Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

380 FREQUENCY MODULATION, Volume IThe lower the value of R, the greater will be the ratio betweenthe unloaded and loaded values of the secondary Q. This means thatwhen the diode current decreases during downward amplitude modulation,the Q of the secondary is able to rise considerably and to compensatefor the decrease in input signal by effectively raising thesensitivity of the detector. The greater the extent to which the Qcan rise when the input signal drops, the greater will be the downwardmodulation capability of the detector. In addition, the value of Raffects not only the operating Q of the secondary, but alsothe operatingQ of the primary. As a result of both of these factors, usinglower values of load resistance increases the extent to which the signalcan drop below its average value without the diodes being biased tocutoff by the stabilizing voltage.If the i-f passband preceding the ratio-detector driver tube is notflat over a range equal to the maximum frequency swing, the amplitudemodulation introduced by this selectivity must be removed by theratio detector. As a result of this, the more the i-f selectivity characteristicdeviates from a relatively flat top, the lower are the loadresistance values which must be used.If itis expected that a receiver using a ratio detector will be usedunder conditions of multipath reception, it follows that the detectormust be capable of removing the amplitude modulation which resultsfrom the partial cancellation of the direct signal by the delayed signalwhich reaches the receiver over a longer path.Experience with multipathreception indicates that it is desirable for the detector to be ableto reject more than 50 per cent downward amplitude modulation toreduce distortion as much as possible. Where very severe multipathisencountered, the phase modulation distortion which accompanies itbecomes appreciable even if all of the a-m distortion is removed.Sensitivity is also involved in the choice of load resistors. Thegreater the value of load resistance R, the greater will be the sensitivityand the less the downward modulation-rejecting capability.Primary L/C Ratio and QThe primary L/C ratio should be as large as possible in order toobtain the greatest sensitivity. The limiting factor is the maximumstable gain between the grid and plate of the ratio detector drivertube. When determining this maximum gain, it should be kept inmind that the gain may increase during downward amplitude modulation,particularly on either side of the center frequency.If the primary Q is high enough so that the operating Q of theprimary is determined essentially by the diode loading, the grid-plate