EasyManua.ls Logo

HP 415E - Page 20

HP 415E
59 pages
To Next Page IconTo Next Page
To Next Page IconTo Next Page
To Previous Page IconTo Previous Page
To Previous Page IconTo Previous Page
Loading...
Section
IV
Paragraphs
4-5
to
4-21
4-5.
RANGE
ATTENUATOR.
4-6.
Тһе
signal
from
А151
is
fed
to
the
first
section
of
the
RANGE-DB
switch,
А251,
and
then
to
the
input
amplifier.
The
second
section
of
A281
is
located
be-
tween
the
input
amplifier
and
the
second
amplifier.
Тһе
RANGE-DB
Switch
positions
are
marked
in
10
db
steps.
4-7.
INPUT
AMPLIFIER.
4-8.
After
passing
through
the
first
section
of
the
range
attenuator,
А251,
the
signal
goes
to
the
input
amplifier
(A3Q1/Q2/Q3/Q4)
which
consists
of
four
transistors
in
cascade.
The
input
signal
is
applied
to
the
base
of
A3Q1
and
the
final
amplifier
signal
is
taken
from
the
collector
of
A3Q4.
The
GAIN
and
VERNIER
controls
are
associated
with
this
amplifier
and
vary
its
gain
over
a
range
of
more
than
10
to
1.
GAIN
control
Ri,
the
coarse
control,
is
a
250K
ohm
variable
resistor
which
adjusts
the
amount
of
negative
feedback
from
the
collector
of
A3Q4
to
the
emitter
of
A3Q1.
VER-
NIER
control,
R2,
is
a
fine
gain
control
and
changes
gain
by
inserting
0
to
5000
ohms
in
series
with
the
output
signal.
4-9,
SECOND
AMPLIFIER.
4-10.
Transistors
A3Q5
and
A3Q6
amplify
the
signal
from
the
second
section
of
the
range
attenuator.
AC
feedback
provides
gain
stability
and
high
input
im-
pedance.
The
output
of
the
amplifier
is
applied
through
the
EXPAND
attenuator,
A282,
to
the
third
amplifier
А398
and
A3Q9.
4-11.
EXPAND
CIRCUIT.
4-12.
The
function
of
the
EXPAND
switch
A282,
is
to
allow
any
signal
level
to
be
measured
on an
expanded
scale
with
continuous
coverage
while
maintaining
the
original
reference
level.
Expansion
is
accomplished
by
applying
a
precise
amount
ої
DC
-offset
current
from
A3Q17
to
the
meter
and
simultaneously
increasing
the
signalto
the
3rd
amplifier.
This
increased
gain
allows
a
2
db
change
in
signal
level
to
deflect
the
meter
across
its
full
scale.
The
offset
current
places
the
zero
signal
indication
off
scale
to
the
left.
4-13.
FREQUENCY
SELECTIVE
CIRCUITS.
4-14,
The
frequency
response
of
the
third
amplifier,
A8Q8
and
A3Q9,
is
shaped
by
negative
feedback.
The
feedback
path
includes
a
Wien-bridge
and
amplifier
АЗОТ.
At
the
null
frequency
of
the
Wien-bridge,
the
negative
feedback
path
is
open
and
the
gain
of
the
am-
plifier
ismaximum.
Off
center
frequency
the
negative
feedback
through
Ње
Wien-bridge
reduces
gain.
The
amount
of
the
"off
resonance"
gain
reduction
depends
on
the
setting
of
the
BANDWIDTH
control,
R3.
4-15.
The
Wien-bridge
is
adjusted
for
a
sharp
null
at
center
frequency
with
BRIDGE
STABILITY
ADJUST
АЗВ29.
Actually,
this
control
is
set
for
a
very
slight
bridge
unbalance
to
produce
just
enough
positive
feed-
back
so
that
signal
current
to
the
base
of
A3Q8
is
supplied
mainly
by
A3Q7.
Thus,
at
resonance,
negligible
signal
current
flows
through
BANDWIDTH
control,
R3,
4-2
Model
415E
andgainis
independent
ofits
setting.
Center
frequency
is
set
by
varying
resistors
R4
and
R5
(these
resistors
are
gangedand
comprise
the
front
panel
FREQ
control).
4.16.
FINAL
AMPLIFIER.
4-17.
The
output
amplifier
consists
of
four
transis-
tors.
The
two
output
transistors,
А3012
and
А3013,
operate
as
a
push-pull
class
B
amplifier
with
both
collectors
AC
grounded.
The
emitters
of
these
tran-
sistors
are
tied
together
and
the
AC
amplifier
output
is
taken
from
this
point
through
a
coupling
capacitor,
A3C28.
Large
negative
feedback
makes
the
gain
of
the
output
amplifier
very
nearly
unity.
The
AC
output
voltage
is
developed
acrossresistor
A3R51:
The
cur-
rent
through
A3R51
is
supplied
by
A3Q12
and
A3Q13
conducting
one
at
atime
onalternate
half
cycles
(Class
B
operation)
and
the
output
signal
sine
wave
is
а
composite
of
this
half-cycle
operation.
In
addition,
the
collector
current
of
A3Q13
can
drive
the
meter
directly.
No
rectifier
diodes
are
needed,
This
meter
driving
current
is
filtered
by
capacitor
A3C26
and
passes
through
the
meter
anda
1000
ohm
resistor,
R6,
to
develop
а
DC
voltage
for
the
recorder
output.
4-18.
GROUND
LOOPS.
4-19,
Тһе
grounding
technique
used
in
the
415E
con-
sists
ої
an
input
connector
ground,
a
circuit
board
ground,
and
output
connector
grounds.
These
are
"floating"
grounds
that
are
tied
together
and
isolated
from
chassis
ground
except
for
a
46.4
ohm
resistor,
ЕТ,
anda
0,05
uf
capacitor,
СІ,
connecting
ground
and
chassis.
A
solid
connection
to
chassis-or-earth
ground
permits
troublesome
ground
loop
currents
to
flow
causing
erroneous
instrument
operation,
For
this
reason,
connecting
grounded
instruments
to
the
415E
output
connectors
сап
cause
erroneous
readings.
Most
recorders
and
oscilloscopes
that
might
be
used
with
the
415E
outputs
have
differential
inputs
available
with
neither
side
grounded
(see
Paragraph
3-11).
4-20.
INPUT
IMPEDANCE.
4-21.
The
Model
415E
is
designed
to
have
an
input
impedance
much
higher
than
that
of
any
crystal
detec-
tor
or
bolometer
normaily
used
with
it.
This
results
in
lower
noise
figure
and
the
highest
possible
input
signal
tothe
415Е.
For
example
with
the
415Е
INPUT
switched
to
LOW,
the
input
impedance
is
approximately
2000
ohms
while
the
output
or
source
impedance
of
a
bolometer
is
approximately
200
ohms.
02152-3

Table of Contents

Related product manuals