Tektronix 585 - Page 24

Applications — Type 585
Fig. 3-2. Measuring the peak-to-peak ac component of an applied waveform.
When sinusoidal waveforms are measured, the peak-to-
peak voltage obtained can be converted to peak, rms, or
average values through the use of standard conversion
factors.
Instantaneous Voltage Measurements
The method used to measure instantaneous voltages is
virtually identical to the method described previously for
the measurement of the ac component of a waveform. The
primary difference is that instantaneous voltage measure 
ments must be made with respect to some reference potential
(usually ground). In this type of measurement, the reference
potential must be used to first establish a reference line on
the graticule. Voltage measurements are then made with
respect to this reference line. In this type of measurement,
the probe Input Selector switch must be placed in the DC
position. The method used to measure instantaneous volt 
ages can also be extended to measurement of the d c com 
ponent of a waveform since the average voltage of a wave 
form can be treated as an instantaneous voltage. The d c
component of any waveform can therefore be measured if
the average voltage can be determined.
In the following procedure, steps are given for establish 
ing the voltage reference line as ground. The same general
method may be used to measure voltage with respect to
any other potential, so long as that potential is used to
establish the reference line. If for example, voltage measure 
ments are to be made with respect to + 10 volts, the refer 
ence line would be established to correspond to that po 
tential.
To measure an instantaneous voltage with respect to
ground perform the following steps (see Figure 3-3):
1. To establish the voltage reference line, select the proper
attenuator for the expected signal voltage, clip the probe
ground lead to the chassis, and touch the probe or attenu 
ator tip to an oscilloscope ground terminal or the voltage
reference required. Adjust the oscilloscope controls for a free-
running sweep. Vertically position the trace to a convenient
point on the crt. This point will depend on the polarity
and amplitude of the input signal, but should always be
chosen so that the trace lies along one of the major divisions
of the graticule. If the trace is widened by stray interference,
ground the probe body near the tip to reduce the inter 
ference. The graticule division corresponding to the position
of the trace is the voltage reference line and all voltage
measurements must be made with respect to this line. (Do
not adjust the vertical positioning control after the reference
line has been established.)
2. Remove the probe or attenuator tip from ground and
connect it to the signal source. Adjust the triggering con 
trols for a stable display. The probe ground lead should
be either coaxial or very short.
3. Using the graticule, measure the vertical distance in
centimeters from the point to be measured to the voltage
reference line.
4. Multiply the distance measured first by the plug-in sensi 
tivity in volts per centimeter, then by the attenuation factor
of the attenuator or probe used.
As an example, assume that when using the P80 Probe
with a 10-1 attenuator you measure a vertical distance of
2.5 centimeters between the reference line and the point to
be measured. In this case, 2.5 centimeters multiplied by
0.1 volt per centimeter gives 0.25 volt. This figure multi 
plied by the attenuation factor of 10 gives the instantaneous
voltage, 2.5 volts. If the point that is measured is above
the reference line, the polarity is positive; if the point is
below the reference line, the polarity is negative.
3-2