Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

376 FREQUENCY MODULATION, Volume Imately unity.Since a will be fixed within relatively narrow limits byconsiderations which will be discussed later,the only variable part inthe expression for S/P is w 2 Q s V4a Q P°, Assuming that Q s x is as largeas practicable, consistent with the unloaded secondary Q being severaltimes the loaded secondary Q, this leaves the factor w 2 /4a °.Q pThisfactor must have a particular value, depending on a, in order to makeS/P approximately unity. The question arises as to whether there isany advantage in making n 2 large and a Q p° proportionately large, orin keeping n? small and making a "Q p proportionately small.To determine the manner in which the primary damping varies forparticular values of w 2 and corresponding values of a Q p°, consider theratio of the shunt damping resistance introduced by the diodes andthe equivalent shunt resistance of the undamped primary.iseqUalt0n*R4Q, *, 0,'cZ, aQ v°This ratiosince X s is not a variable here. Comparing this expression with Equation(1), it follows that the relative amount of diode damping on theprimary does not change with any change in n, Q °, por a, provided thesechanges are made in such a way as to keep constant both the ratioS/P and the percent of critical coupling. Of course, the sensitivitywill change, increasing with increases in n, a, and Q p°. The grid-plategain of the driver tube will also increase as n is increased.It has just been demonstrated that the primary damping cannotbe controlled by varying L p/L s , the unloaded primary Q, or the relativenumber of tertiary turns, if the ratio S/P is to be maintained. FromEquation (1) it is clear that if the primary damping is to be increased,it must be accompanied by an increase in a, in order to maintain theratio S/P constant. This increase in a is frequently undesirable, asdiscussed below.Consider Equation (2) which gives the value of S. Again if P/Sis held constant at approximately unity for optimum a-m rejection,the gain to the secondary increases with increasing values of Q 8 , nand a. For values of a between 0.4 and 1.0, £ increases rapidly witha, as shown in Figure 19. The implication of this, so far as design forhandling maximum downward amplitude modulation is concerned, isthat other things being equal, the initial per cent of criticalcouplingshould be approximately 50. This means that as Q pand Q„ increasewith downward modulation, the resultant increase inthe per cent ofcritical coupling will cause the greatest changes in the gain to thesecondary.