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HP 416A - Page 46

HP 416A
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may
be
operattd
is
small
,
being
determined
solely
by
variations
(with
frequency)
in
the
signal
so
rce
output
power.
Since
the
range
of
variation
in
the
level
of
power
into
the
ncident
detector
is
comparatively
small
(approximately
10
db
when
the
signal
source
is
a
Model
670HM
oscillator)
,
it
is
practical
to
attenuate
the
incident
voltage
ahead
.
of
the
detector"
To
provide
this
desirable
attenuation,
generally
a
20-db
directional
coupler
is
used
in
the
incident
channel.
Because
of
the
wide
dynami
c
range
over
which
the
reflected
detector
may
be
operated
(a
possible
range
of
40
db
,
with
100%
reflection
at
one
end
of
the
range
.
and
the
possibility
of
a
reflection
as
low
as
1 %
at
the
other
end),
much
attenuation
ahead
of
the
reflected
detector
is
not
p r a c
tical
,
since
the
minimum
voltage
into
the
Model
416A
must
be
at
least
3
mic
rov
olts.
By
choosing
a
signal
source
with
sufficiently
high
output
power
,
howeve
r,
a
directional
coupler
which
offers
10
db
attenuation
can
be
tolerated.
NOTE:
With
a
20
-
db
coupler
in
the
incident
channel
and
a
10
-
db
coupler
in
the
reflected
channel,
the
voltage
in
the
incident
channel
will
be
down
10
db
from
the
level
it
should
have
to
maintain
a
true
ratio.
By
setting
the
Model
416A
EXCESS
COUPLER
LOSS
switch
at
10
db
, a
10-db
gain
is
provided
in
the
incident
input
circuit
to
brin
g
th
e
incident
voltage
up
to
the
proper
level.
Po
s
sible
Errors
-
a.
If
the
level
of
power
into
the
detector
c
annot
be
kept
low
enough
to
operate
the
detecto
r in
the
linear
regi
on
of
its
input/output
character-
istic,
the
resulting
departur
e
fr
orn
a
tru
e
squa
re
-l
aw
characteristic
will
be
the
sour
ce
of
tw
o
types
of
possible
error.
(1)
In
order
to
calibrate
a r a t
io
mete
r,
it
is
necessary
to
assume
that
the
detector
follows
some
fixed
det
ec
tion
law.
The
Model
416A
has
been
cal
ibrated
on
a
square
-law
basis.
Where
the
voltages
out
of
the
detectors
do
not
r
eta
in
a
square-law
r
elation
to
theil-
in-
put
y
oltag
es,,,
the
ratio
n:1
e t
e.r
c
annot
indicate
th
e
tru
e
ratio
of
re
-
fleeted
-
to
-
incident
power.
The
magnitude
of
this
error,
of
course
,
is
a
function
of
the
amount
of
deviation
from
a
true
square
-
law
characteristic•
and
at
inputs
up
to
- 5
dbm
will
not
be
great.
(2)
Th
ere
is
another
facto
r
with
re
spe
ct
to
dev
i
ation
from
square
-law
that
c
an
c
ontribute
gr
e
ate
r
error
than
lack
of
tracking
between
met
er
calibration
and
the
proportional
character
of
the
input
volt-
ages.
This
fa
ct
o r
is
lac
k
of
tracking
betwe
en
the
two
detectors
be-
cause
t
he
ir
d
eviat
ions
from
sq
uare
-
law
are
different.
If
the
deviation-
fro
rn-square
-
law
c
ha
r
acteristic
of
the
two
detectors
were
identical,
a
true
ratio
would
be
maintained
,
and
no
appre
c
iable
error
would
be
int
ro
duced
by
this
factor.
Diffe
re
nces
in
crystal
characteristics
exist,
howeve
r,
and
la
c k
of
tracking
between
the
two
detectors
in
the
nonl
i
near
region
of
th
e
ir
input/output
characteristic
is
to
be
expected.
=3
1-

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