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H 52005-301D
Input ,·oltmeter
15.
Connected
in
parallel with the inlets
to
the step attenuator, the
input
meter measures the
\'Oltage
applied to
the instrument.
If
switch
SC is
turned off,
the voltmeter
is
disconnected
and
ceases to monitor the input
to the
instrument.
1
G.
The basic
voltmeter
consists of two germanium
diode
s
MR1 and MR3 in a
voltage
doubling
rectifier circuit, feeding the moving-coil meter M1.
The rectifier circuit is preceded by two
transistors
VTl
and
VT2
arranged as
emitter
followers
in
cascade.
These present
a
higher
effective
input impedance
for
the
voltme
ter and isolate the distorting effects due
to
rectification.
17.
The
first
transistor
VTl
receives
the
input voltage of
the attenuator
via
potentiometer
cir-
cuits
selected
by
the
range
switch
SC.
On
the
most
sensitive
range,
one
potentiometer
alone
is
used,
consisting of Rl in series
with
R
Vl in
one
arm
and the
input
impedance of the first emitter
follower
circuit.
On
each of the
other three ranges this potentiometer is preceded by one other
supplementary potentiometer,
selected by
the switch from the
three
formed
by
R4,
RV4
and
R15;
R3,
RV3
and
R14;
R2,
RV2 and
R13.
The
potentiometer arms
have shunt capacitors, some
trimmers,
whic
h
preserve the required
voltage drop at the
higher frequencies
.
18.
The
transistors
rece
ive
their
d.
c.
supply
via socket SKTB and reverse-polarity protection
diode MR2.
The ,·oltmeter circuit has no d. c.
connection
to the rest of the instrument.
It
ha
s
a reference point with
an a.
c. tie to chassis
via ClO,
and
there
is
a
d. c. blocking
capacitor CG
in series with the signal
input
path.
Output matching
19.
This
part
of
the instrument receives the output
of
the
step
attenuator.
It
consists
mainly
of
a multi-ratio matching transformer and
switching a
rrangements
by
which the taps
on the
s
econdary of
the
transformer
are se lected or
the
trans
former is
bypa
s
s
ed (Fig.
3).
20.
At three positions of the
output
impedance
selector
switch
SA
,
the
output
of
the
step attenua
-
tor
is
applied
to the
primary
of the
matching
t
ransformer T1 via a d.
c.
blocking capacitor C17.
At
these
three positions SA
also connects
in turn three
pairs
of taps
on
the secondary
of the
trans-
former to the balanced output terminals on
the
front
pane
l,
so
that output impedances of
600
n,
150
nand 75
Q
are obtained.
The
s
econdary
is
floating and
has
a centre tap connected
to
another
terminal
on the front
panel.
Chap. 4
Jlag·e
-1
OM
FR
ST
ATTIN
EP
lJA JOII
ooo
,Y
6
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:-
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.
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s.:~
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;
SA
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Fig·.
:J
Output
matc
hing
s
tn
gc (less
switched interna
l
loa
d)
OAL
UNLUL
CtH TR£
TAP
Nov
.
79
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