Tektronix TDS 220 Manuale di riferimento
ECE 488: RF Circuits and Systems Spring 2005
CORNELL UNIVERSITY © WES SWARTZ (03/8/2)
1
The Tektronix TDS 220 Oscilloscope
A Very Brief Introduction
A Tektronix TDS 220 two-channel digital oscilloscope (“scope” or “Oscope” for short) is
pictured below with its controls grouped for easy recognition in the discussion that
follows.
Figure 1: Front panel of a Tektronix TDS 220 oscilloscope.
1. Oscilloscope display
The LCD display replaces the cathode-ray tube used in older oscilloscopes. It adds
digital readouts of various settings and menu options. The example shown here displays
only Channel-1 on a voltage scale of 2 Volts per division (as indicated by CH1 2.00V in
the bottom-left corner of the display). The digit “1” with the arrow along the left edge
indicates the zero base line for the voltage of Channel 1. The time scale is set at 250 ns
(250 nano-seconds or 2.5X10-7seconds) per division. Since there are 10 divisions from
left to right, the display shows a total time of 2.5 µs (2.5 micro-seconds). The downward
pointing arrow at the middle of the screen’s top edge indicates when triggering of the
display occurred (on a rising edge of the square wave). The leftward-pointing arrow
along the right side of the display indicates the voltage threshold that will trigger a sweep
of the trace. The items down the right side of the display indicate the functions of the
side-screen menu buttons for the particular menu (CH1) currently selected. This
ECE 488: RF Circuits and Systems Spring 2005
CORNELL UNIVERSITY © WES SWARTZ (03/8/2)
2
particular menu was selected by the CH 1 MENU button within the Vertical Controls
section. The DC coupling mode means that “Direct Current” voltages are fed in (and not
blocked as in AC
coupling). Higher
frequencies can be
blocked by changing the
BW (band-width) setting
to ON. The 10X probe
setting means that the
selected probe attenuates
the signal by a factor of
ten and that the readings
on the scope will be
adjusted to compensate
for this factor.
A second example for the
display is shown in
Figure 2. Here the same
pulse-train signal is fed
into both channels. Note
the change in the time
base.
2. Vertical controls
The vertical position and scaling of the traces for the
two channels is controlled by the two sets of knobs
shown in Figure 3. The POSITION knobs adjust the
vertical position of the zero voltage level for each
channel. (In extreme positions, the traces can be moved
completely off the display. Beware of this.) The
VOLTS/DIV (volts-per-division) knobs control the
amount of gain applied to the signals and hence
determine the vertical axis scales of the display. These
values are displayed along the bottom edge of the
screen as mentioned above under the display section.
The probe point is attached to the signal source to be
measured, and the black wire lead is attached to a
suitable ground (usually the ground pin of the same IC
to minimize noise pickup). Try adjusting these knobs
while observing the effects on the display.
Figure 3: Vertical Controls.
Figure 2: Example of a two-trace display on the TDS 220.
ECE 488: RF Circuits and Systems Spring 2005
CORNELL UNIVERSITY © WES SWARTZ (03/8/2)
3
The CH 1 MENU and CH 2 MENU pushbuttons cause the appropriate input channel menu
functions to appear on the side-screen menu and can toggle the channel trace on and off.
3. Horizontal controls
The horizontal position and scaling of the waveform traces are
controlled by the two knobs shown in Figure 4. The POSITION
knob moves the traces left and right on the screen. The
SEC/DIV (seconds per division) knob sets the time interval
between each major grid division (approximately one
centimeter) along the horizontal axis of the display. This time
interval is common to all channels currently being displayed. It
is adjustable from 5 nanoseconds to 5 seconds per division.
Finally, the HORIZONTAL MENU button selects additional
horizontal functions for the side-screen menu.
Warning 1: It is entirely possible to move the traces completely
off the screen with the POSITION knob.
Warning 2: Since this is a “sampling” scope, make sure that
the time base is properly set to avoid aliasing of high frequency
signals.
4. Trigger controls
Triggering determines when the painting of a trace across the display
begins. Triggering can be automatic or forced manually depending
on the settings of these controls. Triggering can be repetitive (for
periodic waveforms) or produce just a single sweep for single events.
Automatic triggering begins when the input voltage crosses a selected
voltage level (determined by the LEVEL knob) in the selected
direction (either increasing or decreasing as determined by the RISING
EDGE/FALLING EDGE item in the side-screen menu selected by the
TRIGGER MENU button). The input voltage used to cause triggering
can be either of the input channel signals (CH 1 or CH 2 BNC input
connectors) or a special signal delivered to the EXT TRIG input BNC.
The triggering signal can be shown on the display by pushing and
holding the TRIGGER VIEW button. Improper setting of the trigger
controls can result in an overlay of confusing multiple traces, or no
trace at all. A wonderful feature of the TDS 220 is the automatic
setup feature initiated by the AUTOSET button (see Figure 1).
