18
Subject to change without notice
sweep will, however, not immediately start. Time is required
to perform internal switching, so the next start is delayed for
the so called hold off time, irrespective of the presence of
triggers. The hold off time can be extended from its minimum
by a factor of 10:1. Manipulation of the hold-off time and thus
of the time for a complete sweep period from start to start can
be useful e.g. when data packets are to be displayed. It may
seem that such signals can not be triggered. The reason is
that the possible start of a new sweep does not coincide with
the start of a data packet, it may start anywhere, even before
a data packet. By varying the hold off time a stable display will
be achieved by setting it just so that the hold off ends before
the start of a data packet. This is also handy with burst signals
or non periodic pulse trains.
A signal may be corrupted by noise or hf interference so a double
display will appear. Sometimes varying the trigger level cannot
prevent the double display but will only affect the apparent time
relationship between two signals. Here the variable hold off time
will help to arrive at a single display.
Sometimes a double display will appear when a pulse signal
contains pulses of slightly differing height requiring delicate
trigger level adjustment. Also here increasing the hold off time
will help.
Whenever the hold off time was increased it should reset to its
minimum for other measurements, otherwise the brightness
will suffer as the sweep rep rate will not be maximum. The
following pictures demonstrate the function of the hold off:
Picture 1: Display with minimum hold off time (basic setting).
Double image, no stable display.
Picture 2: By increasing the hold off a stable display is achieved.
Time base B (2
nd
time base), Delaying, Delayed,
Sweep
Consult ”Controls and Readout“ HOR VAR
26
and TIME/DIV.
24
for specifi c information.
As was described in ”Triggering and time base“ a trigger will
start the time base. While waiting for a trigger, after completing
the hold off time, the trace will remain blanked. A trigger will
cause trace unblanking and the sweep ramp which defl ects
the trace from left to right with the speed set with TIME/DIV.
At the end of the sweep the trace will be blanked again and
reset to the start position. During a sweep the trace will also be
defl ected vertically by the input signal. In fact the input signal
does continuously defl ect the trace vertically, but this will be
only visible during the unblanking time.
Let us assume one period of a signal is displayed at a convenient
time base setting. Increasing the sweep speed with TIME/DIV.
will expand the display from the start, so that parts of the signal
will disappear from the screen. It is thus possible to expand
the beginning of the signal period and show fi ne detail, but it
is impossible to show such fi ne detail for ”later“ parts of the
signal.
The x10 Magnifi er (MAG x10) may be used to expand the display
and the horizontal positioning control can shift any part of the
display into the centre, but the factor of 10 is fi xed.
The solution requires a second time base, called time base B.
In this mode time base A is called the delaying sweep and
time base B the delayed sweep. The signal is fi rst displayed by
TB A alone. Then TB B is also turned on which is the mode”A
intensifi ed by B“. TB B should always be set to a higher sweep
rate than A, thus its sweep duration will be also shorter than
that of A. The TB A sweep sawtooth is compared to a voltage
which can be varied such that TB A functions as a precision
time delay generator. Depending on the amplitude of the com-
parison voltage a signal is generated anywhere between sweep
start and end.
In one of two operating modes this signal will start TB B im-
mediately. The TB A display will be intensifi ed for the duration
of TB B, so that one sees which portion of the signal is covered
by TB B. By varying the comparison voltage the start of TB B
can be moved over the whole signal as it is displayed by TB A.
Then the mode is switched to TB B. The signal portion thus
selected is now displayed by TB B. This is called „B delayed by
A“. Portions of the signal can thus be expanded enormously,
however, the higher the speed of TB B, the darker the display
will become: the rep rate will remain that of the input trigger
signal while the duration of TB B is reduced with increasing
speed. The readout display is not affected.
In cases where there is jitter the TB B can be switched to wait
for a trigger rather than starting immediately. When a trigger
arrives TB B will be started by it. The jitter is removed, however,
the effect is also, that the TB B start now can be only from signal
period to signal period, no continuous adjustment is possible
in this mode.
Alternate sweep
In this mode the signal is displayed twice, with both time bases.
An artifi cial Y offset can be added in order to separate the two
displays on the screen. The operation is analogous to Y dual
trace alternate mode, i.e., the signal is alternately displayed by
both time bases, not simultaneously which is not possible with
a single gun crt. TB B operation is the same here.
T r i g g e r i n g a n d t i m e b a s e
period
heavy parts are displayed
signal
adjusting
HOLD OFF time
sweep
Fig. 1
Fig. 2
