Tektronix TDS 220 Manuale di riferimento

ECE 488: RF Circuits and Systems Spring 2005

CORNELL UNIVERSITY © WES SWARTZ (03/8/2)

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)

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)

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)

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)

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

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)

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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