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BRUEL & KJAER 1023 - Modulator Circuits and Operation; VCO Servo Loop and Frequency Conversion

BRUEL & KJAER 1023
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1023.1
Technical Description
Modulators
Two
different
types of
modulators
are being used
in
the
instrument.
The
drawing
shows
the
type
which
is used
to
provide
20
to
220
kHz
for
the
Frequency
Counter
and
for
the
Frequency
to
Current
Converter
for
stabi-
lizing and correcting
the
linearity
of
the
VCO.
f1
is applied to
the
base
of
Vl
which
acts as an
ordinary
amplifier
stage
as
long as
V2
is
turned
ON by a negative f
2.
When
V2
is
turned
OFF by a positive f
2
.
Vl
will
not
transfer
any signal
at
all
to
the
output.
In
this
way
f1
will
be chopped
by
f2'
and
V3
and
V4
are
inserted
to oper-
ate in
the
same
way
as
Vl
and
V2
thus
creating a balanced
modulator.
Also
the
Constant
Level
Output
Modulator
ZM
0200
which
can be con-
nected to
the
Fixed and Variable Osc.
outputs
is of
this
type,
where
a sta-
bil
output
amplitude
can be obtained as a
fixed
amplification
is deter-
mined by
the
ratio
between
RC
and
RE.
The next
drawing
shows
a
more
sophisticated type
than
the
previous
one.
It
is used
to
mix
the
fixed and variable
oscillator
signals
in order to
produce
the
final
output
signal
of
the
instrument.
The demands here are
low
noise,
extremely
low
distortion
and a band-
width
on the
output
from
10Hz
to
20
kHz.
Servo
Loop
for
VCO
In
order to
obtain
a
frequency
change
proportional to
tuning
voltage as
well
as a
high
frequency
stability,
a feed-back
arrangement
has been
made.
From
the
Frequency
Tuning
potentiometer
a
positive
current
is applied
to
an Integrator
the
output
of
which
is used
to
tune
the
Voltage Controlled
Oscillator.
The
1,2
to 1
MHz
output
signal
from
this
oscillator
is mixed
with
a fixed
frequency
of
1,22
MHz
thus
producing
a signal
from
20
to
220
kHz.
A Frequency
to
Current
converter
supplies a
current
proportional
to
fre-
quency, and it
now
appears
that
the
VCO
will
be
tuned
to
a
frequency
which
causes
the
negative
current
output
from
the
converter
to be ex-
actly
the
same value as
the
positive
tuning
current.
Frequency
to
Current
Converter
The basic
definition
of
this
converter
is
that:
Current
i = charge Q x Frequency f and
charge
Q = Capacitors C x Voltage
V.
This indicates
that
the
stability
of
the
system is
given
by
the
charge Q
which
is created
by
means
of
a
very
stabil ref. voltage V
of
-10
V and a
number
of capacitors
C,
which
are
of
different
temperature
coefficient
for max. stability.
1-2
+Ub
-Ub
450mV
ff
i x
-p W
450mV
p-p
400mV
LF
f
yor
TTL
Freq. Tuning
Integrator
1,22
MHz
-10Vref.
V
---~'"'
charge discharge
III
Freq Tuning
~
C
(temp.
comp.)
0-10V
5.75

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