Tektronix 3B5, Инструкция по эксплуатации

Страница: 30 / 187

Circuit Description — Type 3B5
TIME/DIV switch and DLY ’ D SWP MAG switch provide the
correct normal and magnified sweep rate readout on the
readout panel.
Seek Mode Operation
The operating circuits for the Seek Mode of operation
are shown in Fig. 3-3. This includes all blocks which were
operative for Manual Mode (except Manual Operating
Mode Power) plus the automatic circuits in the Seek Circuit,
Counter Circuit and Readout Logic circuit. The Operating
Mode Power circuit applies power to the various circuits
to operate the automatic seek functions. The Sweep Trigger
circuits operate as determined by the front-panel controls
unless the controls are incorrectly set or the applied trigger
signal is not correct. Then, the Seek Circuit switches the
Sweep Trigger circuit to auto trigger operation and a stable
display is presented, if possible. If a trigger signal is not
present in the auto trigger mode, the NOT TRIG'D readout
comes on and the sweep free runs.
The Seek Circuit resets the sweep to the slowest sweep
rate at the start of each seek cycle. Then, the Seek Circuit
compares the trigger pulses produced by the Sweep Trigger
circuit against a window-gate signal derived from the saw­
tooth and the sweep gate signal (see Seek Circuit discussion).
If trigger pulses occur during the window gate, the Seek Cir­
cuit produces an advance pulse which advances the Counter
Circuit to the next faster sweep rate and a sweep reset pulse
resets the sweep so it starts again at the left side of the
graticule at the new sweep rate. Again, the Seek Circuit
compares the trigger pulses against the window-gate signal
and advances the Counter Circuit if trigger pulses are present.
This continues until the sweep rate is fast enough so no
trigger pulses occur during the window-gate signal (see
Seek Circuit discussion for more complete explanation). The
final sweep rate is maintained until another seek command
is received or the operating mode is changed. This complete
seeking action takes place each time a seek command is
received.
Fig. 3-4 shows the logic sequence for Seek Mode operation
including a full explanation of operation. The circled num­
bers on this logic diagram refer to the step in the explana ­
tion which describes the action of this individual circuit.
External Mode Operation
The circuits which are operative for External Mode are
shown in Fig. 3-5. In this mode of operation, most of the
circuits are controlled by external program logic applied
to the circuits from an external programmer through the
front-panel PROGRAM connector. An E is shown beside
each block which is controlled from the programmer. The
programmable capabilities of this instrument are the same
as the front-panel capabilities (except for variable sweep
rate).
The External Mode of operation is established by applying
an external mode command to the Operating Mode Power
circuit. External Power is then applied to those circuits which
control external mode operation and Manual or Seek Mode
operation is locked out. Sweep rate, delayed sweep mag­
nification, delay, horizontal position, trigger mode, source,
coupling, slope and level are externally programmable.
If auto triggering is programmed, the front-panel LEVEL
control determines the starting level of the display.
CIRCUIT DESCRIPTION
Sweep Trigger
General
The Sweep Trigger circuit produces trigger pulses to turn
on the Sweep Generator circuit. This trigger pulse is derived
either from the vertical input signal or an external signal
connected to the EXT TRIG INPUT connector. Control stages
in this circuit select the source, coupling, level, mode and
slope of the trigger signal. Fig. 3-6 shows a logic block
diagram of the Sweep Trigger circuit. A diagram of this
circuit is shown on diagram 1 at the rear of this manual.
Trigger Source and Coupling
Relay K3-1 selects the trigger signal either from an internal
signal from the amplifier unit or an external signal applied to
the EXT TRIG INPUT connector. This relay is controlled by the
Trigger Mode Logic stage. When K3 is not energized, inter ­
nal triggering is provided from the vertical signal. When
K3 is energized, K3-1 connects the external trigger signal
to the input circuit. Trigger coupling is selected by K4.
When K4-1 is open, the trigger signal must pass through
coupling capacitor C3. This capacitor blocks the DC com ­
ponent of the trigger signal and attenuates low-frequency
AC signals below about 50 Hz. When K4 is energized, K4-1
closes and C3 is by-passed to directly couple all components
of the trigger signal to the Input Emitter Follower. DC
coupling is provided only in the EXT-DC position of the
Trigger Function switch, SW50 or when externally program ­
med for external DC. More information on how trigger
source and coupling are controlled is given under Trigger
Mode Logic.
Input Emitter Follower
The Input Emitter Follower, Q23, provides a high input
impedance for the trigger signal. It also provides isolation
between the Sweep Trigger circuits and the triggering source.
The level on the trigger signal at which the trigger circuits
respond is controlled by varying the bias level at the base
of Q23. The bias level is determined by the LEVEL control,
the Auto Trigger P-P Level stage or the external program
trigger level. Further information concerning operation of the
level control circuit is given under Trigger Level Amplifier.
The network D23-D24-D26-C23-C26-R25B in the emitter cir­
cuit of Q23 provides stable triggering in the auto trigger
mode throughout the complete rotation of the Trigger LEVEL
control. This is described in detail under Auto Trigger P-P
Level.
The trigger signal at the emitter of Q23 is coupled to the
base of Q74 through D62. This diode returns the DC level
of the trigger signal to that at the base of Q23 by off-setting
the base-emitter voltage drop of Q23. D62 also provides
temperature compensation for Q23. The trigger signal at
this point follows the level established at the base of Q23
by the trigger signal or the Trigger Level Amplifier circuit.
3-2
®L