Amplitude Modulation : Suppressed Carrier(DSB-SC)

Amplitude Modulation : Suppressed Carrier(DSB-SC) Page 1 of 5 정보를포함하고있는 modulating signal 과반송파신호 carrier signal 을서로곱해서얻어지는변조된신호, 즉, modulated signal 을시간영역에서보면 carrier 의진폭부분에 modulating signal 이위치하여일정한값을갖던 carrier 의진폭이변화함을알수있다. 따라서이런변조방식을진폭변조 (amplitude modulation) 라고한다. modulated signal 의스펙트럼을주파수영역에서보면, carrier 신호의주파수성분은없어지고 modulating signal 의스펙트럼이 carrier 주파수를중심으로상측과하측두군데에생기게됨을볼수있다. 따라서, 진폭변조에서중에서이경우를 DSB-SC 방식이라고한다. DSB-SC: Double Side Band with Suppressed Carrier DSB-SC 방식에서변조된신호를복조하기위해서는변조할때사용한것과동일한주파수및위상을갖는 carrier 가수신측에서필요한데, DSB-SC 방식은수신된신호에 carrier 가포함되어있지않으므로수신측에서 carrier 신호를만들어내는게어려운일중의하나이면서또한중요한문제이다. 본 section 에서는 DSB-SC 방식의변조 / 복조원리및방법에대한이해가요구된다. 5.1 Amplitude Modulation : Suppressed Carrier (DSB-SC) Modulation The process of imposing a low-frequency electrical signal onto a high-frequency signal - The process of imparting the source information(i.e.,modulating signal) onto a bandpass signal(i.e.,modulated signal) with a carrier frequency fc - Type of modulation Amplitude modulation : Suppressed Carrier (DSB-SC) Synchronous detection (coherent detection)(homodyne detection) http://mir.knu.ac.kr/multimedia/chap51.html

Amplitude Modulation : Suppressed Carrier(DSB-SC) Page 2 of 5 Synchronization problem in the local oscillator It is necessary to have synchronization in both frequency and phase between transmitter and receiver when DSB-SC modulation is used To avoid this problem, suppressed-carrier system may have a small amount of carrier reinserted in Φ(t) at the transmitter : " pilot carrier " i) Δw = 0 :Δθ 0 phase error 시 The phase error in the local carrier carries a variable gain factor in the output signal The signal strength will "wax and wane " ii) Δθ = 0 :Δw 0 frequency error 시 We obtain the signal f(t) multiplied by a low-frequency sinusoid If fc fm 때 Δf ~ Δfm comparable For telephone and radio system : Δf 30Hz is acceptable Generation of DSB-SC signals (modulators) i) product modulators ii) chopper modulators (switching modulators) http://mir.knu.ac.kr/multimedia/chap51.html

Amplitude Modulation : Suppressed Carrier(DSB-SC) Page 3 of 5 iii) balanced modulators Two AM modulators are arranged in a balanced configuration to cancel out the carrier balanced modulator - AM modulator (power-law modulators) Demodulation of DSB-SC signals - Synchronous detection Local oscillator must be synchronized to the oscillator in the modulator to achieve proper demodulation i) A simple squaring synchronizer http://mir.knu.ac.kr/multimedia/chap51.html

Amplitude Modulation : Suppressed Carrier(DSB-SC) Page 4 of 5 ii) Phase-locked loop (PLL) The bandwidth of the PLL is controlled by the bandwidth of the low-pass filter iii) Pilot carrier system A method commonly used in DSB-SC modulationto maintain synchronization between modulator and demodulator is to send a sinusoidal tone whose frequency and phase are related to the carrier frequency - Stereo multiplexing in commercial FM / Previous Chapter / 5.1 / 5.2 / 5.3 / 5.4 / 5.5 / Next Chapter / http://mir.knu.ac.kr/multimedia/chap51.html

