FORMsSWRTHYPHASCISPELPRGHDOC h PICP69LINP6Z BOXP6Z dOVLP6Z dSWCL@BlackWhiteRedGreenBlueCyanYellowMagentafSalmonwPeachUBrownellowLight GreenwwwGrey DarkOrangeݙVanillalowGrey LightIDXHIDXDSoftSansdTitleepz,IDXDSoftSansdBlank LinefIDXDSoftSansdLetter HeadergIDXDSoftSans dIndex EntryhTOCH TOCDSoftSansdTitlenz%TOCDSoftSans dBlank LineoTOCDSoftSansdZLevel 1pTOCDSoftSans dZLevel 2qTOCDSoftSerif dLevel 3rTOCDSoftSerif dhLevel 4sTOCDSoftSerif dvLevel 5tTOCDSoftSerif d*Level 6uTOCDSoftSerif dLevel 7vTOCDSoftSerif d8Level 8wIDXPSEC1 PTPAG3pA PRN3KBVtils:APPLICATIONS/FinalWriter/FWFonts/SWOLFonts/Serif/WŬWW~WW { dELX.FormulaText{WPASDSoftSansdDocument TitlePASDSoftSansdSection Namepz??PASDSoftSansdTopic Nameq{??PASDSoftSansdSub Topic 1r|??PASDSoftSansdSub Topic 2s}??PASDSoftSans dBodyPASDSoftSans dIndented BodyTOCP!rOUTPBIBPTOIPENDPDSP2PHHFDASoftSans dSoftSans dPNDASoftSans dSHPP6Z dGRMRStrictly.For Business Writing.For Casual Writing.Custom 1.Custom 2.Custom 3.IDXLConductorsTDINFFCOP3`W9&WtFDTASoftSans_Bold WŬa W+{Utils:APPLICATIONS/FinalWriter/FWFonts/SWOLFonts/5W.ZW& W,W*TW8W,VW+xW,FDTAWorchester Utils:APPLICATIONS/FinalWriter/FWFonts/SWOLFonts/Serif/FDTASoftSansW8W%YW$PWwLWW$oW$WtW_W$PW# W$WtW~GINF "GINF Z BlackGINFUtils:GRAPHICS/PPaint/Brushes/FWriter5/Line2.bshMXRMXRW4_j9BlackFORMTILBMBMHD& D,CMAP0f"@@CAMGBODY<U߻TU߻TU߻Twww<GINF SHD1PW!W!,W!PTMainTBDYRULEMATTR dCHRSElectronic Servicing yr.1 pt.1ATTR dCHRS ATTR hdCHRS ATTR hdCHRS ATTR dCHRSLecture: 020.TATTR xdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRSThu - 20 - February ATTR hdCHRS-1997RULEATTR xCHRSRULEATTR xCHRSFrequency Modulation - (F.M.)RULEMATTRM dCHRSMIn FM the frequency of the carrier is at least 10,000 times the frequency of the signal, which is much more compared to the AM which is only 10 to 20 times larger. For this reason, FM is only possible on frequencies in the V.H.F. and above. FM is only used with sound and the vision signal cannot be carried with FM for domestic use.RULEATTR dCHRSRULEmATTRm dCHRSmStability: This is the ability of the receiver to remain tuned on the station without the need of adjustment.RULE=ATTR= dCHRS=Carrier for radio broadcasting is between 88MHz - 108Mhz VHF RULE?ATTR? dCHRS?Carrier for TV Carrier is always 6MHz above the vision carrier.RULEATTR dCHRSRULE.ATTR. dCHRS.In Frequency Modulation the carrier amplitude is kept constant while the frequency of the carrier varies directly proportional with the modulating voltage. When the modulating voltage is positive, the carrier frequency increases. When the modulating voltage is negative the carrier frequency decreases.RULEATTR dCHRSRULE3ATTR3 dCHRS3This change in carrier frequency is called frequency deviation and the maximum permissible deviation that a radio station can produce should not exceed +/-75kHz and this corresponds to 100% modulation. In the case of TV, the sound carrier can only deviate by +/- 50kHz (which corresponds to 100% modulation)RULEATTR dCHRSRULEATTR dCHRSDifferent FM systems can exist having different ranges of deviation like 25, 50, 75KHz, etc., but the best one will be the one with the highest range of deviation. Figure 3.13 shows diagrammatically what happens in FM. Figure 3.14 shows the effect of larger amplitude voltage of modulating signal then that of figure 3.13. So that in figure 3.14 there is a larger frequency deviation. Figure 3.15 shows the effect of higher frequency or modulating signal than that of figure 3.14. The rate of frequency has measured by 2 times.RULEATTR dCHRSRULEA,ATTR  dCHRS AmplitudeATTR8 dCHRS8 of modulating signal determines the amount of deviationRULEW,ATTR  dCHRS FrequencyATTRN dCHRSN of modulating signal determines the rate at which the