Tektronix TYPE 109, Инструкция по эксплуатации

Страница: 24 / 56

Circuit Description— Type 109
control R614 and a resistor R615. The FREQUENCY control
sets the voltage applied to the base of Q613. Transistor
Q613 is an emitter follower whose output is tied to the
base of another emitter follower, Q627. This second emitter
follower stage has the required current capabilities for con­
trolling the voltage applied to the Pulser Multivibrator stage.
Capacitors C614 and 627 provide additional filtering
of the supply. They reduce the amplitude of the multivi­
brator spikes getting into the supply from the Driver Multi­
vibrator stage.
Driver Multivibrator
The Driver Multivibrator stage is basically a saturating
core square-wave oscillator. Transistors Q725 and Q735,
function as high-speed switching elements. They are used
in conjunction with transformer T750 to provide a saturating
multivibrator circuit. During operation, the core materials
of the transformer exhibit rectangular hysteresis loops de­
signed to produce a square-wave output.
This stage operates from the voltage set by the FRE­
Q U EN C Y control. Bias for the two transistors is obtained
from R744. Initial operation of the multivibrator depends
on a slight unbalance between the transistors. When power
is first applied to the instrument, one of the two transistors
conducts first. For descriptive purposes, assume that Q725
conducts.
As Q725 conducts, current passing through one half of
the primary of T750 induces a negative voltage at the base
of Q725 and positive voltage at the base of Q735. This
causes Q725 to conduct more heavily and holds Q735 in
cutoff. This regenerative action continues until the core of
T750 saturates.
When the transformer core saturates, the driving voltage
at the bases of the two transistors is reduced. This causes
current flow through Q725 to decrease, which in turn causes
the induced voltages at the bases of the two transistors to
shift polarity. With a negative voltage on the base of
Q735, this transistor then conducts thereby inducing a still
greater negative voltage at its base. This action then re­
peats as Q725 and Q735 alternately saturate the core in
opposite directions.
The operating frequency of the Driver Multivibrator stage
is determined by the amount of time required for the core
to saturate and switch the multivibrator. The amount of time
for this action to take place is dependent on the number
of turns in T750 and the drive voltage on the transformer
primary.
Since the number of turns of T750 are fixed, the multi­
vibrator frequency can be changed by varying the drive
voltage.
To accomplish this, FREQUEN CY control R614,
when adjusted, changes the operating voltage on transistors
Q725 and Q735. By changing the transistor operating volt­
ages, core saturation time changes, thus changing the multi­
vibrator frequency. By adjusting the FREQUENCY control
the multivibrator frequency can be varied between approxi­
mately 275 and 360 cycles per second.
Resistor R750 reduces the amplitude of the switching trans-
sients or “ spikes” . Some instruments below S/N 202 and
all instruments S/N 202 and up were modified by adding
C750. This capacitor blocks the dc path between collectors
of Q725 and Q735, decreases the ripple and spike ampli­
tudes at the emitter of Q627, and improves the 10-volt
power supply regulation at low-line voltage. In addition, as
an indirect effect, the capacitor increases the operating
frequency of the multivibrator slightly.
Driving the Mercury Switch Reed
Transformer T750 provides approximately a 200-to-l step
down into a one-turn secondary (see Fig. 4-2) which couples
the energy to the mercury switch, and serves as both a
switch body (or housing) and a coaxial return. The one-
turn secondary permits a large current to flow and thus
a large magnetic field to be generated to drive the reed
of the mercury switch.
B ia s M a g n e t
P rim a ry W in d in g s
o f T 7 5 0
M e rc u ry Sw itch
B od y
O n e -T u rn S e co n d a ry
o f T 7 5 0
F ig . 4 - 2 . Left fro n t v ie w o f the T yp e 1 0 9 sh o w in g the p u lse r sectio n.
A permanent magnet is used to provide magnetic bias
for the reed of the mercury switch. The field set up by the
one-turn secondary adds to or subtracts from this bias and
causes the reed to move from one contact to the other.
The double set of contacts causes the frequency of the out­
put pulses to be twice the frequency of the multivibrator, or
nominally 640 cps.
The permanent magnet is adjusted so the period of
closure with each contact is at least 250 /xsec or longer.
Use of the mercury switch eliminates contact bounce at the
start of the pulse and the resulting irregularities in the
generated pulse. Use of high pressure in the mercury
switch prevents precontacting ionization.
105-Volt Power Supply
The secondary winding of T601 that connects to terminals
6 and 7 supplies the ac voltage to full-wave rectifier D662A,
B, C and D. The output of the rectifier is applied to an
RC filter network consisting of C661A, R661, and C661B.
The output of this filter is applied to a voltage regulator
tube, V679.
4-2