os-cillator tuned to the peak of the transformer trans-former used. . Connected in Parallel. Tubes A and ('. are connected in parallel across the loop, the tuned iVj"' dividing ):':twoen them. The fraction of titv .RiP"l mf": taken by A passes on to Il9 fiFSt (?t(:f!0I' : at radb frequency. The fraction of the signal energy taken by C is mixed with the long wave oscillations; therefore there-fore the output of O (being the subtrahend sub-trahend of the radio frequency and long wave frequencies) becomes automatically au-tomatically the desired heterodyne obtained ob-tained by manual tuning In the conventional con-ventional super. This Is passed on to the lirst detector, B, by Hie usual pick-up. As an example, let us say the long wave oscillator is set for 50 kilocycles. kilo-cycles. The tuned signal is. say of (100 kilocycles. The fraction passing through A is handled at radio frequency. fre-quency. The fraction, taken by 0 is heterodyned on the grid of this tube, ami becomes GOO-oO, or i50 Ke. This output is passed on to the lirst detector, de-tector, whose output becomes conventionally con-ventionally 'GOO-ooO, or f.0 kilocycles, the frequency desired for amplification amplifica-tion via the intermediate frequency transformers Simple Method of Mixing. The capiieily-impedance bridge is shown in the diagram as a simple imMbod of mixing the fixed component and the signal frequency without troublesome. ' reaction between tubes A and C. A number of other ways suggest themselves separate local oscillator, or a common feed !.'-ok. The quartz crystal suggests a fas- Cinaling Held of experiment, This automatic frequency changer may apply lo any tuning circuit for long-wave transformer amplilicntihn. In achieving its automatic feature and doing away with the troublesome double dou-ble beat of the super, it sacrifices the inherent selectivity of the tunable o-cillator. The floating beat-note sys tern is onlv us selective as its tuner. For this reason one or more stages' of tuned r.f. amplification may advantageously ad-vantageously be used ahead of the "first detector." -r- . |