deviation is performed.RULEATTR dCHRSRULE]ATTR] dCHRS]What happens in FM if the amplitude and the frequency of the modulating signal are increased:RULE0,ATTR hdCHRS ATTR/ hdCHRS/A) The amount of frequency deviation increasesRULE.,ATTR hdCHRS ATTR- hdCHRS-B) The rate of frequency deviation increasesRULE8,ATTR hdCHRS ATTR7 hdCHRS7C) The amount and rate of frequency deviation increaseRULE9,ATTR hdCHRS ATTR8 hdCHRS8D) The amount and rate of frequency deviation decrease:RULEATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ANSWER: CRULEATTR hdCHRSRULE ATTRhdCHRSSide Frequencies of FMRULEHATTRH hdCHRSHIn FM the theoretical number of side frequencies is infinite. These side frequencies are apart from each other by the same value of frequency and are multiples of the modulating frequency (fig 3.16). Luckily, as the side frequencies are further apart from the carrier frequency, they loose their amplitude and become negligible.RULEATTR hdCHRSRULEdATTRd hdCHRSdSo the practical number of significant side frequencies is limited and can be calculated as follows:RULEATTR hdCHRSRULEnATTRn hdCHRSn1) Find the modulating index by the formula carrier frequency deviation -> (always 75,000Hz)RULE"ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRS ATTR hdCHRSmodulation signal frequencyRULEzATTRz hdCHRSzExample, if carrier frequency deviation = 75kHz, and modulation signal frequency is 15kHz, modulation index = 75 / 15 = 5.RULEATTR hdCHRSRULEATTR hdCHRS2) The modulation index is looked up in Bessell's tables and the corresponding table gives the number of significant side frequencies. For the above example, an index of 5 gives a table with 16 frequencies, 15kHz apart, 8 on the left and 8 on the right. The tables do not only give the number of frequencies, but also the amplitude for every single frequency, and in FM, the amplitude of some side frequencies can be larger from the carrier frequency, in comparison with the AM which can reach only up to a max of half the carrier amplitude. So the total band width of the 16 significant side frequencies spaced by 15kHz each is 16 x 15,000 = 240 kHz (the maximum band width). Therefore the total band width of the transmitter in this case is 240kHz. (figure 3.18a)RULEATTR hdCHRSRULE7ATTR7 hdCHRS7If the modulating signal is 7,5kHz and the carrier frequency deviation is 75kHz, the modulation index is of 75 / 7.5 =10. The Bessell table of 10 corresponds to 28 significant side frequencies separated by an equal space of 7.5kHz. hence the bandwidth of the transmitter is 28 x 7.5kHz = 210 kHz. (figure 3.18b)RULEATTR hdCHRSRULEATTR hdCHRSRULE ATTRhdCHRSAdvantages of FM over AMRULEATTR hdCHRSThe main advantage of FM is that it is a clean, noisless means of radio modulation compared to AM, which bears a lot of interferences. RULEATTR hdCHRSRULEATTR hdCHRSElectrical noise and interference (caused by sparks, welding, etc.) have are abler to Amplitude Modulate a carrier. In the case of an AM wave, noise and interference create a new amplitude, which is different from the original modulating signal. The result is that at the receiver de-modulator, anything that is Med will be heard as noise on the load speaker. So in this case noise and interference cannot be seperated from the information (modulating signal)RULEATTR hdCHRSRULEaATTRa hdCHRSaIn FM, although the carrier will also suffer amplitude modulation by intereference and electrical noise, the frequency is not affected. At the FM reciever de-modulator, the output given is due to the frequency variation (not amplitude) of the carrier, so no noise and interference is heard in the loadspeaker, and hence a clean transmission is obtained.RMSTRULEATTR dCHRSLMSTRULEATTR dCHRS