Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

RATIO DETECTOR 379at a particular signal level. Above and below this level, a balanceda-m component is present. As indicated in Figure 12, the magnitudeof the residual amplitude modulation isnot large except at low levelswhere the diode perveance is low. The level at which optimum a-mrejection isobtained may be varied by altering the circuit constants,particularly the fraction of the total rectified voltage which is stabilized,or the ratio of secondary and tertiary voltages.Secondary InductanceIt is desirable to make the secondary inductance-capacitance(L/C)ratio as high as possible, consistent with keeping the secondary tuningcapacitance high enough so that stray capacitances and variations intube capacitances do not have an excessive effect on the tuning.Usinga high L/C ratio has the advantage of increasing the sensitivity andat the same time tends to reduce the peak diode currents. At a frequencyon the order of ten megacycles these considerations pointtoward the use of a secondary tuning capacitance on the order of 25to 75 micromicrofarads. Higher values result in lower sensitivity forthe same amount of a-m rejection without any significant advantageas far as stability of tuning and independence of tube replacementare concerned.The advantage of making the secondary a bifilar winding has beendescribed. If a single center-tapped winding is used instead of abifilar winding, it is desirable to use a fixed slug to obtain the samecoupling between the two halves of the secondary and the primary.Another circuit variation involves the use of two equal fixed condensersacross the secondary to obtain an i-f center-tap.The secondary Q should, in general, be as high as possible, consistentwith the desired peak separation. At frequencies on the orderof ten megacycles, this will mean a Q on the order of 75 to 150. Useof the higher values of Q will result in improved sensitivity for agiven a-m rejection. The peak separation, however, will be lower thanif smaller values of secondary Q were used.Load ResistorsThe value R of the diode load resistors will depend on the amountof downward amplitude modulation to be handled which in turn dependsupon the i-f selectivity characteristic, the amount of multipath transmissionanticipated, and the importance of obtaining the maximumsensitivity.In general, R will be selected so as to reduce the operatingsecondary Q to a value approximately one-fourth or less of its unloadedvalue.