In the other direction vertically The resultant displacement of this

spot develops the resonance curve. The pattern is stationary, because the sweep is in step with the frequency variation and the position of the spot is recurrent during each cycle.

Actual visual alignment devices employ this basis of developing the resonance curve. There are, of course, certain differences between the design of the practical units and the explanation here given, but the basis of operation, as here presented, is substantially that experienced in practice. Although this reference and explanation covers what is the i-f. band, let it be known that it is applicable to the r-f. band and also to the a-f. band. As far as the latter is concerned, the shape of the resonance curve differs, as a result of the difference between an a-f. system response characteristic and an i-f. or r-f. system response characteristic, but the basis of operation remains unchanged. What has been said, as will be shown later, is applicable to all types of frequency modulator circuits and to single and double pattern systems.

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