Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

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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.
294 FREQUENCY MODULATION, Volume Iaudio cycle. If the signals have approximately equal amplitudes, nearlycomplete cancellation results when they go out of phase. This changein amplitude modulates the resultant carrier since it causes a suddendrop or deep hole in the resultant carrier amplitude, as shown byD - I00,000~ ft = 4000" D- 75,000" fJ =4000~D 100,000" fJ = IOOO" D =75,000" fJ = 1000*D = 75,000" fJ = 400"Pig. 23—Oscillograms showing distortion.
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FREQUENCY MODULATIONVolume IEdited
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1000-3000 watt FM transmitter—194
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:FREQUENCY MODULATIONVolume ICONTEN
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FREQUENCY MODULATION NOISECHARACTER
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NOISE CHARACTERISTICS 3w na - (u—
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NOISE CHARACTERISTICS 6to the ratio
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NOISE CHARACTERISTICS 7where C = ca
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•NOISE CHARACTERISTICS 9nF naF in
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NOISE CHARACTERISTICS 112G-)F1 aJ.
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ecurrent impulse. It isNOISE CHARAC
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NOISE CHARACTERISTICS 15constant sh
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NOISE CHARACTERISTICS 17/ 1 d(t)- (
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NOISE CHARACTERISTICS 19modulation
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.NOISE CHARACTERISTICS 21vary direc
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NOISE CHARACTERISTICS 23and frequen
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1NOISE CHARACTERISTICS 25curves for
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NOISE CHARACTERISTICS 27not conveni
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NOISE CHARACTERISTICS 29lation rece
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NOISE CHARACTERISTICS 31theoretical
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NOISE CHARACTERISTICS 33sponding or
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NOISE CHARACTERISTICS 35modulation
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NOISE CHARACTERISTICS 37signal was
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NC1SE CHARACTERISTICS .39Since the
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NOISE CHARACTERISTICS 41tude modula
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NOISE CHARACTERISTICS 43frequency m
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phase modulation maintained constan
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FREQUENCY MODULATION 47band pass of
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FREQUENCY MODULATION 49IV.Interfere
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FREQUENCY MODULATION 61frequency in
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FREQUENCY MODULATION 53maximum perm
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FREQUENCY MODULATION 55Frequency Mo
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BAND WIDTH AND READABILITY INFREQUE
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BAND WIDTH AND READABILITY 59The re
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BAND WIDTH AND READABILITY 61Listen
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BAND WIDTH AND READABILITY 63The cu
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VARIATION OF BANDWIDTH WITH MODULAT
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VARIATION OF BANDWIDTH 67
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VARIATION OF BANDWIDTH0.0606 4998 7
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iVARIATION OF BANDWIDTH 71Example:
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ItVARIATION OF BANDWIDTH 73where 6
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whereVARIATION OF BANDWIDTHmEn=—
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A PRETUNED TURNSTILE ANTENNA*tByGeo
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IMPULSE NOISE IN F-M RECEPTIONSByVe
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THE RATIO DETECTOR*!ByStuart Wm. Se
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—PUSH-PULL FREQUENCY MODULATED CI
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APPENDIX IFREQUENCY MODULATIONA Bib
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FREQUENCY MODULATION BIBLIOGRAPHY"F
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FM BIBLIOGRAPHY 511"Input Impedance
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APPENDIX IIFM STATION PLACEMENT AND
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STATION PLACEMENT AND FIELD SURVEY
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