Differential Measurements - Tektronix 222 Operator's Manual

Making Measurements-222 DSO Operators
In the example illustration, the horizontal distance between the cross-
over points for 10% and 90% is about 0.9 division. Multiply that by the
SECIDIV readout to obtain a rise time of approximately 45 ns. A trig-
ger position of MID is shown in the example to display the complete
rising edge of the signal. If fall time is to be measured, use negative
Trigger SLOPE to view the falling edge of the signal.
5.3 FLOATING MEASUREMENTS
The vertical inputs and probe reference leads of the
222
Digital
Storage Oscilloscope are completely isolated from the chassis,
external ground, and from each other. This feature makes it ideal for
making waveform measurements directly across circuit components
without regard to the ground reference. This type of measurement is
not possible using a grounded oscilloscope without special isolation
adapters. The double-insulated construction also permits safe use of
the
222
for making elevated voltage measurements (up to the safe
input limits given in the Specifications).
If either the
EXT
TRIG COMM connector or the RS-232
cable is connected to an external earth ground or
external power is applied, the oscilloscope chassis
cannot be elevated.
5.4
DIFFERENTIAL MEASUREMENTS
The differential vertical inputs make it possible to perform in-circuit
measurements while rejecting common-mode noise found in many
power supply and low-level signal circuit. A typical use for rejecting
common-mode noise is shown in Figure 5-5.
In 5-5A, the common-mode noise completely hides the audio signal
of interest. With the 222 isolated differential vertical input, the under-
lying signal, in this case the dial tone on a typical telephone system,
is free of the noise as seen in 5-5B.
5-6 REV JUN 1989