Tektronix 2A61 Instruction Manual page 10

Operating Instructions—Type 2A61
(B) Signal applied to —INPUT.
(C) Resultant display.
Fig. 2-2. Typical CRT displays showing differential rejection of common mode signal. Resultant waveform (C) displays the difference in
signals A and B.
Setting the DC BAL Control
The DC BAL control may need to be set occasionally dur-
ing regular operation of the Type 2A61. Proper DC balance
minimizes trace shift when rotating the MILLIVOLTS/DIV
switch from one position to another.
To set the DC BAL control, proceed as follows:
1. Plug the Type 2A61 into the left-hand plug-in compart-
ment of a Tektronix 560-series Oscilloscope and tighten the
securing knob. Turn the oscilloscope power on and allow
about 5 minutes for the system to reach operating tempera-
ture.
2. Free run the oscilloscope time base at a sweep rate
of 2 ms/div.
3. Set the Type 2A61 front panel controls as follows:
INPUT SELECTOR GND
MILLIVOLTS/DIV 5
VARIABLE CALIB
FREQ RESPONSE
FROM 60 Hz
TO 60 Hz
FILTER OUT
4. Vertically position the trace to the CRT graticule center
line using the Type 2A61 POSITION control.
5. Reset the MILLIVOLTS/DIV switch to .01.
6. Adjust the DC BAL control to return the trace to the
graticule center line.
2-4
7. Reset the MILLIVOLTS/DIV switch to .5 and repeat steps
4, 5 and 6 until there is no trace shift while rotating the
MILLIVOLTS/DIV switch between .5 and .01.
Fig. 2-3. Example of a measurement situation in which CMRR is only
1000:1.