Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

MULTIPATH DISTORTION 293If the maximum deviation is reduced, such that the two vectors donot go out of phase so often, the distortion will be much less,becausethe number of peaks will be reduced and the one vector will not rotateso fast relative to the second. Figure 22 shows the output at 1000cycles per second with a maximum deviation of 15,000 cycles persecond. The time delay is 1/30,000 second as before. The phase of thetwo radio-frequency carriers was chosen such that they were in phaseat the undeviated frequency. This assumption also reduces the distortion.Upon reducing the deviation further, or decreasing the time delayof the retarded signal, the irregularities will disappear.Experimental Study of Distorted OutputThe distortion in the audio output from the receiver was studiedby using the circuit of Figure 8 with a frequency-modulation receiverconnected just ahead of the oscilloscope. The signal was passed throughthe intermediate-frequency amplifier, limiter, and discriminator, andthe distortion was studied with the oscilloscope. Figure 23 shows aseries of oscillograms taken with different audio frequencies and deviations.It is easily seen that high deviations and high audio frequenciescause the most distortion.Reducing either one will decrease the distortion.These curves cannot be compared directly with the theoreticalones previously shown, since the time delay is not the same. The timedelay used experimentally was limited to 16.2 microseconds by theavailable length of cable, while the theoretical curves were based on atime delay of 33.3 microseconds.Naturally, the increased time delaycauses the one signal to lag further behind and thus causes greaterdistortion. Crosby 8 has also shown many oscillograms of this type ofdistortion, taken over a path from Kansas City to Riverhead, LongIsland. These show the characteristic sharp breaks in the audio outputand the effect of noise.ConclusionThe calculations and experimental study show that serious distortionof a frequency-modulation signal can be caused by multipathtransmission over relatively short distances. This distortion is liableto be encountered in the vicinity of large buildings or other objectswhich reflect and absorb the waves and thus cause interference.Thesebuildings or other large objects can be near either the transmitter orthe receiver and cause such reflections. The two signals will be inphase part of the time and out of phase part of the time during the8See page 401 of footnote reference 5.