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..
fn9
input
signat.
at
a
nominal
_14
dBm,
is
applied
through
toroid
transformer
T1041
to
the
Oaii
of
common-emitter
amplifier
el040.
Gain
adjustment
Rl0i!0,
in
the emitter
circuit.
sets
the
posi
Vn
impfin"i
gain.
The
output
is
transformer
coupled,
by
Tt0i0,
to
the
base
of
feedback
amptifier
OiOSO.
ihis
circuit
include.s
emitt€r
d€generation
through
resistor
RlOg4
and
collector-to-base
feedback
throulh
resistor
Rl041.
The
collector
feedback
helps
to
prorid"
a
well-deftned
output
impedance
of
50
ohms.
input
impedance
is
a
function
of
transformer
T1040
ana
relistor
R1042
across
the
primary
winding.
.
Fro.m
the
final
amplifier,
the-signal
is apptied
through
a
band-pass
filter
with
about
2
O-e insert
on
toss.
The
*6
dBm
output
signat
from
th€
fitter
is afptied
through
coaxial
connector
J6g2
to
the
Log
Amplinii.
LOc
AMp
and
DETECTOR
(Diagram
22)
The
Logarithmic
(Log)
Amptifier
and
Detector
accepts
input
signals
from
the Vi
circuits.
The
signals
are
amplified
so
the output
is
proportional
to
the
1-oga-
rithm
of
the input.
The
output
ii
ttren
applied
to
a
linear
detector
which
outputs
a video
signal.
By
controlling
the compression
curve_
characteristi-c",
"""h
OB
ctranfE
in
th€
input
signal
level
r€sutts
in
an
equal
increment
of
change
in
the output.
In
the 10
clBldiv
mode,
each
divi-
sion
of
displacement
on
the screen
represents
a
10
dB
change
of
input
signal
level.
Log
Amplifier
Circuits
,
Tlgr".
circuits
togarithmicaily
amptify
the
input
signat
from
the VR
circuits
and
apply
the output
signal
to-the
Detector
circuit.
The
Log'Amplifier
consists
of
seven
ac-coupled
amplifier
stages.
Each
stage
has
two
gain
values
that
depend
on
signal
amplitude.-
In
additionlhe
first
three
stag€s
have
an
extra
automatically
sel€cted
gain
value.
The
combined
circuits
provide
high gain
for
lowJevel
signals
and
low gain
for
high-level
-ignals.
For.the,output
signal
to
be
ploportional
to
the log;ithm
of
the_input,
more
gain
is
reguired
for
a chang-e
from
-80
dBm
to
-79
dBm
than
a
change
from
_1
dBm
to
0
dBm. For
a
given
stage
of
the cir:cuit,
the
gain
starts
at
approximately
l0
dB
for
a
low{evel
signal
and
decreases
to
unity
as
the
input
signal
level
increases.
In
_the
first
three
stages,
the
gain
becomes
tess
than
unity as
th€ signal
amplitude
increases.
^ _lnput
signaf
levels
nominalfy
range
between
-84
dBm and
*6
dBm.
As
the
signat
tev-et
increases.
the
gain
decrease
begins
with
the final
stage
and
proc€eds,
in
succession,_back
through
the
remaining
six
stages
to
the first.
Each
stage
initially produced
approximately
10
dB of gain.
That
gain
was
reduced
to
unity, so
the
total
gain
reduction
is
Z0
dB. With
further
increases
in
input
signaf
level,
three
more
gain
change
Theory
of
Operadon
-
4g4A/4g4Ap
Service,
Vol.
1
st€ps
take
place.
The
gain
of
th€
first
three
stages
is
reduced.
betow
unity
approximately
7
dB
for
each
itage.
This
reduction
starts
with
the
first
stage
and procee-ds
to
the third,
to
provide
an additionat
gain
reduction
of
approximatety
20
dB.
-
As
the Input
signal
increases
from
_g4
dBm
to
19
d?r,
the
gain
through
the
amptifier
decreases
toga_
rithmically
so
that
the output
signal
is
exactly
propor-
tional
to
the
logarithm
of
the
input.
This
ii
accom-
plished
through
a
system
of
seiies
diode
limiting
in
each
stage,
with
a
second
set
of
diodes
for
extra
timit-
ing
in each
of
the first
three stages.
The
following
description
of
a
simple
three-stage
log
amplifier,
with one
gain
step
in
each
stage,
proviOls
ai
aid
-to
understanding
the
concept
of
a
logarithmic
ampfifier.
Figures
7-g,
T-9,
and
7-10
show
an
example
amplifier
and
illustrat€
its operation.
The
gain
of
each
stage
is.3.16
(10d8)
up
to
an
output
level-of
1V
peak,
then
unity
for output
tevets
greater
than
t V
peafi
tnai
is,.
each
stage
uses one
breakpoint.
That
breakpoint
voltage
is
used for
ease
of
illustration;
the actual
break-
point
voltage
is
significanfly
lower.
The
amplifier
is shown
in Figure
7-g.
The
source
has
a
step
att€nuator
that allows
the
input
signal
to
be
incremented
in 10
dB
steps.
Table
7-5
shows
the
pro-
gression
of
gain
reduction
above
1
V at
each ampiifier
output.
Note
that with
each
input
level
change
of
10
dB,
the
output
change
at
point
4 is
0.694
V.
The
gain
curv€
for one
stage
is shown
in
Figure
7-9.
Also
note
that
when
the
level
at
point
1 is increased
beyond
1.V,
it
is
beyond
the togging
range
of
the
ampiifier.
Similarly,
if
th€ input
level is
decreased
10
dB
betow
the
minimum
inpul
level,
the
output
increment
is
different.
A
curve
of
the logging
range
is
shown
in
Figure
7-10.
POINT
1
POINT
2
POINT
3
POINT 4
44 16-99
Flgure
7{. Block
diagram
of
a
tfrree
stage
log
ampllfier.
7-29