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Homework 3, ECE438, Fall 2016, Prof. Boutin
Hard copy due in class, Wednesday September 14, 2016.
The goal of this homework is to to understand the relationship between a signal and a sampling of that signal, viewed in the frequency domain. This time, we are looking at signals beyond pure frequencies.
Question 1
Consider the signal $ x(t)=4 \text{sinc } ( \frac{t-3}{5} ). $
a) Obtain the Fourier transform X(f) of the signal and sketch the graph of |X(f)|.
b) What is the Nyquist rate $ f_0 $ for this signal?
c) Let $ T = \frac{1}{4 f_0}. $ Write a mathematical expression for the Fourier transform $ X_s(f) $ of $ x_s(t)= \text{ comb}_T \left( x(t) \right). $ Sketch the graph of $ |X_s(f)| $.
d) Let $ T = \frac{2}{f_0}. $ Write a mathematical expression for the Fourier transform $ {\mathcal X}_d(\omega) $ of $ x_d[n]= x(nT) $ and sketch the graph of $ |{\mathcal X}_d(\omega)| $.
Question 2
Electrocardiogram signals are very susceptible to interference from the 60 Hz power present in the room where the patient is being monitored. You are going to design a high-pass digital filter to eliminate the 60 Hz interference and everything at frequencies below 60 Hz.
a) Sketch the CTFT of the (analog) high-pass filter that is needed in this case
b) You want to obtain a discrete-time representation of the electrocardiogram signal that preserves all the signal. Assuming that the highest frequency in the electroelectrocardiogram signal are at 2200 Hz, what criteria would you use to select the sampling frequency?
c) Pick a specific value of sampling frequency that satisfy the criterion you wrote in b). Then sketch the graph of the DTFT of the resulting sampling of the electrocardiogram signal. For purpose of illustration, you may assume that the CTFT of the original electrocardiogram signal has a triangular form.
d) Using the same specific value of sampling frequency as for c), sketch the graph of a digital high-pass filter that would remove the interfering frequencies (those that correspond to 60Hz or below in the analog world) from the digital signal.
Hand in a hard copy of your solutions. Pay attention to rigor!
Presentation Guidelines
- Write only on one side of the paper.
- Use a "clean" sheet of paper (e.g., not torn out of a spiral book).
- Staple the pages together.
- Include a cover page.
- Do not let your dog play with your homework.
Discussion
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