BK Precision 5105B, Instruction Manual, Page: 18 / 48

18
Subject to change without notice
After the time base deflected the trace from left to right the
trace will be blanked so the retrace is invisible. The next sweep
will, however, not immediately start. Time is required to per-
form 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 conincide 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
can not prevent the double display but will only affect the
apparent time relationship between two signals. Here the va-
riable 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:
Fig. 1: Display with minimum hold-off time (basic setting).
Double image, no stable display.
Fig. 2: By increasing the hold-off a stable display is achieved.
Time base B (2
nd
time base). Delaying, Delayed
Sweep. Analog mode
Consult ”Controls and Readout“ HOR 30 and TIME/DIV. 28 for
specific information.
As was described in ”Triggering and time base“ a trigger will
start the time base. While waiting for a trigger – after runout
of the hold-off time – the trace will remain blanked. A trigger
will cause trace unblanking and the sweep ramp which deflects
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
retrace to the start position. During a sweep the trace will
also be deflected vertically by the input signal. In fact the input
signal does continuously deflect the trace vertically, but this
will be only visible during the unblanking time. This is, by the
way, one marked difference to DSO operation where the input
signal is only measured during the acquisition time, for most
of the time the DSO will not see the signal. Also, in analog
mode the signal itself will be seen on the screen in real time,
whereas a DSO can only show a reconstruction of the signal
acquired some time later.
In analog mode thus the display will always start on the left.
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 fine
detail, but it is impossible to show such fine detail for ”later“
parts of the signal.
The x10 Magnifier (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 fixed.
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 first displayed by
TB A alone. Then TB B is also turned on which is the mode
”A intensified 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 comparison voltage a signal is generated anywhere
between sweep start and end.
In one of two operating modes this signal will start TB B
immediately. The TB A display will be intensified 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 as the rep rate will remain that of the
accepted signal triggers while the duration of TB B is reduced
with increasing speed.
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 start on 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 artificial 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. What was said above about how TB B
can be started holds also here.
T r i g g e r i n g a n d t i m e b a s e