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National Company Advertisement: IF Circuits
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When designing a receiver or transmitter using discrete components rather than connectorized components or packaged integrated circuits, where the interfaces are at or near 50 + j0 Ω, adding frequency selectivity beyond that provided by the generic response requires inserting separate filters. If you are designing the entire signal path, including the biasing, feedback (if any), and stage interfaces from scratch, you can include features that increase frequency selectivity. In the 'old days' with vacuum tubes and interstage coupling transformers being commonplace, the addition of a few capacitors made response peaking a simple advantage to implement. National Company Advertisement Number nineteen of a series
This scheme was abandoned by us in later models, because we felt that it was not particularly desirable. Its only virtue is its ability to act as a selectivity control and this we feel can be better accomplished by other means. Its disadvantages are numerous, among them being the mechanical complication of changing coupling by a panel control, and the pronounced effect of the adjustment on gain and other circuit constants. Variable selectivity presents a problem quite different in broadcast receivers than in communication receivers. In the former, the aim is "High Fidelity," which requires a sub-normal selectivity. In amateur receivers, the object is to eliminate interfering signals which necessitates abnormally high selectivity. Because of this difference, two entirely different types of control are desirable.
To come back to amateur receivers, however, the control should be in the direction of increased selectivity, not decreased. We think that the only practical answer to this is the Single Signal Filter, such as used in our communication receivers and discussed in detail on this page last March. In this filter, a knob on the front panel gives a variable admittance to the I.F. amplifier of from several kilocycles down to a few cycles. We realize keenly that it is unwise to say that any development is valueless, and we do not wish to be dogmatic about it. However, we believe our comments above pretty well sum up the situation as matters stand. So for the present anyway, we will stick to fixed coupling. It seems the best way, particularly as it gives us the assurance that after they leave the factory, our I.F. units will stay at the optimum coupling value for best gain and selectivity. James Millen Here are all the National Company advertisements I have:
Posted October 27, 2016 |
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Last march we mentioned that much interest is being shown in I.
F. transformers having variable coupling as a means of controlling selectivity. This idea is particularly
interesting to us, because as far back as May 1933 we advertised variable coupling as a feature of our
earliest model air-dielectric-tuned I.F. transformer. In this unit, the adjustment was made by a screw
which moved the coils closer or farther apart. 
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