Amplitude Modulation with Large Carrier (AM) Page 1 of 5 진폭변조방식중에서 DSB-SC 는복조를위해수신측에서 carrier 를만들어내어야하므로시스템이복잡해지고 cost 가올라가게된다. 방송시스템과같이송신측은몇개안되지만수신측이엄청나게많을경우에는수신기를간단히만들어 cost 를낮추는것이전체적으로훨씬경제적이라할수있다. 따라서, 수신측에서 carrier 를만들어낼필요없이진폭변조된신호를간단히복조할수있도록하기위해서송신측에서 DSB-SC 신호에다가큰값의 carrier 신호를포함하여전송하게되는데, 이를 DSB-LC 또는 DSB-TC 방식이라고한다. DSB-LC: Double Side Band with Large Carrier DSB-TC: Double Side Band with Transmitted Carrier 이때, 송신측에서보내는큰값의 carrier 신호는정보의전송에는아무런관계가없고단순히수신측에서의복조를간단히하기위해서보내는것이므로, 송신측에서보면 power 효율면에서 DSB-SC 방식과는비교도안되게아주나쁜방식이된다. AM 방송에서사용하는변조방식이이에해당된다. 본 section 에서는 DSB-LC 방식의변조 / 복조원리및방법에대한공부와 DSB-SC 방식과비교하여 power 효율면에서의차이에대한확실한이해가요구된다. 5.2 Amplitude Modulation with Large Carrier (DSB-LC or AM) Assumtion A general AM signal waveform can be written as http://mir.knu.ac.kr/multimedia/chap52.html

Amplitude Modulation with Large Carrier (AM) Page 2 of 5 - Example : tone modulation Carrier and sideband power in AM http://mir.knu.ac.kr/multimedia/chap52.html

Amplitude Modulation with Large Carrier (AM) Page 3 of 5 - Example : tone modulation f(t)=cosωmt The fraction of the total power contained in the sideband becomes, Since m 1, the transmission efficiency of an AM(DSB-LC) system is at best 33% In contrast, the transmission efficiency of a DSB-SC system is 100% Generation of DSB-LC signals i) Chopper modulator(rectifier modulator) http://mir.knu.ac.kr/multimedia/chap52.html

Amplitude Modulation with Large Carrier (AM) Page 4 of 5 ii) Modulator using nonlinearities (power-law modulator) Detection of DSB-LC signals i) Envelope detector ii) Rectifier detector http://mir.knu.ac.kr/multimedia/chap52.html

Amplitude Modulation with Large Carrier (AM) Page 5 of 5 The rectifier detector is not as efficient as the envelope detector / Previous Chapter / 5.1 / 5.2 / 5.3 / 5.4 / 5.5 / Next Chapter / To The Contents / SiteMap / Demo http://mir.knu.ac.kr/multimedia/chap52.html

Frequency-Division Multiplexing (FDM) Page 1 of 1 Multiplexing, 즉, 다중화란하나의채널을통해서여러개의정보를동시에전송하는것을말한다. 예를들면, TV 신호에는영상신호 / 오디오신호 / 문자신호등여러정보가방송국 1 개의채널을통해모두수신된다. FM 스트레오방송에서도 Right/Left 2 개의신호를 1 개방송국의 carrier 신호를통해서전송된다. 이와같은다중화는각정보를적절한 subcarrier 신호로 modulation 하여주파수영역에서서로겹치지않도록만든다음에그전체를하나의신호로간주하여원하는 carrier 신호로 modulation 하여전송함으로써가능하다. 이와같은방식의다중화를 FDM(frequency division multiplexing), 즉, 주파수다중화라고한다. cf: TDM(time division multiplexing) 5.3 Frequency-Division Multiplexing (FDM) Quadrature multiplexing Using the orthogonality of sines and cosines makes it possible to transmit and receive two different signals simultaneously on the same carrier frequency It permits two message signals to occupy the same frequency band But it requires precise phase synchronization of transmitter and receiver / Previous Chapter / 5.1 / 5.2 / 5.3 / 5.4 / 5.5 / Next Chapter / To The Contents / SiteMap / Demo http://mir.knu.ac.kr/multimedia/chap53.html

