Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

MULTIPATH DISTORTION 287antenna was fairly well shielded from the direct wave from the transmitter,and it received signals which may have been reflected fromother buildings.Figure 13 shows several curves of the audio outputand the corresponding variation in the carrier voltage during the cycle.They are all for a 300-cycle audio tone and ± 75-kilocycle deviation, forvarious antenna positions. The audio output should be a pure sinewave, and the intermediate-frequency voltage should show the intermediate-frequencyselectivity curve, as shown by the dotted lines.Although there were several irregularities along the straighter portions,the most serious distortion occurs at the ends of the swing. Thefirstintermediate-frequency voltage curve shows that there were severalpaths of nearly equal voltage. In the second case, the secondarypaths were still present, but the relative voltage was much less, asshown by the decreased departure from the selectivity curve. Since the^wIFVoltageFig. 13—Distortion for various antenna positions.signal strength was fairly low, some of the distortion was probablydue to the inability of the limlter to remove allthe deep holes in thecarrier.Many other field tests in New York City and northern New Jerseyhave shown this distortion. The time delay is usually about 7 to 12microseconds. This means that, for two-path transmission, there willbe one or two holes in the carrier. Since the limiter tends to fail atthese holes, the distortion can often be reduced by detuning thereceiver until the hole in the carrier coincides with the discriminatorzero-balance frequency. Since the discriminator output is zero at thispoint, variations in input voltage cause a minimum of distortion. Itsometimes happens, when the direct and indirect waves are nearlyequal, that severe distortion isencountered in the audio output of theset even though the input signal, as indicated by the voltage developedat the limiter grid, would normally be sufficient to secure limiting.