Amplitude Offset And Attenuation; Trigger And Gate Control; Pulse Outputs - Wavetek 145 Instruction Manual

4.2 AMPLITUDE OFFSET AND ATTENUATION
The selected waveform is inverted and amplified in the pre-
amplifier. The preamplified waveform is sent to the output
amplifier.
The output amplifier is an inverting amplifier with a current
limiting output stage for short circuit protection. The de
offset control provides the offset to the selected waveforms
center reference. The de offset can be set by voltage at the
external de offset connector. The output amplifier establishes
the generator O dB attenuation reference. An output atten-
uator decreases this reference amplitude in operator selected
20 dB steps. The attenuator consists of three voltage dividers.
Attenuation between the steps is provided by the attenuation
vernier.
4.3 TRIGGER AND GATE CONTROL
Generator operation is controlled by allowing or preventing
the timing capacitor to charge. Figure 4-3 shows in detail
this portion of the circuit. For continuous operation, the
trigger amplifier maintains a positive level above the positive
peak developed by the charging capacitors. This reverse
biases (turns off) the start/stop diode, and the trigger ampli-
fier does not interfere with continuous operation.
. ,When the trigger amplifier outputs some level below the
positive peak. charging level, the diode is forward biased
(turned on) to sink the integratin_g current from the current
source, preventing the capacitors from charging to the posi-
tive peak. This stops waveform generation and holds the
triangle output at some de level called .the trigger baseline.
The trigger baseline is the level where a triangle waveform
cycle starts and where it stops. This baseline is directly ap-
plicable to the triangle waveform and thus affects the sine
wave. The square wave levels, output via the hysteresis
switch, are not affected by the triangle baseline levels.
The norm_al trigger baseline is zero volts, analogous to 0°
phase of a sine or triangle waveform. The trigger start/stop
control offsets the trigger amplifier output and can change
the baseline for starting and stopping a sine or triangle
waveform from its negative peak
(-90°)
to its positive peak
(+90°) range. At the extreme positive peak level setting
though, the diode is again reverse biased and generator
operation
goes continuous.
Wt'len charging level is being held, the positive current
generator still varies its output with corresponding frequency
control inputs. These varying currents must be sunk through
the diode to keep the timing capacitors from varying their
charge, and thus varying the trigger baseline. The baseline
compensation circuit monitors the output from the positive
current generator to control the trigger amplifier and thus
4-2
control the necessary compensating current through the
diode.
The trigger control logic determines that after a waveform
starts, it always stops at a complete cycle and at the same
phase at which it started. The trigger control logic latches
the trigger amplifier for an enabling output from the time
the cycle starts to when the negative peak of the last cycle
is reached (just one cycle in the trigger mode). Upon reach-
ing the negative peak, the timing capacitor continues
charging positive again, but stops upon reaching the trigger
baseline. A square wave from the hysteresis switch synchro-
nizes the last negative peak time for unlatching the trigger
amplifier for its trigger baseline output.
The generator mode control circuitry (not shown) deter-
mines whether the trigger control logic is to be fired for just
one cycle, or is to be held on for the duration of the trigger
input. When in gate mode, the trigger is directly coupled for
controlling the trigger control logic. In the trigger mode,
the squaring circuit output is converted by a one-shot to a
narrow pulse which fires the trigger control logic.
The squaring circuit is a level detector that generates a
square pulse for the duration of a trigger signal above the
set trigger level. The pulse is also generated for the duration
the manual trigger switch is held down in gate mode, and
fires one cycle in triggered mode .
4.4 PULSE OUTPUTS
The pulse outputs are based on the squan, wave from the
basic generator circuit (see figure 4-1); the ulse frequency
is controlled by the frequency dial, freque; ·..::y vernier and
VCG voltage in the same manner as the
V\
1veforms. The
square wave is first modified to the sync pulse by a one-shot
circuit; then the normal/double/delayed pulse selector switch
sets or inhibits AND gates to distribute the sync pulse to
the delay one-shot and the width one-shot circuits. When
the switch is in normal position, the sync pulse is gated to
the width one-shot; the delay one-shot is bypassed. When
the switch is in delayed position, the sync pulse is gated to
the delay one-shot only. With the switch in the double posi-
tion, the sync pulse is gated to both the delay and width
one-shots.
Pulse width of the width and delay one-shot pulses can be
varied by the front panel width and delay controls, respec-
tively. The resulting pulse is gated by the selection of a pulse
width value rather than the square wave ( nu ) detent on
the pulse width switch. The pulse or the basic generator
square wave, as selected by the pulse width control, is sent
to a buffer circuit and output as TTL, TTL, ECL and ECL
pulses. The pulse or square wave is also routed to another
buffer which is set by the selection of PULSE, PULSE or a