HP 120A - Page 38

Sect, HI Page 4
b. Trigger Generator - The Trigger Generator is
designed to be triggered on the negative slope of
the synchronizing signal. In the AUTOMATIC position
the bias on the input pentode stage is adjusted so that
the pentode is conducting when waiting for a synchro 
nizing signal and the triode section is cut off. In ad 
dition, the bias point is automatically adjusted to an
optimum point where the Trigger Generator will trig 
ger on most waveforms. A negative trigger pulse is
needed to start the action of the sawtooth generator.
When the pentode section of the Schmitt trigger is
turned off and the triode is turned on, a negative
pulse is produced. This pulse is differentiated in the
output circuit of the Schmitt trigger and fed into the
sawtooth generator to start the generator. Thus, an
input signal which crosses the lower hysteresis lim 
it will start the sweep. When the TRIGGER LEVEL
control is turned to its extreme counter-clockwise
position (AUTO, position) the switches associated
with the TRIGGER LEVEL control convert the Trig 
ger Generator into a free-running multivibrator which
operates at a frequency of approximately 100 cps.
Thus the Trigger Generator in effect generates its
own trigger in the absence of a signal and presents
a trace on the face of the cathode-ray tube. As soon
as a synchronizing signal is received the applied sig 
nal takes control of the synchronization.
In the variable TRIGGER LEVEL control position the
feed-back for self-triggering is disconnected and the
bias level is adjustable. As this control is turned
clockwise from the AUTOMATIC position, first the
feedback is disconnected and then the control varies
the bias level. As the bias is made more and more
positive it will require a more negative signal to trig 
ger the Trigger Generator. A differentiating circuit
has been placed in the plate circuit of the final tube
(triode) of the Trigger Generator to convert the out 
put into a sharp spike.
c. Start-Stop Trigger - This sharp output spike
is then fed into the Start-Stop Trigger. This
is another Schmitt trigger but it has rather wide
hysteresis limits. The wide hysteresis limits are
needed so that the generated sawtooth can be fed
back to the start-stop trigger and thus terminate it 
self. The integrator output is fed back via the hold-
off cathode follower to drive the start-stop trigger
past the upper hysteresis limit. When this point is
reached the Trigger changes state, causing the Inte 
grator Switch to conduct Then the voltage discharges
through the resistance-capacitance network of the
circuit. This voltage is not permitted to reach the
lower hysteresis limit, however. A voltage is ap 
plied by the Hold-Off Cathode Follower to prevent
this. Since this hold-off voltage is developed later
on in the circuit, it will be explained later.
d. Integrator Switch - The Integrator Switch is
controlled by the square-wave output of the Start -
Stop Trigger. The Integrator Switch consists of two
triodes, one of which is connected as a diode. While
the circuit is awaiting a trigger, the diode is nor 
mally conducting, thus shorting out the Sawtooth In 
tegrator. When a negative synchronizing signal is
received, the Trigger Generator converts it into a
negative pulse operating the Trigger Generator which,
in turn, puts out a pulse. The negative pulse applied
to the plate of the diode causes it to cut-off permitting
the Sawtooth Integrator to commence operation. The
triode section of the Integrator Switch serves to hold
the output of the Sawtooth Integrator at a definite volt 
age (approximately 50 volts) so that the sawtooth will
always start from the same point.
e. Sawtooth Integrator - The Sawtooth Integrator
consists of a triode, and a pentode, Miller-type
integrator, which generates essentially a linear, pos 
itive, rising, waveform which sweeps the trace a-
cross the face of the cathode-ray tube at a linear rate.
The rate at which this sweep takes place is deter 
mined by the values of R and C in the grid circuit of
the pentode. These values are varied for each range
of the SWEEP TIME switch. When the Integrator
Switch opens, the voltage applied to the resistance -
capacitance combination tries to charge the condenser
through the resistance. However, the condenser is
connected between the grid and plate of an amplifier.
As the voltage across the condenser starts to change,
the change, amplified by the tube and the output po 
larity reversed, is applied to the grid thus keeping the
voltage relatively constant. Since there is a constant
voltage across the resistance, a constant charging
current must be flowing into the conden'ser. Since the
charging current is constant, and the values of the
capacitor and charging circuit are constant, the volt 
age increase across the condenser is highly linear.
The rising output waveform of the Sawtooth Integrator
is fed through two neon bulbs to the triode section of
the 6U8. The signal is then cathode-coupled through
another bulb to the Hold-off Cathode Follower which
is used for isolation. Neon bulbs are used to drop the
voltage down to the proper levels while at the same
time furnishing a direct-coupled path for the signal.
In general, the neon bulbs are shunted with a condens 
er to improve the high-frequency response of the cir 
cuit. A resistance is also used in series to prevent
the possibility of the neons oscillating.
f. Hold-Off Cathode Follower - The rate at which
the rising output Sawtooth Integrator reaches the
upper hysteresis limit of the Start-Stop Trigger will
be determined mainly by the resistance and capac 
itance in the grid circuit of the Sawtooth Integrator .
However, after this signal has triggered the Start-
Stop Trigger, this voltage will be returned to its
original value fairly rapidly by the Integrator Switch.