BK Precision 1403A User Manual

CIRCIIIT
DESCRIPITON
5
MHz
bandwidth. Front
panel
vertieal
posi-
tioning.
is
aeeomplished by
potentiom
eter
VRz,
I
eOS,
as
it shifts
the de
bias of
Q110
and
Q111.
The vertieal defleetion
plates
of
the CRT are
fed from
the
eolleetors of
Q110
and
Q111,
when
NOR-DIR switeh
SS
is
set
to
NOR. At
this
point,
the
waveform
is about
10
volts
per
division.
With
NOR-DIR switeh
55 set to
DIR, the
vertieal defleetion
plates
of
the CRT
are
eonneeted
direetly
to the V DIR terminal and
ground.
A sample of
the
vertieal final amplifier
output
is available to
the
horizontal
ehannel
eireuits for internal syne.
HOruZONTAL CHANNBL CIBCITrIII
Refer
to
Fig.
4 for
a simplified
diagram
of
the
horizontal
ehannel
eireuits and to
the
sehematie
diagram
at
the rear of
the
manual
for full
eireuit
details.
Sweep Generator
The
sweep
generator
eireuit eonsists of a
reeurrent sweep sawtooth
oseillator
built
around
transistors
Q112
and
Q113.
The
frequeney
at whieh
the sawtooth
oseil-
lator operates
is
determined
by the
RC
time
eonstant of
the eomponents
in
the
emitter
eireuit of
Q113
(Ca,
C4, C5, VR4 and
R141).
SWEEP
RANGE switeh, 32,
seleets the
eapaei-
tanee eomponent of the
RC eireuit:
when
set
to the 10-100
position,
C3 is seleeted,
provid-
ing
a eapaeitanee
of 0.47
UF;
when set
to the
100-1 k
position,
C4
is
seleeted,
providing
a
eapaeitanee
of 0.047
pF;
when set to
the
1 k-
10 k
position,
C5 is seleeted,
providing
a
eapaeitanee of 390
pF;
and, when set to the
10 k-100
k
position,
the stray
eapaeitanee
of
the
eireuit
is
seleeted. In
eaeh
ease, the
SWEEP
VARI/EXT GAIN eontrol, VR4,
and
its
series resistor
R141, are seleeted as the
resistanee
eomponent
of
the
RC
eireuit.
The
RC time
eonstant
is
greatest
when VR4
is
fully
eountereloekwise
and its resistanee
is
highest.
The
RC
time eonstant is redueed and
the
sweep
frequeney
is inereased
as
V.R4 is
turned
eloekwise.
Fig.
5 is a
partial
sehematie
diagram
of the
sweep
generator
eireuit with
waveforms
to
help illustrate operation
of the
eireuit.
First,
let us eonsider
the
free
running
manner
of operation
where
no
syne
signal
is
applied.
During the
negative-going
ramp
portion
of
the sweep
waveform
(at
Ql13
emitter),
Q112
eonduets
and establishes
a
de
voltage
at
the base
of
Q113,
eutting
it off.
With
Q113
eut
off, the seleeted
sweep
range
eapaeitor
eharges toward
-15
volts through
VR4 and
R141. The
rate of eharge
is deter-
mined by the RC
time eonstant
as
previously
deseribed. As the
eapaeitor
eharges,
the
voltage at the emitter
of
Q113
ramps
nega-
tively.
The voltage eontinues
to ramp
nega-
tively,
whieh deereases
the bias
on
Q113,
until
the threshold
of
eonduetion
is reaehed.
The
negative-going
ramp
ends
when the
threshold
of eonduetion
of
Q113
is
reaehed.
Q113
eonduets and
quickly
diseharges
the
sweep
range
eapaeitor
to
the de
value
estab-
lished by the voltage divider
of R138,
Q113,
VR4,
and R141. This eonduetion
of
Q113
develops a
negative
pulse
at
the eolleetor,
whieh
is fed
baek through
C108
to the
base of
Ql12,
cutting
it
off.
The RC
time eonstant
of
C108
and R135 determine
the eutoff
period
for
Ql12.
This is the very short
interval when
the sawtooth
waveform
returns to
its
positive-
most
point.
The negative
pulse
at
the
base of
Q112
is inverted
to a
positive
pulse
at the
eolleetor,
and is direetly eoupled
to the base
of
Q113
to
eontrol
the
sweep
eapaeitor
dis-
eharge
period.
At the end
of
this
pulse,
Q113
is again
eut off, and the
next
negative-going
ramp begins. This eyele
is
eontinuously
re-
peated.
The sawtooth
waveform
ramps
from
approximately
+2
volts to
-2
volts, and
is
limited
to the
most linear
portion
of
the sweep
eapaeitor eharge eyele.
Now let us eonsider
eireuit
operation
when
a
syne
signal is
applied.
Refer
again
to
the
waveforms
in Fig. 5. Syne
signals
are
fed to
Q112,
superimposed
on the
sweep
reset
pulse,
inverted
and applied
to the
base of
Q113.
A
negative-going
edge
of the
syne
input
signal
is
inverted
to
a
positive-going
edge at
the
base
of
Ql13.
If this superimposed
positive
edge
oeeurs
just
before
the end
of a
free-running
sweep,
it will bias
Q113
into
eonduetion
and
b
a
Yt

BK Precision 1403A Specifications

General

Brand	BK Precision
Model	1403A
Category	Test Equipment
Language	English