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ECE 4371, Fall, 2014 Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 7 Sep. 17 th, 2014
Outline Analog vs. Digital ADC/DAC: gateway between analog and digital domains –Sampling Theorem –Quantization –Most important part in communication system –Most important during interview –Read books carefully Examples
Claude Elwood Shannon, Harry Nyquist
Sampling Theory In many applications it is useful to represent a signal in terms of sample values taken at appropriately spaced intervals. The signal can be reconstructed from the sampled waveform by passing it through an ideal low pass filter. In order to ensure a faithful reconstruction, the original signal must be sampled at an appropriate rate as described in the sampling theorem. –A real-valued band-limited signal having no spectral components above a frequency of B Hz is determined uniquely by its values at uniform intervals spaced no greater than seconds apart.
Sampling Block Diagram Consider a band-limited signal f(t) having no spectral component above B Hz. Let each rectangular sampling pulse have unit amplitudes, seconds in width and occurring at interval of T seconds. A/D conversion f(t) T f s (t) Sampling
Impulse Sampling with increasing sampling time T
Introduction Equation number is not the same as in the book
Interpolation If the sampling is at exactly the Nyquist rate, then
Practical Interpolation Sinc-function interpolation is theoretically perfect but it can never be done in practice because it requires samples from the signal for all time. Therefore real interpolation must make some compromises. Probably the simplest realizable interpolation technique is what a DAC does.
Under Sampling, Aliasing
Avoid Aliasing Band-limiting signals (by filtering) before sampling. Sampling at a rate that is greater than the Nyquist rate. A/D conversion f(t)f(t) T f s (t) Sampling Anti-aliasing filter
Aliasing 2D example
Example: Aliasing of Sinusoidal Signals Frequency of signals = 500 Hz, Sampling frequency = 2000Hz
Example: Aliasing of Sinusoidal Signals Frequency of signals = 1100 Hz, Sampling frequency = 2000Hz
Example: Aliasing of Sinusoidal Signals Frequency of signals = 1500 Hz, Sampling frequency = 2000Hz
Example: Aliasing of Sinusoidal Signals Frequency of signals = 1800 Hz, Sampling frequency = 2000Hz
Example: Aliasing of Sinusoidal Signals Frequency of signals = 2200 Hz, Sampling frequency = 2000Hz
Natural sampling (Sampling with rectangular waveform) Figure 6.10
Bandpass Sampling (a) variable sample rate (b) maximum sample rate without aliasing (c) minimum sampling rate without aliasing
Bandpass Sampling A signal of bandwidth B, occupying the frequency range between f L and f L + B, can be uniquely reconstructed from the samples if sampled at a rate f S : f S >= 2 * (f2-f1)(1+M/N) where M=f 2 /(f 2 -f 1 ))-N and N = floor(f 2 /(f 2 -f 1 )), B= f 2 -f 1, f2=NB+MB.