Presentation on theme: "TEL312 Electronic Communications Fundamentals Modulation Modulation is the process by which information (message) is transformed Into waveforms that are."— Presentation transcript:
TEL312 Electronic Communications Fundamentals Modulation Modulation is the process by which information (message) is transformed Into waveforms that are compatible with the characteristics of the channel. Baseband Modulation (for digital modulation) is to use certain pulse waveforms to Carry information. Bandpass Modulation: the desired Information signal modulates a sine wave (or Carrier).
TEL312 Electronic Communications Fundamentals Why Modulate? Improve transmission efficiency. e.g. – 3kHz voice, wavelength is c/f which is about 60 miles. Using a 60 mile antenna is impractical. Allows efficient usage of bandwidth e.g. Allows multiple signals to use one channel (FDMA) Help minimize interference effects e.g. Spread Spectrum Techniques Simplify design to meet circuit design requirements e.g. Intermediate Frequency (IF) can help simplify filter and amplifier design at different frequencies.
TEL312 Electronic Communications Fundamentals Analog Modulation TWO MAIN TYPES OF MODULATION SCHEMES ( BOTH BANDPASS): AM – Amplitude Modulation If m(t) is sinusoidal, we have tone-modulated AM: FM – Frequency Modulation:
TEL312 Electronic Communications Fundamentals AM – Amplitude Modulation (Ref. Tomasi, Chapter 4 – 6) THREE MAJOR TYPES OF Amplitude MODULATION Conventional Amplitude Modulation Full AM, also known as Double Sideband Transmitted Carrier AM (DSB-TC) Double Sideband Suppressed carrier (DSB-SC) modulation Single Sideband (SSB) modulation
TEL312 Electronic Communications Fundamentals AM Type I – DSB-TC AM
TEL312 Electronic Communications Fundamentals Modulator DSB-TC AM GENERATION Special case: message is a tone Carrier signal
TEL312 Electronic Communications Fundamentals Frequency Translation of AM DSB-TC
TEL312 Electronic Communications Fundamentals Modulating Index (m) a measurement for the degree of modulation and bears the relationship of the ratio of what is the degree of modulation required to establish a desirable AM communication link? to maintain. IF, envelope distortion will occur and the waveform is said to be over-modulated. Under this circumstances, is large enough, resulting the non-proportionality of ----hence distortion of the desire message signal!!
TEL312 Electronic Communications Fundamentals In practice, the modulation index of an AM signal can be computed from Amax and Amin. A min A max Amax : is half the peak-to-peak value of the AM signal Amax(pk-pk) /2 Amin : is half the peak-to-peak value of the AM signal Amin(pk-pk) /2 Am : is half the difference of Amax and Amin. Ac : is half the sum of Amax and Amin. The values for A max and A min can be obtained directly from the oscilloscope. The evaluation of the modulation index m can be achieved by invoking the following expression :
TEL312 Electronic Communications Fundamentals Power distribution in full AM The power in a sinusoidal signal is proportional to the square of its amplitude. Transmitted power = the carrier power + the power in the sidebands Carrier power : Sideband power: Total transmitted power: where is the proportionality constant
TEL312 Electronic Communications Fundamentals The power in the sidebands depends upon the value of the modulation index. The greater the percentage of modulation, the higher the sideband power. Maximum power appears in the sidebands when the carrier is 100% (m=1) modulated. The power in each respective sideband, is given by Each sideband is one-fourth, or 25 percent, of the carrier power. Since there are two sidebands, their power put together to give 50 percent of the carrier power.
TEL312 Electronic Communications Fundamentals Power Efficiency
TEL312 Electronic Communications Fundamentals Generation of AM – DSB-TC (Square law Modulator) Nonlinearity (square-law) BPF at +2B + Square law of nonlinearity: where are constants.
Square-Law Detector Although above is described as a modulator, it can also be used as a demodulator provided that the BPF is replaced by a low pass filter (LPF) with cutoff frequency at (i.e. bandwidth of ) and a local carrier signal oscillator. Detection of AM – DSB-TC