Figure 4: Horizontal
Controls.
Figure 5:
Trigger
Controls
ECE 488: RF Circuits and Systems Spring 2005
CORNELL UNIVERSITY © WES SWARTZ (03/8/2)
4
Whenever the display is crazy, try pushing this button before calling your TA.
Although the trigger signal determines when the trace is to be displayed, the neat thing
about these digital scopes is that this time does not need to correspond to the beginning of
the trace. The buffering of the input samples allows the trace on the display to begin
before the actual time of the triggering event. The time at which the trigger occurred is
marked by a downward-pointing arrow along the top edge of the display, as mentioned
earlier.
Manual triggering is initiated by the FORCE TRIGGER button.
The LEVEL knob serves another function called HOLDOFF which sets how much time
must elapse before another trigger can occur. The SET LEVEL TO 50% button causes the
trigger level to be set at the midpoint between the peaks and valleys of the triggering
signal.
Another important trigger item selected by the side-screen menu buttons is the source of
the triggering signal. Don’t select a source that is not connected!
5. Handy buttons
The cluster of three buttons in the top-right corner of the TDS 220 will prove to be quite
useful. The AUTOSET button has already been mentioned under Trigger Controls, but it
does much more. In fact, whenever you first connect it to a new point on your circuit,
you should push AUTOSET to have the scope automatically adjust the vertical and
horizontal settings as well as the trigger. Then you can optimize the display using the
separate vertical and horizontal controls. Since this function selects the lowest numbered
active channel for triggering, I recommend that channel 1 be used as the trigger source
whenever you are not using the external trigger port.
The RUN/STOP button toggles between freezing the display and allowing it to free-run.
The HARDCOPY button can send a copy of the display to a printer, if one is connected.
6. Automated measurements using digital readouts
The TDS 220 oscilloscope can produce digital readouts of most of the relevant input
signal parameters, including frequency, period, mean (average) voltage, and peak-to-peak
voltage. (Note that the “amplitude” of a sine wave is the zero-to-peak voltage, not the
peak-to-peak value.) The following steps give an example:
1. Push the MEASURE button to see the Measure side-screen menu.
2. Push the top side-screen menu button to toggle the selection to SOURCE.
3. Select
CH1 for the sources in the next four menu boxes.
ECE 488: RF Circuits and Systems Spring 2005
CORNELL UNIVERSITY © WES SWARTZ (03/8/2)
5
4. Push the top menu button to
select TYPE.
5. Select FREQ, PERIOD,
MEAN and PK-PK for the
four measurement types.
The values of the selected
items are then shown in the
menu and are updated
periodically. An example of a
display produced with the
above setup sequence is shown
in Figure 6.
6. Manual measurements using digital readouts
The TDS 220 can display lines (called cursors) that can be manually placed to measure
voltages and times. Suppose you want to measure the peak-to-peak voltage of a sinusoid
that has some superimposed noise spikes. You can get a measure of this by setting the
cursors (via the upper knobs in the Vertical Controls section) at positions that ignore the
noise. (The automated measurement will give the extreme value from the lowest valley
to the highest peak.) The peak-to-peak measurement would proceed as follows:
1. Push the CURSOR button to see the Cursor menu.
2. Push the top menu button to
select Voltage (toggling
through Time and Off).
3. Push the Source menu button to
select CH1.
4. Use the
CURSOR 1 knob to
place a cursor at the bottom of
the valleys as shown in Figure
7.
5. Use the
CURSOR 2 knob to
place the remaining cursor at
the top of the peaks.
Figure 6: Example of automated measurements with
digital readouts.
Figure 7: Voltage measurements using the cursors.
Horizontal
Cursors
ECE 488: RF Circuits and Systems Spring 2005
CORNELL UNIVERSITY © WES SWARTZ (03/8/2)
6
The side-screen Cursor menu now shows:
• The voltage at Cursor 1.
• The voltage at Cursor 2.
• Delta shows the voltage difference between the two cursors, which is the desired peak-
to-peak voltage measurement.
Now, suppose you want to measure the time between two events displayed on the scope.
Again, you will use the upper knobs in the Vertical Controls section to set the positions
of the cursors. Proceed as follows:
1. Push the CURSOR button to see
the Cursor menu.
2. Push the top menu button to
select Time.
3. Push the Source menu button to
select CH1.
4. Use the CURSOR 1 knob to place
a cursor at the start of the time
period to be measured, as shown
in Figure 8.
5. Use the CURSOR 2 knob to place
the remaining cursor at the end of
the time period.
The side-screen Cursor menu now
shows:
• The time at Cursor 1.
• The time at Cursor 2.
• Delta shows the time difference,
which is the length of the period,
and the frequency (reciprocal of
the period).
Acknowledgements: My thanks to some unknown author for producing the pictures of the TDS 220 used here.
Figure 8: Time measurements using the
cursors.
Vertical Cursors
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