Model 185B Section IV
Paragraphs 4-29 to 4-35
= +0.2v
-12.6V
CRI
TUNNEL DIODE CRI fb, TUNNEL
DIODE
-
LD-L-545
A. ASTABLE E. BISTABLE
Figure 4-3. Tunnel-Diode Operation
decreasing, so the current difference between diode 4-30. The circuit described in paragraph 4-29 can
current and inductor current flows into diode junction be made monostable by increasing transistor current
capacity. Since diode junction capacitance is small, to a point where the tunnel diode is always receiving
this difference current causes a sharp voltage rise a current greater than its peak current. In this case,
across the junction capacity and hence across the CR1 is stable at point W on the curve shown in figure
diode. This voltage rises rapidly to point D on the 4-3B. Momentarily reducing tunnel diode current
E-I curve. At point D, diode conduction current to less than that shown at point T in figure 4-3B will
equals the current supplied from the constant-current cause the tunnel diode to go through a cycle from T
source, and so the voltage stops rising. At this time to R to S to V to W. The circuit will remain at point
the voltage across the diode exceeds the supply volt- W until triggered by another pulse.
age. Since current through the inductor is constant
only during switching, current starts to decrease, and 4-31. TRIGGERING CIRCUITS.
diode voltage starts to fall toward operating point C.
When diode voltage reaches point E, tunnel diode CR1 4-32. GENERAL.
again enters its negative resistance region. Inductor
4-33. The following discussion deals primarily with
current is again held constant, and diode junction ca- the synchronizing circuits in Model 185B. Unless
pacity is forced to discharge to point A. The process
otherwise noted, all reference to ?signal? in the fol-
now begins over again. lowing circuit explanation will refer to the triggering
c. For a large value of load resistance, pulse rise pulse under discussion rather than the signal to be
time (time BD) is determined by diode junction ca- viewed. All reference to horizontal-scan voltage to
pacitance and peak current. Pulse width (time DE) the crt refers to the voltage at the input to the hori-
and hold-off time (time AB) are determined by the zontal amplifier. Refer to the schematic diagram,
inductor, diode resistance, and power source resist- figure 5-13 and figure 4-4 in the followingdiscussion.
ance. Assuming the inductor acts as a constant cur-
rent source for the times involved, the size of the 4-34. TRIGGER INPUT.
load resistance determines the slope of lines BD 4-35. The trigger signal is connected to TRIGGER-
and EA. ING INPUT connector 5101. TRIGGERING switch
SlOl is adjusted to one of three positions, depending
4-29. BISTABLE OPERATION. Figure 4-3B shows on frequency and amplitude of input trigger.
a bistable circuit of the type used inModel 185B trig-
a. SENSITIVE: for small amplitude trigger signals
gering circuit. In this case there is a PNP transistor (less than 150 mv) below 100 mc. In this position of
with tunnel diode CR1 as part of its collector load.
TRIGGERING the input trigger is passed to the ramp
CR1 is biased at point R on the E-I curve, and is
stable at this point. If a positive pulse is fed to the gate generator.
emitter of Q1 (or a negative pulse to the base), the b. NORMAL: for trigger signals of amplitude
current through Q1, and hence the tunnel diode cur- greater than 150 mv and frequency below 100 mc. In
rent, will increase, As the current passes the high this position of TRIGGERING the input trigger passes
current knee of the curve, the tunnel diode enters its through a 20-db attenuator consisting of R158, R159
negative resistance region. Diode current therefore before being passed to the ramp-gate generator.
begins to decrease. Transistor Q1 acts as the con-
stant current source, however, and prevents current c. HIGH FREQ: for trigger signals above 100 mc.
from decreasing; the current difference flows into In this position of TRIGGEFUNG, trigger is applied
the junction capacity. As a result, diode voltage rises to a count-down circuit before being passed to ramp-
rapidly to point V, then when the input pulse falls gate generator. A free-running tunnel diode oscil-
again to zero, diode voltage falls to U where it re- lator consisting of diode CR120, inductor L101, and
mains until a negative reset pulse causes the current low-impedance bias voltage source R155, R156 (fig-
to drop below Iv. ure 4-5A) is used to count down the input frequency
01255-1 4- 3 |
ACEC
