Tektronix 323, Инструкция по эксплуатации

Страница: 45 / 170

Circuit Description— Type 323
Sweep Gate output to
the Unblanking Amplifier, The cath­
ode-ray tube is thereby unblanked when the sawtooth starts
rising and is turned off at the instant retrace is initiated.
Unblanking can be disabled by injection of a positive signal
through the EXT BLANK connector.
Sweep Generator
A knowledge of N-channel Field Effect Transistor (FET)
operation and Tunnel Diode switching action is necessary
for understanding the Sweep Generator circuit. The FET
operation can be understood by simply comparing a triode
vacuum tube to it, with the cathode, grid and plate com­
paring to the source, gate and drain respectively. Like the
vacuum tube, the FET has high input impedance and only
leakage current flows in the gate circuit.
Refer to the tunnel diode voltage-current graph contained
in Fig. 3-6. A tunnel diode switching circuit is designed to
take advantage of the fact that a tunnel diode has two
stable states. A tunnel diode operating in its low voltage
state to the left of point B will stabilize to a point on the
curve which satisfies circuit voltage and current require­
ments. If a signal input causes the voltage or current to
exceed the value at point B, the tunnel diode will switch
(pass through the unstable negative resistance region) and
stabilize to the right of C at a point which will again satisfy
circuit requirements. It is then operating in its high voltage
state, and will remain there as long as its voltage and cur­
rent remain greater than indicated by point C. If the cur­
rent or voltage falls below that value, the diode will again
switch through the negative resistance region and return
to its low state. The difference between low and high state
operating voltages is commonly in the vicinity of ’/ 2 volt.
Circuits are designed to permit quiescent operation in
either the high or low state at points such as A and D.
Transients can then be used to cause switching action to
occur, leaving the tunnel diode in the switched state after
the transient has expired.
Refer to the schematic diagram of the Sweep Generator
(and to the Timing Switch schematic as necessary). A
summary of the purpose of components, and their status
during a complete sweep cycle, is contained in Table 3-1.
TABLE 3 1
O perating Status o f Sweep Generator Components
Component Purpose
Status1
Quiescent Sweep
Rising
Retrace H old off
D301 Holdoff Diode On
D303 Tunnel Diode Low state High Slots Low state Low State
D305
Gate Diode On
OH
On On
D309
Disconnect
Diode On OH On On
D342/D343 Trigger Dis­
abling Diodes
Off On
m . . .
. ....
On OH
D350 Blanking Diode On Off
On
On
D353
Unblanking
Diode
O ff Or.
||| «| g§
O ff Off
Q3C5
Sweep Start/
Stop Switch
Off Saturated
.................
O ff O ff
Q311, Q317,
Q326
Sweep
Amplifiers On On On On
Q343
Quiescent
Position
Switch
Saturated O ff
m
Saturated
Q356 Unblanking
Switch OH On O ff OH
Q363
Unblanking
Amplifier OH Or.
OH
OH
Q370
Plate Charging
Switch
OH
$qfur«ted OH OH
Q373
Plate Discharg­
ing Switch Saturated
O ff Saturated Saturated
3-10
®
1 Shaded areas indica te d e via tion from quiescent condition.