Single-Sideband(SSB) Modulation Page 1 of 3 진폭변조된신호를전송하기위해서는, DSB-SC 경우에서보면 carrier 주파수를중심으로원래 modulating signal 이가지고있는대역폭의 2 배를필요로하게된다. 정보를전송하는입장에서보면 carrier 의상측또는하측의어느하나만전송하더라도상관없으며, 그렇게할경우전송에필요한대역폭을반으로줄일수있게된다. 이를위해개발된방식이 SSB 방식이다. (SSB: Single Side Band) SSB 방식의신호를만들기위해서는보통 DSB-SC 신호를만들어한쪽을 filtering 하게되는데, carrier 주파수를중심으로 abrupt 한특성을갖는 filter 를만들수없으므로 modulating signal 의스펙트럼이저주파영역에는거의존재하지않는또는무시해도좋은그런신호의변조에만주로사용된다. ( 음성신호의경우 20 Hz 부터 20,000 Hz 까지의주파수성분을가지고있지만, 보통음성통신에서는 300 Hz 에서 3.3 khz 정도의성분만으로도정보를전달하는데별무리가없다. 따라서, 아마추어무선등의음성통신에 SSB 방식이이용되기도한다.) 본 section 에서는 SSB 신호의표현방법및변조 / 복조방식에대한이해가요구된다. 5.4 Single-Sideband(SSB) Modulation Generation of SSB signals i) Filtered method http://mir.knu.ac.kr/multimedia/chap54.html

Single-Sideband(SSB) Modulation Page 2 of 3 Demodualtion of SSB signals - Synchronous detection i) Phase error 시 cos(ωct + Δθ) A phase error in the locally generated carrier will result in phase distortion in the output However, phase distortion in SSB demodualtion is quite tolerable for voice communication ii) frequency error 시 cos(ωc + Δω)t Frequency errors give rise to spectral shifts as well as to phase distortion in the demodulated output - envelope detector http://mir.knu.ac.kr/multimedia/chap54.html

Single-Sideband(SSB) Modulation Page 3 of 3 If the carrier is much larger than the modulating signal, i.e., A >> f(t), then SSB-LC signal can then be demodulated using an envelope detector / Previous Chapter / 5.1 / 5.2 / 5.3 / 5.4 / 5.5 / Next Chapter / To The Contents / SiteMap / Demo http://mir.knu.ac.kr/multimedia/chap54.html

Vestigal-Sideband(VSB) Modulation Page 1 of 1 대역폭이아주큰 modulating signal 을진폭변조할경우에, 변조된신호의전송에필요한대역폭을줄이기위해 SSB 방식이사용된다고앞에서설명하였다. 그러나, modulating signal 이저주파영역의신호성분을많이포함하고있을경우에는원하는대로의 filtering 이불가능하므로 SSB 방식의적용이어렵게된다. TV 신호의경우오디오및영상신호를포함한전체의대역폭이 4,5 MHz 에해당되는데이를 DSB-SC 방식으로변조할경우에전송에 9 MHz 의대역폭이필요하게되어주파수대역의낭비가심하게된다. 그러나영상신호의경우 d.c 성분을포함한저주파대역의성분을많이가지고있으므로 SSB 방식을적용할수가없다. 이를해결하기위해개발된방식이 VSB 방식이다.(VSB: Vestigial Side Band) carrier 주파수를중심으로 odd symmetry 특성을갖는 filter 를이용함으로써 TV 신호의경우 6 MHz 의대역폭으로신호를전송하고수신측에서원래의정보를복원할수있게된다. 5.5 Vestigal-Sideband(VSB) Modulation Consider a modulating signal of very large bandwidth having significant low-frequency contents For example, television siganl, image signal, facsimile data, high-speed digital data Practical SSB systems have poor low-frequency response VSB modulation / Previous Chapter / 5.1 / 5.2 / 5.3 / 5.4 / 5.5 / Next Chapter / To The Contents / SiteMap / Demo http://mir.knu.ac.kr/multimedia/chap55.html