CHOPPED
Note
When
it
is
necessary
to observe a single
transient
at
two parts
of
a
circuit another
procedure
must
be
followed. In the foregoing
case,
one of the
signals
triggers the
sweep
and that amplifier remains
conducting for the
sweep duration. At the end of the sweep the
amplifiers are switched
and the
other
signal
then triggers the sweep
and that amplifier
remains conducting for the
sweep duration.
Each of the signals is
being displayed every
other sweep cycle. If you attempted to observe
a
single transient
in this manner the transient
will
pass through whichever
amplifier happens
to
be conducting
and
will
trigger the sweep.
This will display the transient as seen by
whichever
amplifier is conducting but when
the
amplifiers
are switched at the end of that
sweep there will be no further signal to
trigger
another sweep until the next
transient occurs.
The
problem here is to be able to observe
the
transient using both amplifiers during a
single sweep
cycle.
Turn the
MODE
switch
to CHOPPED. Now
the
two amplifiers are being switched on and
off independently of any signal. The
switching
rate
is approximately 100 kc so each amp
lifier
is
conducting for about
5
μ
sec
and then
is cut off while the other amplifier conducts
for
an
equal length of time.
It will
usually
be very difficult if
not
impossible
to trigger the sweep internally from
the signal so the triggering controls should
be
set for
external triggering.
The external
triggering signal
should bear a definite time
relationship
to the
displayed
signals.
It
is
possible that the
displayed
waveform could end
up
being
the
100 kc switching waveform.
This
is possible particularly if the
two
traces
are positioned
very
far
apart.
The
two signals to be observed
will
be
con
nected
to the
two inputs and both
waveforms
will
be displayed
during one sweep
cycle. Tran
sients
as
short as
1
msec
duration can be
well delineated, with
about
100
elements
in
each
trace. As before,
the
independent
control
of each amplifier will
allow you
to position,
attenuate,
or invert the
waveforms
so they
can be easily compared.
ADDED ALGEBRAICALLY
In
many applications, the desired signal
is
superimposed
on
an undesired signal such as
line
frequency hum, etc. The Algebraic Output
of the Type
CA unit (with the MODE switch
in
the ADDED
ALGEBRAICALLY
position)
makes it possible
in many cases to improve
the
ratio
of desired to undesired signal.
Connect
one input to a source containing both the
desired
and undesired signal. Connect the
remaining input to a source containing
only
the
undesired signal.
Place the MODE switch
in
the ADDED ALGEBRICALLY
position.
Set
the POLARITY
switches to opposite polarities
(depending
upon the polarity
of the desired
signal). By careful adjustment, especially at
low
frequencies,
of the VARIABLE
controls
and/or
the GAIN ADJ.
controls the
amplitude
of the undesired signal displayed can
be
reduced
by a factor
of 20 compared to the amplitude
of the
desired signal.
2-4
Operating
Instructions - Type CA
AA
