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H
52005-301D
TABLE
2
Transformer
voltage
step-down ratios
Output
OUTPUT
Terminals
Voltage
Voltage
impedance
IMPEDANCE
connected
step-down
step-down
Q
switch
position
to load
r
atio
dB
(R)
600
600
Q
UNBAL
UNBAL
1:1
0
600
BAL 600
Q
BAL
1:1
0
150
BAL
150
Q
BAL
1:0.5
6
75
BAL
75
Q
BAL
1:0.35
9
37.5
BAL
150
Q
One
BAL
and
1:0.25
12
CENTRE
TAP
18.75
BAL
75
Q
One BAL
and
1:0.177
15
CENTRE TAP
50.
When the
output
impedance
and the
matchi
ng
load are
less
than 600
~.
the
output
vo
ltage is
the input
voltage multiplied
by
the
same
factor as for
600
n
(Fig. 3)
reduced by
the voltage step-
down
ratio
of the output matching
transformer
(Table
2).
OUTPUT
TO
A MISMATCHED LOAD
51
If
the load
connected to the attenuator
has
an impedance
differing from any of the output
impedances obtainable on the
instrument, it
is possible to find the
vo
ltage
across
such
a
load
by
applying a
correction to
the value of
voltage
that would be developed
if
the
load
matched.
This
correction can
be derived
from the general expression for the voltage developed
across
a load
fed from
a source of known
impedance.
The voltage across an
unmatched
load,
E
xRL
Ro +RL
where
E is
the
e.m.f. or open-circuit voltage of the
source,
R 0
is
the
output impedance of
the
source
and
RL is
the
load
impedance
.
52.
When the load
matches the source,
the voltage
developed across it
is half
the value
of
E,
and
if
the load is
greater or
less than
the output
impedance
,
the voltage across the
load
is
greater
or
less
than
half of E.
This
variation in voltage relative to the value when the load is
matched, expressed
in
dB, is a correction
that can
be
applied
to
the
expression for
V
0
m above
.
The
output voltage
across
the
load, V
0
=
(A +
R
+ M) dB below Vin
where
A is
the
total
attenuation indicated by
the
dials in dB,
R is the
voltage step-down
ratio of
the output matching
tr
ansfo
rme
r,
in
dB
(Table
2),
M
is the correction,
in
dB, depending on
the r
elation
of the
load RL
to
the
output
impedance
Ro
(Fig.
4)
a
nd
Vin
is the
input voltage,
read
on
the
input meter, applied from a
GOO
Q
generator
.
Note
.
.
.
(1)
The corrections given
for
a mismatching load
are not
valid
unless
t
he
actual
output
impe-
Nov.
79
Chap.
3
Page
11
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