Operating Instructions— Type 1A1
to trigger on high-frequency asynchronous signals that are
vertically positioned apart on the CRT with POSITION con
trols. If jitter occurs, it can be reduced and sometimes
eliminated by positioning the displays close together or
superimposing them. This not only reduces jitter, but may
also increase the display brightness.
If you use the AC or AC Slow triggering mode, stable
internal triggering on asynchronous signals above 1 kHz
is more difficult to obtain and the jitter will be greater. At
sweep rates faster than 0.5 ms/cm, the dual-trace display be
comes noticeably brighter as the traces are vertically posi
tioned closer together, and dimmer as the traces are more
widely separated. These effects are normal and are caused
by the problem of triggering on the alternate-mode com
posite trigger waveform. The waveform is very similar to
the one described for chopped-mode operation.
The alternate-mode composite trigger consists of the asyn
chronous signals applied to the Type 1A1, superimposed on
the DC positioning levels of the alternate-mode switching
waveform. The switching waveform portion of the compos
ite trigger is a low-frequency squarewave whose amplitude
is governed by the setting of the POSITION controls and
DC components (if any) of the applied signals. By itself,
the switching waveform viewed on a test oscilloscope re
sembles the waveshape shown in Fig. 2-1 B when the traces
are positioned 2 cm apart. Repetition rate of the switching
waveform is one half of the sweep repetition rate.
When the alternate-mode composite trigger is internally
AC coupled to the oscilloscope trigger input circuit, the trig
ger circuit may not respond instantly to the signals super
imposed on the alternate-mode switching signal. The de
lay is caused by the recovery time of the trigger input cir
cuit as each cycle of the low-frequency switching waveform
couples into the input stage of the trigger circuits. Since
AC coupling is used in all the Triggering Coupling switch
positions (AC, AC Slow, AC Fast, AC LF Reject) recovery
time is dependent on the RC time constant of the trigger in
put circuit.
In conclusion, trigger circuit recovery time is shorter, hence,
the sweep repetition rate is higher and the display is bright
er, if AC Fast or AC LF Reject triggering mode is used. In
either of these triggering modes, a smaller value coupling
capacitor is used in the oscilloscope trigger input circuit
as compared to the value used in the AC or AC Slow trig
gering mode. Trigger recovery time can be shortened and
triggering will be more stable if high-frequency waveform
displays are vertically positioned closer together or super
imposed rather than positioned further apart.
Displaying Two Synchronous Signals, 250 Hz and
Above.
To show true time and phase relationship between
two synchronous signals, 250 Hz and above, use alternate
mode operation and trigger on Channel 1 only. In practice,
for displaying signals between 250 Hz and 10 kHz you can
choose either alternate- or chopped-mode operation since
TABLE 2-2
Dual-Trace Operation
Applied Signals
(one to Channel
1 and other to
Channel 2)
Type 1A1
MODE Switch
Setting
Oscilloscope Trig
gering Source
Switch Setting
Oscilloscope Trig
gering Coupling
Switch Setting
Display shows
true time
relationship
between signals
1. Two n o n -re p e titiv e
signals or two low-fre
quency synchronous sig
nals (below 10 kHz). Ap
ply reference signal to
Channel 1.
CHOP
Plug-In3 or Ext (con
nect
coaxial
cable
from CH 1 TRIGGER
connector on oscillo
scope).
AC or AC Slow or
AC Fast or AC LF Re
ject.
Yes
Use sweep rates up to
10/.is/cm. Higher sweep
rates reduce resolution.
2. Two asynchronous sig
nals, any frequency with
in full bandwidth of the
system.
ALT
Norm Int or Int4
AC or AC Slow for
frequencies below 1
kHz.
AC Fast or AC LF
Reject for frequencies
above 1 kHz.
No
3. Two synchronous sig
nals, 250 Hz and above.
ALT
Plug-In3 or Ext (con
nect c o a x ia l cable
from CH 1 TRIGGER
OUT c o n n e c to r to
Trigger Input connec
tor on oscilloscope).
Norm Int or Int4
AC or AC Slow or
AC Fast or AC LF
Reject.
AC Fast or AC LF
Reject.
Yes
Apply reference signal
to Channel 1.
No
3Plug-lrt position is the Channel 1 only internal signal available at pin 5 of the Type 1A1 interconnecting plug to the oscilloscope. If your
oscilloscope is not wired to permit use of this trigger source, use the Ext position and CH 1 TRIGGER OUT signal.
‘ Norm Int or Int switch position is the internal trigger takeoff signal from the oscilloscope vertical amplifier. In dual-trace operation this trigger
is a composite of the applied signals superimposed on the DC positioning levels of the channels as they are switched.
2-10
