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Philips PM 5167 - Page 42

Philips PM 5167
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41
3.1.2.6.
Impedance
converter
3
consists
of
a
complementary
emitter
follower
U3/326
and
327
which
decouples
the
square-wave voltage
at
U3/649
attenuated
to
2 V
pp
and
causes an
output
impedance
of
50
n.
3.1.2.7
.
The
amplitude
controi
operates
at
frequencies
higher
than
3 kHz
and
compensates
the
rise
of
the
triangular-
voltage ampl
itude
at
R 1 for.
The
rise
of
the
triangular·voltage
amplitud~
is
due
to
the
short
switching
intervals
of
comparator
1 (ca.
20
ns),
during
which
the
tr
i
angular
voltage keeps its
momentary
rise
direction
.
At
10M
Hz
the
triangular
voltage reaches a rise
speed
of
lOV/50
ns,
which
wou
Id
cause an ampl
itude
increase
of
10
V
.20
ns/50
ns = 4
V.
The
amplitude
controi
reduces
the
amplitude
of
the
square-wave voltage
at
Rl
by 4 V
at
10
MHz,
which
results
in a linear
amplitude
response
of
the
triangular
voltage.
The
FET-pair
U3/323
performs
the
high-ohmic
decoupling
of
the
triangular
voltage.
One
of
the
two
transistors
is
used
for
offset·
and
temperature
compensation,
the
adjustment
of
which
is
achieved
by
U3/681
11682.
The
high-pass
filter
U3/637,
690,
514
suppresses
the
controi
effect
below
3 kHz.
In
the
controller
used
as
integrator
(U3/311
to
313
and
336
to
338),
the
actual
value
of
the
triangular
voltage
obtained
by
peak
rectifying
(diodes
U3/403,
410)
is
compared
with
the
nominal
values
at
the
base
contacts
of
transistors
U3/313
and
338.
The
controller
reduces
the
square-wave
amplitude
at
R 1 by
means
of
the
transistors
(U3/309,
310
and
333,
335)
used
as
adjusting
elements.
The
adjustment
of
the
nominal
value
is
carried
out
by
select-on·test
resistors
U3/619,
663
.
3.1.2.8.
The
hold-condition
is
obtained
by
means
of
transistors
U3/321,
350
,
322, 351.
When
U3/16
is
connected
to
U3/17
via switch
run/hold
(807),
all
transistors
mentioned
are
conductive.
No
current
passe s
then
through
the
switching array.
The
connected
capacitor
C(U2/501
to
510
and
512)
remains
charged,
voltage
remains available
at
connection
point
13
and
switching
array 1
stops
oscillating.
Transistors
U3/304,
329
and
303,328
enables
the
single-shot
operation
to
be
performed.
If
-5
V
is
applied
to
connection
point
U3/20
all
transistors
mentioned
are
conductive.
The
switching
array 2
no
longer oscillates.
U3/304
is
connected
in
paralIei
with
U3/306
so
that
I R
is
applied
to
U3/648
and
649,
independent
of
the
position
of
the
switching array. I n
the
same
way
U3/329
is
connected
in paralIei
with
U3/334.
As
a result
of
this
uR
becomes
-5
V.
The
triangular
oscillation
is
held
until
the
triangular
voltage uC reaches
the
potential
at
R
1.
This
condition
is
stable
and
the
comparator
is
balanced
.
If
a voftage
more
positive
than
-5
V
is
applied
to
transistor
U3/339
from
potentiometer
START
LEVEL
via
point
8/U3,
this
voltage
determines
the
voltage
at
Rl
(U3/659
+
660)
.
The
voltage
at
C
becomes
the
same
potential.
In
this
way
any
start
potential
for
the
triangular signal
is
possible.
3.1.3.
Controi
unit
U5
The
controi
unit
supplies
the
charging
currents
for
the
oscillator.
The
controi
unit
fig.
22
consists
of:
- a
current
source
for
potentiometer
FREQUENCY,
-
two
pairs
voltage·current
converters,
-
four
cu
rrent
mirrors,
- a
symmetry-control
stage
and
- a single-shot logic.
3.1.3.1.
The
current
supplied
by
power
source
U5/311
determines
the
voltage
at
potentiometer
FREQUENCY
. By
means
of
potentiometer
FREQ
.
OFFSET
the
emitter
resistance
U5/646
of
power
source 311
and,
as a result
of
this,
the
voltage
at
potentiometer
FREQUENCY
can
be
changed
by
±5
%.
The
frequency
ranges 1 Hz
to
1 M are
adjustable
via
potentiometers
U2/601
to
607
.
3.1.3
.2.
Voltage-current
converters
U5/351
and
·
U5/352
are
controlled
by
the
voltage
between
input
SWEEP
IN
and
potentiometer
FREQUENCY.
The
emitters
of
transistors
301
and
305
are
brought
to
the
same
potential
difference
by
feedback.
The
frequency-determining
resistor
R =
(U2/608
to
611
and
6141161611628)
or
(617
to
620)
wh ich
is
selectable
with
switch
FREQ.
MUL
TIPL.,
is
located
between
these
two
emitters.
The
last
mentioned
resistor series applies
for
the
time-asymmetric
operation,
the
switch-over being
performed
by
Ul/\.
Resistor R
determines
the
emitter
currents
and
consequently
the
collector
currents
of
transistors
U5/301
and
U5/305,
which
correspond
to
the
charging
currents
IC
and
IC* (fig . 7).

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