Tektronix 310A, Instruction Manual, Page: 34 / 132

Circuit Description-Type 310A
signals from DC to four megahertz. When the Trigger
Coupling-Mode switch is set to the AC or AUTO positions,
the trigger signal must pass through C2. This capacitor
blocks the DC component o f the trigger signal and attenu­
ates AC signals below about 30 hertz.
Slope Comparator
V I0 A and VIOB are connected as a comparator to pro­
vide selection o f the slope and level at which the sweep is
triggered. O utput signal from the Slope Comparator stage is
always obtained from the plate o f V I0 A and the sweep is
started from the negative-going portion o f the signal at this
point. To provide selection o f the trigger slope, TRIGGER
switch SW2B connects the trigger signal to the grid o f
V I0 A fo r positive-slope triggering or to the grid o f V I OB
fo r negative-slope triggering. Circuit operation is as follows:
For positive-slope triggering, the trigger signal is applied
to the grid o f V10A and a reference voltage from TRIG
LEVEL control R22 is connected to the grid o f VIO B. R13
establishes the cathode current o f both V10A and V10B. In
this configuration the tube w ith the most positive grid con­
trols conduction o f the comparator. For example, w ith a
positive-going trigger signal, V10A conducts when its grid
level rises more positive than the grid o f
V I OB as estab­
lished by the TRIG LEVEL control. When V10A conducts,
its plate level goes negative. Notice that the o utp ut signal
from the Slope Comparator stage is 180° out o f phase w ith
the input trigger signal. The sweep is started on this
negative-going output, which is coincident w ith the
positive-going portion of the input trigger signal. However,
the displayed waveform on the CRT may not start on the
selected slope at fast sweep rates since, there is no delay
line in the Vertical A m plifier circuit.
TRIG LEVEL control R22 sets the grid level o f VIOB
fo r positive-slope triggering. This in turn determines the
level on the trigger signal at which the comparator switches.
With the TRIG LEVEL control set near midrange and a
positive-going signal applied, VIOB conducts until the
applied trigger signal raises the grid o f V I0 A more positive
than the grid o f V I OB (tube w ith the most positive grid
always controls conduction o f the comparator). VIOB is
then reverse biased and V10A is forward biased to produce
the negative-going output as discussed previously. V10A
remains on and VIOB o ff until the applied trigger signal
drops the level at the grid of V10A more negative than the
grid o f V I OB. Then the circuit returns to the original condi­
tions. Now, assume that the TRIG LEVEL control is turned
clockwise to produce a display which starts at a more posi­
tive level. A more positive level is established at the grid of
V10B by the TRIG LEVEL control. Now the trigger signal
must rise more positive before the grid of V10A is positive
enough to bias this tube on to produce the negative-going
output which triggers the sweep. The resultant CRT display
starts at a more positive point on the displayed signal, since
the sweep is started later. When the TRIG LEVEL control
is turned counterclockwise toward —, the effect is the
opposite to produce a resultant CRT display which starts at
a more negative point on the trigger signal.
To start the sweep on the negative slope o f the trigger
signal, TRIGGER switch SW2B reverses the connections to
the grids of V IO A and VIOB. Now, the trigger signal is
connected to the grid o f VIOB and the reference level from
the TRIG LEVEL control is connected to the grid of
V10A. The TRIG LEVEL control establishes the point at
which the comparator switches as discussed fo r positive-
slope triggering. Assume that V10B is not conducting w ith
a positive-going signal applied. As the grid o f V10B goes
positive w ith the applied trigger signal, V10B w ill be
forward biased when its grid voltage exceeds the level at the
grid of V10A set by the TRIG LEVEL control. When this
occurs, V10A shuts o ff and its plate rises positive. Note
that the signal at the plate o f V IO A is now in phase with
the applied trigger signal. Since the sweep is always started
on the negative-going portion of the signal at the output of
the Slope Comparator, the sweep w ill not start until the
trigger signal goes negative to turn VIOB o ff and V10A on.
Then, the output o f this stage goes negative to trigger the
sweep.
In the AUTO position o f the Trigger Coupling-Mode
switch SW3B, the TRIG LEVEL control is by-passed to
ground. This sets the grid o f the reference tube o f the com­
parator at ground level. The signal must also pass through
C2 in the AUTO mode. Now, triggering occurs very near
the average level o f the applied trigger signal.
Trigger Multivibrator
Normal Triggering. The output from the slope Compara­
to r stage is connected to Trigger M ultivibrator stage V40A
and V40B. These tubes are connected as a Schmitt bistable
m ultivibrator fo r normal triggering (Trigger Coupling-Mode
switch SW3D set to AC or DC). To understand the opera­
tion o f this circuit, assume that the TRIG LEVEL control is
set near midrange, TRIGGER switch set to a + position and
the circuit is ready to receive a trigger signal. These condi­
tions produce a positive output level from the Slope
Comparator stage (see Slope Comparator discussion) which
holds V40A in conduction and its plate is negative from the
supply voltage. The quiescent level on the plate o f V40A is
determined by divider R41-R46-R48 between +300 volts
and —150 volts. This divider also sets the quiescent level at
the grid of V40B negative enough so that V40B is held off.
The plate o f V40B rises positive toward the supply voltage
to set the quiescent output level o f this stage.
When a trigger signal applied to the Sweep Trigger circuit
produces a negative-going output from the Slope Compara­
to r stage, V40A is reverse biased. The plate of V40A rises
positive and the voltage at the grid o f V40B is pulled posi­
tive through divider R46-R48. V40B is forward biased and
its plate drops negative as the current through V40B
increases. This negative-going change at the plate o f V40B is
connected to the Sweep Generator circuit to start the
sweep. Capacitor C46 improves the response o f this circuit
fo r fast changes to produce an output signal w ith a fast rise.
This stage remains in the condition just described until the
output from the Slope Comparator stage returns positive
and V40A is again forward biased. Then the grid o f V40B
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