Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

150 FREQUENCY MODULATION, Volume Iapproach zero asymptotically as the frequency was increased, and theresultant curvature of the output voltage vs. input frequency wouldproduce undesirable distortion.The characteristic impedance of the filter is high compared to R .The filter is terminated at its output and where it may be connectedto the grid of an audio amplifying tube.(6) It has been found experimentally that a small bias voltage(approximately 1.5 volts) in series with the charge diode d xeffectivelyovercomes the contact potential of both diodes and contributes somewhatto improved linearity of the device.n- osrecroi?i: Appro*Fig. 3Experimental Determination of Detector LinearityOne may expect representative linear performance from a monitordetector built according to the data presented. There may, however,be circumstances wherein an accurate knowledge of the capabilitiesof the device are required. For this reason a brief description of thecalibration method employed is included here.Figure 4 shows a circuit in which the monitor detector (withoutthe heterodyne oscillator of Figure 3) is represented diagrammaticallyby a box with input and output terminals. A signal generator isemployed (in conjunction with a calibrating crystal oscillator) toprovide a point-to-point determination of d-c detector output vs.frequency.inputAn L. & N. Kohlrausch slide wire, fed from a 2-volt storagecell, is used in conjunction with a sensitive galvanometer in a potentiometercircuit which permits the d-c output of the detector to be