Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

386 FREQUENCY MODULATION, Volume IThe manner in which the a-m rejection of a ratio detector varieswith the frequency of the amplitude modulation is shown in Figure 26.This detector (see Figure 8) has a time constant of 0.1 second. Thecharacteristics apply (a) to a balanced detector with the center of theload resistor grounded and (b) to the unbalanced form of thecircuit which is the same as Figure 8, but with one end of thestabilizing condenser grounded. It should be noted that for the sametime constant, the balanced circuit gives much better rejection of lowaudio frequencies. This is to be expected because in the case of thebalanced circuit, fluctuations in the stabilizing voltage do not causeany disturbance in the output, whereas in the case of the unbalanced(0) BALANCED CIRCUIT-0-r-©4ID)UNBALANCED CWCUirFig. 26—A comparison of balanced and unbalanced circuits with respectto their effectiveness in rejecting low-frequency amplitude modulation.The time constant of the load circuit is the same for both.circuit they do contribute to the output. For a given value of timeconstant in the stabilizing voltage, then, it follows that the balancedcircuit will give better rejection of low-frequency amplitude fluctuations.TolerancesIn general, the ratio detector circuit will require more attentionto tolerances than is necessary for a balanced discriminator. Theprincipal factor which must be controlled closely is the effectivecoupling between the primary and secondary winding, since this controlsthe ratio between the secondary and tertiary voltages, which inturn affects the a-m rejection. This coupling is determined not only