Applications
Observing Coincidence
of
Digital Signals
With digital signals applied to
CH
1 and
CH
2,
the Add waveform is high when
both
signals are high, low when both are low, and at
an
intermediate level when
one
signal
is
high
and
the other
is
low. By inverting
CH
2,
you can observe the
coincidence of one
signal and NOT the other. To observe coincidence of TTL
signals:
1.
Connect the signals of interest to
CH
1 and
CH
2.
If the coincidence of
interest has one
signal high and the other one low, invert
CH
2.
2.
Display
CH
1,
CH
2,
and Add. Set both VOL TS/DIV
to
2 V and both inputs
to
GND.
Position both channels on screen and the Add trace one division
above the bottom of the
graticule. Then deselect
CH
1
and
CH
2.
3.
Set both inputs to
DC.
Set Trigger mode
to
Auto and Source to Vert. If the
coincidence of interest
is
high-high, set trigger SLOPE to
+.
If the
coincidence
is
low-low, set SLOPE to
-.
(If
CH
2
is
inverted, consider the
inverse of the
CH
2 signal
in
the high-high
or
low-low combination.) Press
INIT@50%, then carefully adjust the trigger level to respond to the high-
high
or
low-low state combination. (Trigger level readout doesn't operate
with Add Source.)
4.
Now you
can
observe and measure coincidence durations and other time
intervals. Channels 3 and 4 can show relationships
to
other signals.
Measuring Off-Ground Signals
And
Cancelling
Interference
The
Add
mode
can
measure voltage between a pair
of
pOints
where neither
point
is
ground. The technique can cancel interfering signals or uninteresting
components of a
signal through common-mode rejection.
1. Display the signal on
CH
2 at the point you consider low, common, or
inverse
and
display on
CH
1 the high or active signal.
2.
Set both VOL TS/DIV equally and for three to eight divisions of amplitude
on
the larger of the pair of signals.
3.
Select the Add display, Invert
CH
2,
connect the
CH
1 probe temporarily to
the
CH
2 point, and adjust either the
CH
1 or
CH
2 VAR control, if
necessary,
to
minimize the amplitude of the Add display.
4.
Move the
CH
1 probe back to the active signal and observe the desired,
differential signal.
You may
be
able
to
increase vertical sensitivity by one VoltslDiv step, keeping
CH
1 and
CH
2 equal, without serious distortion. If the common mode signal has
the same repetition rate as the
signal of interest,
CH
1 or
CH
2 may be usable
as
the trigger source. Vert Trigger Source
is
often more desirable because it responds
only to the differential signal. Figure 3-2 shows
an
example.
2465A/2455A/2445A Operators
3-5
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