Tektronix 7A18A Instruction Manual page 16

Operating Instructions—7A18A
Comparison Measurements
In some applications it may be desirable to establish arbi-
trary units of measurement other than those indicated by
the VOLTS/DIV switch. This is particularly useful when com-
paring unknownsignals to a reference amplitude. One use
for the comparison-measurement technique is to facilitate
calibration of equipment where the desired amplitude does
not produce an exact numberofdivisions of deflection. The
adjustmentwill be easier and more accurateif arbitrary units
of measurementare established so that the correct adjust-
mentis indicated by an exact numberof divisions of deflec-
tion. The following procedure describes how to establish
arbitrary units of measure for comparison measurements.
To establish an arbitrary 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 unit sweep rate to display several cycles
of the signal.
2. Set the VOLTS/DIV switch and the VARIABLE con-
trol to produce a display which is an exact numberofverti-
cal 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 unknownsignal can be measured accu-
rately at any setting of the VOLTS/DIV switch, the ampli-
tude of the reference signal must be known. If it is not
known, it can be measured before the VARIABLE
VOLTS/DIV control is set in step 2.
4. Divide the amplitude of ithe reference signal (volts) by
the product of the vertical deflection (divisions) established
in step 2 and the setting of the VOLTS/DIV switch. This is
the vertical conversion factor.
Vertical reference signal
Conversion = ampliltude (volts)
Factor vertical VOLTS/DIV
deflection X switch
(divisions) setting
5. To measure the amplitude of an unknownsignal, dis-
connect the reference signal and connect the unknown sig-
nal to the input connector. Set the VOLTS/DIV switch to a
setting that provides sufficient vertical deflection to make an
accurate measurement. Do not readjust the VARIABLE
control.
2-6
6. Measure the vertical deflection in divisions and calcu-
late the amplitude of the unknownsignal 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 V, a VOLTS/DIV setting of 5 volts and the VARIABLE
control adjusted to provide a vertical deflection of four divi-
sions. Substituting these values in the vertical conversion
factor formula (step 4):
30 V
Vertical Conversion Factor =
i c a l Conversi aX5V
Then with a VOLTS/DIV setting of 2 V, the peak-to-peak
amplitude of an unknownsignal which produces vertical
deflection of five divisions can be determined by using the
signal amplitude formula (step 6):
Signal
Amplitude ~ 2VX1.5 X5 - 15 volts
Dual-Trace Phase Difference Measurements
Phase comparison between twosignals of the samefre-
quency can be made using the dual-trace feature of the
7A18A. This method of phase difference measurement can
be used up to the frequencylimit of the oscilloscope system.
To make the comparison, use the following procedure:
1. Set the CH 1 and CH 2 coupling switches to the same
position, depending on the type of coupling desired.
2. Set the DISPLAY MODE to ALT or CHOP. In general,
CHOP is more suitable for low frequencies and ALT is more
suitable for high frequencies. Set the TRIGGER SOURCE to
CH 1.
3. Connect the reference signal to the CH 1 input and
the comparison signal to the CH 2 input. Use coaxial cables
or probes which havesimilar time delay characteristics to
connectthe signals to the input connectors.
4. If the signals are of opposite polarity, set the CH 2
POLARITY switch to invert the channel 2 display. (Signals
may be of opposite polarity due to 180° phase difference; if
so, take this into accountin the final calculation.)
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