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1、15-1 An AM broadcast transmitter is tested by feeding the RF output into a 50- (dummy) load. Tone modulation is applied. The carrier frequency is 850 kHz and the FCC licensed power output is 5,000 W. The sinusoidal tone of 1,000 Hz is set for 90% modulation.(a) Evaluate the FCC power in dBk (dB abov
2、e 1 kW) units.(b) Write an equation for the voltage that appears across the 50- load, giving numerical values for all constants.(c) Sketch the spectrum of this voltage as it would appear on a calibrated spectrum analyzer.(d) What is the average power that is being dissipated in the dummy load?(e) Wh
3、at is the peak envelope power?Solution：(a) FCC power:500010lg6.99()1000dBK :cos20001000900.9501cosmmmccbLetm tAtfHzAs tAm ttQ是调制负载上通过的电压为：( )7071 0.9 cos (2000)cos2 850,000 s ttt(c) )cos(2)707(9 . 0)cos(2)707(9 . 0cos707)(ttttsmcmcc21 2500050cA 707cAV2(d) 50负载上的平均功率： 222AVGreal21 0.921505020.95000 1
4、70252cAstPw (e) 222 PEPP1max50001 0.9180502 50cAm tw5-2 An AM transmitter is modulated with an audio testing signal given by , where ,and . 120.2sin0.5cosm ttt1500fHz2500 2fHz100cA Assume that the AM signal is fed into a load.50(a) Sketch the AM waveform.(b) What is the modulation percentage?(c) Eva
5、luate and sketch the spectrum of the AM waveform.Solution: 12100(1 0.2sin0.5cos)coscas tttt2 222221110.9( )( )12222cccs tAAm tA 35-4 Assume that an AM transmitter is modulated with a video testing signal given by ,where f1=3.57MHz. Let Ac=100. 10.20.6sinm tt (a) Sketch the AM waveform.(b) What are t
6、he percentages of positive and negative modulation?(c) Evaluate and sketch the spectrum of the AM waveform about fc.4Solution: 1110.20.6sin3.57;100100 1cos100 0.80.6sincosmcccam ttffMHzAs tm tttt maxmin140 100%pos.mod.40%10010020%neg.mod.80%100ccccAAbAAA A 04015ccmcmcfS fffjffffff5-5 A 50,000-W AM b
7、roadcast transmitter is being evaluated by means of a two-tone test. The transmitter is connected to a 50- load, and 5, where f1=500 Hz. Assume that a perfect AM 1111coscos2m tAtAtsignal is generated.(a) Evaluate the complex envelope for the AM signal in terms of A1 and 1.(b) Determine the value of
8、A1 for 90% modulation.(c) Find the values for the peak current and average current into the 50- load for the 90% modulation case.Solution:(a) )2cos(cos1 2236)(1 )(V2236)50(2000,501112ttAtmAtgAACcc(b) 1111111cos2ccos2oscosAttm tAtAtto find m(t)min : x() = cos +cos2( )sin2sin20dx d sin4sincos125. 1)5
9、.104(5 .104 x 111min104.51.12min0.905.8omtAmAxAt max1MIN1maxmin 1122361222361 1.1253.1250.900.57622cAAAAAAAAA另解：6(c)maxmax maxmps111223612(0.8)5813.6volts116.272 50 ( )22361 0.8(coscos2) cos0 AvccAAIAIs ttttformaxmax maxmps111223612(0.576)4811.9volts96.238 50 ( )22361 0.576(coscos2) cos0 ccAAIAs ttt
10、tfor另解： mpsA 0IAv5-12 SSB signals can be generated by the phasing method shown in Fig. 5-5a, by the filter method, of Fig. 5-5b, or by the use of Weavers method Weaver, 1956, as shown in Fig. P5-12. For Weavers method (Fig. P5-12), where B is the bandwidth of m(t),(a) Find a mathematical expression
11、that describes the waveform out of each block on the block diagram.(b) Show that s(t) is an SSB signal.Figure P5-12 Weavers method for generating SSB.Solution:7)cos(212cos)(1BttBtV )21()21(21)()cos()()()()(313BfMBfMfVBttmtVtmtV )21()21(21)()sin()()()()(424BfMBfMjfVBttmtVtmtV elsewhere , 021|,)21()21
12、(21elsewheref , 021| ),( )(45 fBfBfMBfMBffV fVLikewise elsewhere , 021|,)21()21(21 )(6 fBfBfMBfMfV tBftvtVc212cos)()(59 elsewhere 21|21|, 0,)21 21()21 21(21|21|,)21 21()21 21(4121 21 21)(559fBBffBBffMBBffMBBffBBffMBBffMBffVBffVfVccccccccccccccccccffBfBBffBfffBBffBBfBBffBBBff21|21| Likemise21 21 21 2
13、121 21 21 21|21:|AsideThus,elsewhere , 0,)()(41,)()(41)(9fffBfBffMffMBfffffMBffMfV cccccccc8Likewise elsewhere 21|21|, 0,)21 21()21 21(4121|21|,)21 21()21 21(4121 21 21)(6610fBBffBBffMBBffMBBffBBffMBBffMBffVBffVjfVccccccccelsewhere , 0,)()(41,)()(41)(10fffBfBffMffMBfffffMBffMfV cccccccc=s(t) = v9(t)+v10(t)putout)()()(109fVfVfs)(McfB-Bf)( fScfBfcBfcS(t) is a USSB signalUSSBfffBfffMBfffffMfS ccccccelsewhere , 0),(21),(21)(5-13 An SSB-AM transmitter is modulated with a sinusoid , where 15cosm tt.1112,500,and 1cffHzA(a) Evaluate .
