Fields and Waves I Design Projects Spring 1995
FIRST DESIGN PROJECT from Spring 1995:
The following contains information on the first design project.
Design Project 1, first page
Design Project 1, second page
The following will display the first design problem using ghostscript.
In the first design project, it was necessary to measure the speed of the motors
built by the many student groups. To do this, the motor coils were specified to be
sufficiently open to allow a laser beam to pass through its center. The speed was
then determined by counting the number of times per second that the beam was broken
by the coil. The circuit used to detect the beam can be displayed using ghostscript
by clicking on the following:
SECOND DESIGN PROJECT From Spring 1995:
This last design project consists of four parts, but you only need to do two
of them.
1) Design an electromagnetic wave experiment to determine the dielectric
constant of a common material. The experiment is to be done at optical
frequency and it must be something you can actually do. Your design
must include information on the expected value for the permittivity as
determined from some generally available reference.
2) Do the experiment you designed in part (1).
3) Design a device that will block a signal from some channel on a cable TV
system. The method you are to use involves inserting a "T" in the coax and
connecting a finite length of open circuited cable.
4) Do the experiment you designed in part (3).
Some technical information will be posted here to assist in these designs.
For example, the frequencies of all cable channels will be posted.
Note that the course syllabus requires that you have different partners
this time. To satisfy this requirement, you should change your group by
at least one member.
Some Info:
In this project, you are to design a simple device that will significantly
attenuate the signal to some cable channels while leaving others largely unaffected.
It is not the intention of this project to achieve total blockage of any channels,
since modern televisions are capable of producing some kind of picture out of almost
any level of coherent signal. However, it should be possible to achieve a noticeable
change. To model the effect of inserting the T and open circuit line, you will
need to know the frequencies of the various cable channels. Conventional TV signals
send video, color, and sound separately. The video signals require the largest bandwidth
and are sent at the following frequencies (in megahertz).
2 55.25 3 61.25 4 67.25 5 77.25
6 83.25 14 121.25 15 127.25 16 133.25
17 139.25 18 145.25 19 151.25 20 157.25
21 163.25 22 169.25 7 175.25 8 181.25
9 187.25 10 193.25 11 199.25 12 205.25
13 211.25 23 217.25 24-52 ... 53 397.25
54 73.25 55 79.25 56-60 ... 61 115.25
62 403.25 63 409.25 64-85 ... 86 547.25
where in the last three rows the missing channels are 6 megahertz apart for the indicated
range. This information was taken from the Jerrold CATV Reference Guide RD-15 which contains
a lot of useful information on cable systems.
Comments on Fall 1994 Second Design Project:
I had students investigate blocking a tv signal by inserting
a short section of 300 ohm cable in a 75 ohm cable feed system. I did an
experiment based on this idea with my own tv set and got some qualitative
data on changing picture quality. There are three m-files in my public
directory that I generated to analyze this experiment. The file "zin.m"
calculates and displays the input impedance of a section of 300 ohm line with
a load impedance of 75 ohms (the remainder of the normal cable was properly
terminated). Zin and the reflection coefficient at the input of the 300 ohm
line are displayed. (For the three mi-files just type help and then the name
of the file to see some comments on what the file is supposed to do.) The
second file I wrote is called "plotref.m" It plots the transmitted power even
though it says it plots the reflected power, and thus its name. It also plots
the qualitative data I obtained on picture quality when I inserted the 300 ohm
line into the cable feed. The third file is called "zincap.m" which adds a
capacitor at both the input and the output to the 300 ohm cable to represent
the rather poor quality connections that are possible between a coax and a
two wire line. plotref can be run again after zincap to see how the trans
power now compares with the measured data. Since the data is only very
qualitative, good agreement is not to be expected. However, you can see that
the major features of the data are found in either the results from zin or
zincap. Note that between these three files are several ideas that are more
than a little relevant to the design project this semester.