December 1952 QST
Number nineteen of a series
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.
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.
The most advanced thought on variable selectivity for broadcast receivers inclines toward some form of non-mechanical semi-automatic device. A number of schemes have been suggested for doing this, and it is probable that these will be incorporated in some of the newer receivers for the coming season. In one system, the coupling is controlled by the strength of the received signal, on the theory that strong local stations can override interference by mere volume, and consequently do not require complete elimination of unwanted signals by high selectivity. The controlling circuit is similar to a conventional AVC circuit, but is operated by the audio signal rather than the carrier. Of the many schemes suggested for obtaining the actual selectivity change without moving parts, only one will be mentioned; namely, connecting a variable load resistance across one of the I.F. transformers. This is quite practical if a specially designed I.F. transformer is used. Since the plate resistance of the 58 varies with the suppressor voltage applied, this tube may be used as the variable load resistor.
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.