Tektronix 2247A Operator's Manual page 72

. ,0
Basic Applications
Set the VOL TS/DIV control so that the displayed signal will have a good
vlewln~ a~plitude. For the assumed signal shown In Figure 5-8, a setting of
1 V / div will produce a four to five division display amplitude. Position the
ground cursor (using the VERTICAL POSITION control) at about three
g_ratlcule dlvlsl~ns below the center graticule line. ( For negative-going
signals, two divisions above the center graticule would about center the
display; and for bipolar signals, centering the ground trace Is appropriate.)
1
Zeroing the Trigger LEVEL control may be useful if the cursor is positioned
out of the viewing area. Simply switch the Input COUPLING of the selected
channel to GND and switch the Trigger MODE to AUTO LEVEL. Once
zeroed, set the Trigger MODE to NORM so that the Trigger LEVEL you set
will be maintained. (If AUTO LEVEL Is left on, the Trlgge~ LEVEL will con--
tinue to follow the applied signal; ground In this case.) Use the Trigger
LEVEL control to position the trigger level cursor to about 3
v
(measured
fro~ th~ gr~und cursor). For the assumed signal, this level is ample to
avoid tnggenng on the lower amplitude signals In the display. For other
w~veforms, the user must determine what trigger level is needed to obtain
triggering on a specific waveform amplitude.
Apply the signal to the appropriate input channel connector and set the
Input COUPLING to DC. The waveform display will now appear (assuming
the A INTENSITY Is set to a viewing level), and It will be triggered on the
larger amplitude pulses of the signal.
Once triggering Is obtained, the A SEC/DIV setting may be set to a faster
sweep speed to expand the triggering pulse for making any measurements
wanted.
Use of the Add Mode
With the VERTICAL MODE set to ADD, the resulting waveform fs the
algebraic sum of the signals applied to the Channel 1 and Channel 2 Inputs
(CH 1 + CH 2). A plus symbol (+) appears In the readout between the CH 1
and CH 2 VOL TS/DIV setting readout to Indicate that ADD Is active. If the
~H 2 INVERT feature Is turned on (INVERT button lit), the waveform
displayed Is the difference between the signals applied to the Channel 1 and
~hannel 2 Inputs. Neither the CH 1 nor CH 2 waveform needs to be
displayed to obtain the ADD trace, but any or ALL vertical input channel s
may be displayed at the same time If wanted by the user.
Wh~n the VOL TS/DIV switches of CH 1 and CH 2 are both set to the same
setting, the total deflection factor In the ADD mode Is equal to the deflec
tlon factor Indicated by either VOL TS/DIV readout. The I~ VOL TS--+I
5-22
2247 A Operator
Basic Applications
cursors may be used to make voltage measurements on the ADD trace If
either CH 1 or CH 2 Is displayed along with the ADD trace (and, of course,
both CH 1 and CH 2 at the same VOL TS/DIV setting). If any voltage
measurement function Is active, turning off CH 1 and CH 2 to display the
ADD trace by Itself causes the message "MEASMT SOURCE: CH 1 OR 2
ONLY" to appear and cancels the measurement. If calling for a voltage
measurement with ADD displayed and neither Cl-l 1 nor CH 2 displayed, the
CH 1 VERTICAL MODE Is turned on and the measurement is initialized to
the signal applied to the CH 1 Input.
Two common uses for ADD mode are: ( 1 ) providing a de offset to bring an
ac signal riding on top of a large de voltage within the graticule viewing area
and (2) canceling out a large line-frequency signal component to view some
small feature riding on the waveform In greater de tall using common-mode
rejection.
The followlng general precautions should be observed when using ADD
mode.
a. Do not exceed the Input-voltage rating of the oscilloscope or probe.
b. Do not apply signals that' exceed the equivalent of about eight times
the VOL T.S/DIV switch setitngs, since large voltages may distort the
display. For example, with a VOL TS/DIV setting of O. 5 V, the
v, o ltage applied to that channel should not exceed 4 V.
c . Use CH 1 and CH 2 POSITION control settings which most nearly
position the signal on each channel to mid-scr,en, when viewed
separately. This ensures the greatest dynamic range for ADD
mode signal displays. -
1
I
d . To have similar responses for both channels
I
use the
1
same Input
COUPLING for both CH 1 and CH 2. '
The following procedure shows how to eliminate an unwanted ac Input-
power frequency signal component from the displayed signal.
a. Perform the "Initial Setup" In "OPERATOR CHECKS AND ADJUST-
MENTS" Section 4, and center the bas~line trace vertically.
b. Apply the signal of Interest containing the unwanted line-frequency
component to the CH 1 Input connector.
c. Apply a line-frequency signal to the CH 2 input connector. To maxi-
mize cancellation, the signal applied to the CH 2 Input must be
exactly In phase ( or exactly 180
°
out of phase) with the frequency
component to be canceled from the CH 1 signal.
2247 A Operators
5-23