Radio today (Sept 1935-Dec 1936)

transformer. The greater the effective resistance of the circuit, the lower the amplitude of the response curve for any given signal input and the broader the shape of the curve. Response curves are spoken of as being symmetrical and assymmetrical (or non-symmetrical). Without any reference curves to guide the operator, the curve desired is that which is symmetrical in response, irrespective of the number of peaks; it may be a single peak or a double peak. A symmetrical response curve is one which shows equal response to frequencies which differ by like amount from the peak or resonant frequency on both sides. With this in mind, the operator working upon such tuned circuits can seek to attain symmetry of response with meter type indicators or with cathode-ray tube apparatus. Lining 'em up An example of a single-peaked response curve which displays symmetry is shown in Fig. la. This is an oscillogram of a response curve, peaked at 460 kc. and being swept by a signal which varies in frequency from 450 kc. to 470 kc. Note that the response of the circuit to frequencies within 5kc. below the peak frequency, is substantially the same as for frequencies within 5 kc. above the peak frequency. This oscillogram is not necessarily an indication of what would be a perfectly-aligned circuit with respect to the frequencies passed, for as you can see, response at 5 kc. below and above is substantially zero, but it is a good example of a symmetrical response curve. The correct curve, assuming desired 5 kc. response each side of the peak, would be of this shape; that is, symmetrical, but response at 5 kc. difference above and below the peak frequency, would be much greater. Such symmetry can be secured with meter type indicators, by first aligning for maximum response and then noting the response at various frequencies within the desired band pass range, above and below the peak frequency. Operating with meter type indicators is by no means as effective as with visual devices which show the actual response curve, but if a man does not possess such equipment, he must work with what he has, hence this information. An example of a symmetrical response curve for an over coupled circuit is shown in Fig. lb. There are two peaks, rather than one, but the response of the circuit, to fre quencies which differ by like amount above and below the peak frequency is substantially the same. Such a curve can be said to be representative of the response when an i-f. system is adjusted for high-fidelity operation. Once more we desire to call to your attention that we are speaking about the shape of the curve and not its frequency response with respect to the band pass required in a highfidelity system. Note that the peaks occur are frequencies which represent like differences from the peak, above and below the peak frequency. Flatten 'em out If the adjustment specifications call for such peaks to occur at 7.5 kc. higher than the peak frequency and for 7.5 kc. lower than the peak frequency, and the amplitude of the peaks at these two frequencies is the same, the response will be symmetrical or sufficiently so, to be classed as being satisfactory. There is of course much more to the adjustment of such a circuit, in that the extent of the dip at the peak frequency can be more or less than is shown, with respect to the amplitude of the peaks. If a flat-top characteristic is desired, then the dip would be the minimum consistent with the maximum response over the entire band. The experienced operator can develop such a response curve with meter type indicators, without using the cathoderay tube, but the time required would be many, many times that when cathoderay equipment is used. Trick tuning Symmetry is possible with a triplepeaked curve, just as it is with a doubleor single-peaked curve. See figure lc. As a matter of fact, the triplepeaked response curve was obtained with the equipment which developed the double-peaked curve mentioned before. The change from one curve to the other is purely a matter of tuning adjustment of the respective trimmers. An example of a nonsymmetrical or a symmetrical response curve with three peaks is shown in figure Id. The peak frequency is the same as in figure lb, but the response of the circuit to frequencies higher than the peak is not the same as at frequencies which differ by like amount from the peak, but are lower than the peak frequency. The lack of symmetry evident in figure Id, is not the maximum. 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