Wavetek 143 - Page 20

The triangle wave from the triangle amplifier is coupled
through a buffer amplifier and made available to the func-
tion selector switch. The buffer amplifier provides a low
impedance to drive the sine converter circuit. The sine con-
verter, using the nonlinear characteristics of its diodes,
converts the triangle wave into a sine wave.
The square wave from the sync amplifier, processed through
a one-shot and the sync out buffer, is externally available at
the sync out connector. The sync pulse, then, is a TTL level
pulse output of the generator frequency.
4.22 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 dc
offset control provides the offset to the selected waveforms
center reference. The dc offset can be set by voltage at the
external dc 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.
43 TRIGGER AND GATE CONTROL
Generator operation is controlled by allowing or preventing
the timing capacitor to charge. For 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
4-2
(turned on) to sink the integrating 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 dc 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 normal 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 oper-
ation goes continuous.
When 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
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.