PHY 132 LAB : Oscilloscope
Introduction
In this lab, we consider AC signals, and use of the oscilloscope. This material is
out of sequence with PHY131, but the concepts are not difficult. Text Reference:
Young & Freedman 26.1.
The oscilloscope (scope) is an indispensable tool for measuring electrical signals. It
gives a display of voltage (vertical) vs. time (horizontal). These scales are
accurately calibrated, so you can obtain numbers from the display. There are a
zillion knobs, most of which we can ignore for now. You do need to understand the
following:
1.
Gain and position
of Y and X axes. These are marked as volts/division
and
sec/division. 1 division is a 1cm box on the screen. A value is determined from
the display as volts = (# divisions) * (volts/division). You can generally read
this to 0.1 division. Take care that the CAL knobs are clicked into the calibrated
position.
2.
Input coupling
: A/C or D/C or GND. D/C feeds the signal directly into the
scope. A/C feeds only the A/C part after stripping the average (D/C) value.
GND shorts the input providing an accurate zero reference, for positioning the
display.
3.
Focus/Intensity
: adjust these to get a clear sharp line, but don’t burn the screen.
Wiring tips and Digital Multimeters:
See notes appended.
Theory:
AC signals frequently take the form of a sinewave, which we write as
v(t)=V
0
sin(
ω
t) + V
DC
eq. 1
This has amplitude V
0
, and angular frequency defined as
ω
= 2
π
/T = 2
π
f
eq. 2
where T is the period (sec) and “f” is the frequency, in cycles per second or Hertz (Hz). It
is possible also to have a DC offset V
DC
.
PHY132 Labs (Bennett, JCHS)
-1-
08/21/06

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