Tektronix 7a15a Instruction Manual page 11

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Vertical
distance
Reference line
Fig . 2-2 . Measuring instantaneous voltage with respect to same
reference .
Comparison Measurements
In some applications it may be desirable to establish
units of measurement other than those indicated by the
VOLTS/DIV
switch . This is particularly useful when
comparing unknown signals to a reference amplitude . One
use for the comparison-measurement technique is to facili-
tate calibration of equipment where the desired amplitude
does not produce an exact number of divisions of deflec-
tion . The adjustment will be easier and more accurate if
arbitrary units of measurement are established, so that the
correct adjustment is indicated by an exact number of divi-
sions of deflection . The following procedure describes how
to establish arbitrary units of measure for comparison
measurements .
To establish a vertical deflection factor based upon a
specific reference amplitude, proceed as follows :
1 . Connect the reference signal to the Input connector .
Set the time-base sweep rate to display several cycles of the
signal .
2 . Set the VOLTS/DIV switch and the VARIABLE
control to produce a display which is an exact number of
vertical divisions in amplitude .
Do not change the
VARIABLE control after obtaining the desired deflection .
3 . To establish an arbitrary vertical deflection factor so
the amplitude of an unknown signal can be measured
accurately at any setting of the VOLTS/DIV switch, the
amplitude of the reference signal must be known . If it is
Vertical
Conversion
Factor
Operating Instructions-7A15A/7A15AN
not known, it can be measured before the VARIABLE
control is set in step 2 .
4 . Divide the amplitude of the reference signal (volts)
by the product of the vertical deflection (divisions) estab-
lished in step 2 and the setting of the VOLTS/DIV switch .
This is the vertical conversion factor .
reference signal amplitude ( volts)
vertical deflection X VOLTS/DIV
(divisions) setting
5 . To measure the amplitude of an unknown signal,
disconnect the reference signal and connect the unknown
signal to the Input connector . Set the VOLTS/DIV switch
to a setting that provides sufficient vertical deflection to
make an accurate measurement . Do not re-adjust the
VARIABLE control .
6 . Measure the vertical deflection in divisions and calcu-
late the
amplitude of the unknown signal
using the
following formula :
Signal VOLTS/DIV
vertical vertical
Amplitude
setting
X conversion X deflection
factor (divisions)
EXAMPLE : Assume a reference signal amplitude of 30
volts, a VOLTS/DIV setting of 5 V and the VARIABLE
control adjusted to provide a vertical deflection of four
divisions .
Substituting these values in the vertical conversion factor
formula (step 4) :
Signal
Amplitude
Vertical Conversion 30 V
= 1 .5
Factor 4 X 5 V
Then with a VOLTS/DIV setting of 2 V, the peak to peak
amplitude of an unknown signal which produces a vertical
deflection of five divisions can be determined by using the
signal amplitude formula (step 6) :
= 2 V X 1 .5 X 5 =
15 volts
2-5