Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

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FREQUENCY-MODULATION FIELD TEST 143changing the length of the connected noise transmitting antenna, whichwas located about 50 feet from the receiving dipole. FM-15 representsa deviation ratio of 1.875 when the audio band width is 8 kc. As Crosbyhas shown 8 , the noise spectrum advantage of FM is 6 db for impulsenoise. The FM-15 threshold is shown. The FM-75 threshold is notshown because at the time the measurements were made a-c hum withing8I
144 FREQUENCY MODULATION, Volume Inoise peak amplitude. Once an ignition peak has risen to the valuerequired to control the receiver and blank out the signal a furtherrise in the noise level will not occur until the peak increases in breadth,or duration, or until there is a sufficient rise in certain low amplitudecomponents of ignition noise having fluctuation noise characteristics.The peculiar shapes of such curves below the threshold values are dueto the wave shapes and crest factors of ignition noise.Results of Observations at Temporary Receiving StationsThe observations at the temporary receiving stations confirmedthe measurements made at Bellmore. In going to progressively greaterdistances the FM-75 noise threshold distance was passed and FM-15became superior. Then the FM-15 threshold distance was passed andAM became superior, although it was very noisy. At the limit of A-Mintelligibility both FM-75 and FM-15 were completely smothered bynoise. The service limits were all in excellent agreement with thoseshown by the bar chart, Figure 20, subject to spasmodic interferencefrom passing automobiles. This type of interference was at all stationsthe predominating one, but was only intermittent and not as troublesomeas might be expected.Observations of Diathermal Interference in AM and FMDiathermal machines vary considerably in their characteristics,some types using raw a.c. and others using partially filtered powersupplies.Observations were made of interference on the three typesof modulation using raw a-c machines. It was concluded from theseobservations that this type of interference is characterized by thetransmission of a band of frequencies about 15 kc wide. With amplitudemodulation the background interference is essentially equal to thecarrier-to-noise ratio. With fairly weak interference from diathermyexactly centered on the desired carrier, FM-75 reception is 20 to 25db superior to AM and FM-15 is 10 to 12 db superior. However, withthe diathermy 5 kc off the desired carrier, AM was approximatelyequal to FM-15 and was in some cases superior to FM-75. Withthe diathermy carrier at the edge of the AM and FM-15 passband, theinterference is highly attenuated. Under these conditions the FM-75interference was extremely severe. With the diathermy carrier welloutside of the passband of FM-15 or AM, the interference isnoticeableonly when the diathermy amplitude is extremely high. Under normalreceiving conditions it would not be heard. However, if the diathermyunder these conditions is within the FM-75 passband, the interference
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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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CHARACTERISTICS OF THE PYLON FM ANT
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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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Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

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