Unformatted text preview: am is not shining in anyone's eyes. You should now see a pulse
(blue trace) on the screen - “zoom in” on the pulse using Volts/Div and Sec/Div knobs. Immediately to the
right of the big pulse (the pulse coming out of the laser) is a smaller pulse of roughly the same shape and
polarity (upward pulse) – this is the reflection off a cable which is not terminated (no terminator in Tconnector). The pulse is best viewed at 1 V/Div. Sketch both pulses in the hand-in sheet.
NOTE: If you don't see any pulses, first make sure that the “M Pos:” value at the top of the screen
reads “0.000s”. Then raise the Trigger Level knob until you see the pulse(s). This adjusts the
triggering – the scope's capability to properly track a single waveform (remember, these waveforms are
traveling in time as signals are constantly being emitted by the laser box) and hold it steady for you to
examine and measure.
Also, ensure that the scope is triggering off CH 2 – this will be the case if the color of the “Trig'd” label on
the top of the screen (say, green) is the same as the color as the “CH 2” label at the bottom left of the
screen. If not, press the Trig Menu button at the right of the scope and choose CH 2 from the selection
buttons to the right of the screen.
If you're still having trouble locating the pulse, press the DEFAULT SETUP button and go back to Step
5. Now grab a paper clip and bend it so that it forms a U-shape. You will use the two ends of the clip to
“short” the inner and outer connectors on the open end of the CH2 T-connector. The waveform on the
screen should change radically – sketch this new waveform in your hand-in sheet. Remove the paper clip.
6. Let us now terminate this cable. Gently insert a 50Ω terminator into the T open end and observe the
reflection pulse – what happens to it? Write in hand-in sheet.
7. With the 50Ω terminator still in place, replace the shorter coaxial cable with the longer coaxial cable...
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- Summer '09
- Light, Coaxial cable, pulse, hand-in sheet