Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

RATIO DETECTOR 381gain will rise to a higher value during the a-m cycle. This increasein grid-plate gain is desirable from the viewpoint of increasing theability of the detector to reject downward amplitude modulation, sothat in practice the primary Q is made as high as possible, consistentwith obtaining the required peak separation and not exceeding themaximum allowable grid-plate gain during downward amplitude modulation.Primary Impedance VariationDuring the a-m cycle two opposing effects take place which alterthe primary impedance and hence affect the gain between grid andplate of the driver tube. When the input signal falls, for example,the impedance coupled into the primary by the secondary increasesbecause the Q of the secondary rises. This tends to lower the gainfrom the grid to the plate of the driver tube. On the other hand, thediode loading across the primary decreases and this effect tends toraise the gain from the grid to the plate. From the viewpoint ofrejecting the maximum downward amplitude modulation, it. is desirablethat the circuit parameters be so related that the net change ingainbe such as not to result in a decrease in gain to the primary when thesignal level falls.CouplingThe value of coupling used, along with the relative number ofturns used in the tertiary winding, is the principal factor in determiningthe ratio S/P. The importance of maintaining this ratio in thevicinity of unity, and the effect of varying the ratio by changing thecoupling, or by any other means, has been described. In general, thecoupling should be adjusted to be approximately one-half critical.Since the absolute coefficient of coupling which corresponds to criticalcoupling is itself dependent upon the load resistance and on the tertiaryinductance, the final coupling adjustment must be made after the otherparameters are fixed because the per cent of critical coupling is affectedby changes in these.Tertiary InductanceIn the notation used here, the tertiary inductance is specified interms of the parameter n where the tertiary voltage is equal to 1/ntimes the primary voltage. With a specified value of coupling in mind,usually about 0.5 critical, the number of turns on the tertiary isadjusted so that the required S/P ratio is obtained. If the requiredS/P ratio is obtained with too few turns on the tertiary so that the