Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

368 FREQUENCY MODULATION, Volume Idriver tube.Both the rectified output voltage and the a-f voltage areclosely proportional to the average signal input.One way of showing the amplitude-rejecting properties of thisdetector is by means of the output characteristic curves shown inFigure 11. These are taken with a battery across the stabilizing condenserto hold its voltage constant at the normal value which waspresent just before the battery was applied. The input signal is thenreduced in strength until the diodes are cut off.Note that the outputremains essentially constant over a band of more than 150 kilocycleswhile the signal is reduced to one-third of its initial value. The peakseparation, however, is reduced as the input signal is reduced.Another more convenient way of showing the amplitude rejectionis to use an f-m signal which is simultaneously amplitude modulated.Pig. 11—Effect of reduction in the input signal. The stabilizing voltage isheld constant at a level corresponding to the 0.3-volt input signal. Note thereduction in peak separation for decreasing input signal.When this type of signal is applied, no stabilizing battery is requiredsince the stabilizing condenser will maintain the rectified voltageconstant for a-m frequencies above about 100 cycles. As explained indetail later, the extent to which the characteristics in Figure 12 differfrom a line, indicates the relative amount of residual amplitude modulationin the f-m output.In arriving at the design of this detector(Figure 8), a number ofcircuit parameters, in addition to those previously mentioned, areinvolved. These include the unloaded and loaded values of primaryand secondary Q, the per cent of critical coupling between primary andsecondary, and the grid to plate gain under various conditions. Theyare tabulated below:Primary: Unloaded Q = 70Operating Q (with diode loading) = 40Secondary : Unloaded Q = 89.Operating Q (with diode loading) = 21