Table of Contents
Advertisement
21
Trigger circuit
Compared with trigger unit A of the basic oscil-
loscope this trigger circuit is of a simpler design in
that the trigger amplifier consists of along-tailed
pair circuit and the pulse shaper is aSchmitt-trigger.
(Fig. 18)
The source selection, slope selection and mode selec-
tion as well as the TV synchronisation separator are
identical to the ones of the basic oscilloscope (also
see Fig. 43 and Fig. 44 of the basic oscilloscope).
The trigger signal can be derived from either the Y-
deflection signal or the mains supply (via 27BU-X).
In the first case the trigger signal is applied sym-
metrically to trigger amplifier B1'—B1"; in the two
latter cases it is applied asymmetrically. The am-
plified signal is routed from the anode of B1' to the
Schmitt-trigger. The latter switches when the applied
signal reaches a certain level. Thus, a square wave
originates at the anode of B3" which is differentiated
by'the C24—R66 network.
The resulting negative spikes will switch over the
S.G.M.V. (B4"—B6') to start the time base generator,
but the positive spikes are suppressed by diode GR7
to prevent undue resetting of the S.G.M.V.
The level at which the Schmitt-trigger will operate
is set by potentiometer R59 "Trigger Sens". The
moment at which the trigger signal will reach that
level depends on the d.c. potential at the B1" con-
trol grid. When mode-switch SK1 is in position
"DC", "LF" and "HF" this potential can be varied
by means of "Level" potentiometer R2. In the "TV.
Line" and "TV. Frame" positions this potential is
determined by voltage divider R41 and R42; in the
position "Aut.", the control grid is earthed via R43.
The polarity of the trigger signal can be selected with
switch SK2.
The mode of operation of the time-base generator is
determined by the potential at the S.G.M.V. control
grid (B4") i.e. the free-running operation or the
triggered operation.
When switch SK1 is in the "DC", "LF" or "HF"
position, the potential can be varied with "Stab."
potentiometer R3. If the potential is low enough to
underpass the S.G.M.V. lower hysteresis level the
free-running mode of operation is obtained; otherwise
the triggered mode of operation is obtained.
When the "Stab." control is in the "Preset" position,
switch SKS is thrown and the potential of the
S.G.M.V. control-grid is determined by the setting
of "Preset Stab." potentiometer R113. This also
applies to the "TV. Line" and "TV. Frame" position
of SK1.
When SK1 is in the "Aut." position, the potential is
controlled by the automatic stability control circuit
built around B2. If no trigger signal is present, the
B2p anode voltage is made low enough to obtain the
free-running mode of operation by adjusting the
"Aut. Free-run" potentiometer R28. If a trigger
signal is present, it is rectified by diodes GR2 and
GR3.
A negative voltage then arises at the B2p control grid,
which causes the anode potential to rise slightly so
that a triggered mode of operation is obtained.
The B2p anode voltage is clamped in the positive-
going direction by diode GR4 to a d.c. level, present
at the cathode of cathode-follower B2t and which is
adjusted by "Aut. Trigg. Level" potentiometer R33.
This is to keep the time base generator in the trig-
gered mode when the amplitude of the trigger signal
ahead of the control grid of B2p increases.
Advertisement
Table of Contents
