Propagation Delay Time Measurement:
An example of propagation delay in a divide-by-eight
circuit was given in the previous paragraph. Significant
propagation delay may occur in any circuit. This os-
cilloscope has features which simplify measurement of
propagation delay. Fig. 5 shows the resultant waveforms
when the dual-trace presentation is combined into a single-
trace presentation by selecting the ADD or S U B position of
the MODE switch. In the ADD position the two inputs are
algebraically added in a single-trace display. Similarly, in the
S U B position the two inputs are algebraically subtracted.
Either position provides a precise display of the propagation
time (Tp). Using the calibrated time base (CAL), Tp can be
measured. A more precise measurement can be obtained if
the Tp portion of the waveform is expanded horizontally by
pulling the 5X MAG control. It also may be possible to view
the desired portion of the waveform at a faster sweep speed.
CH
1
CH 2
E X P A N D T H I S PORTION
FOR MORE P R E C I S E
T I M E M E A S U R E M E N T
Fig.
4.
Using ADD or SUB modes for propagation time measurement
Digital Circuit Time Delay Measurement:
Since a dual-trace oscilloscope has the capability of
comparing the timing of one waveform with another, it is
necessary in designing, manufacturing and servicing digital
equipment. In digital equipment, many of the circuits are
frequency dividers as previously described, but waveforms
are often time-related in many other combinations. Fig. 5
shows a typical digital circuit and identifies several of the
points at which waveform measurements are appropriate.
Under the operating condition, waveforms will vary accor-
ding to the input or operating mode. Fig. 6 shows the
relationship between the normal waveforms to be expected
at each of these points and their timing. If the correct time
relationship with respect to other waveform is unknown,
measurement of individual waveforms means nothing. The
dual-trace oscilloscope allows this comparison to be made.
In an application, waveform No. 3 would be displayed on
CH1 and waveform No. 4 thru No. 8 and No. 10 would be
displayed on CH2. Waveforms No. 11 through No. 13
would probably be displayed on CH2 in relationship to
waveform No. 8 or No. 4 on CH 1. No. 8 or No. 10 is an ex-
cellent sync source for viewing all of the waveforms.
With No. 8 or No. 10 used as external sync source, any
of the waveforms may be displayed without readjustment of
the TRIG LEVEL control. Waveforms No. 4 through No. 7
should not be used as the sync source because they do not
contain a triggering pulse at the start of the frame. It would
not be necessary to view the entire waveforms as shown in
Fig. 6 in all cases. In fact, there are many times when a
closer examination of a portion of the waveforms would be
appropriate. In such cases, it is recommended that the sync
remain unchanged while the sweep speed be increased or
X5 MAG control used to expand the waveform display.
A D D
SUB
9
