HP 8753D - Page 425

Conversion
Compression
Figure
6-96.
Conversion
Loss
and
Output
P
ower
as
a
Function
of
Input
P
ower
Level
Conversion
compression
is
a
measure
of
the
maximum
RF
input
signal
level
for
which
the
mixer
will
provide
linear
operation.
The
conversion
loss
is
the
ratio
of
the
IF
output
level to
the
RF
input
level,
and
this
value
remains
constant
over
a
specied
input
power
range
.
When
the
input
power
level
exceeds
a
certain
maximum,
the
constant ratio
between IF
and
RF
power
levels
will
begin
to
change
.
The
point
at
which the
ratio has
decreased
1
dB
is
called
the
1
dB
compression
point.
Notice
in
Figure
6-96
that
the
output
power
increases
linearly
with
the increasing
input
signal
level,
until
mixer
compression
begins
and
the
mixer
saturates
.
Y
ou
can
measure
conversion
compression using
the
same
basic
test
congurations
that
are
used
to
measure
the
conversion loss
.
T
o
set
up
for
a conversion
compression measurement,
rst
measure
the
conversion
loss
of
the
mixer
under
test.
Set
up
for a
CW measurement
at
the
frequency
of
interest.
Sweeping
the
RF
drive
level
over a
25 dB
span soon
shows the
power
level
at
which
the
conversion
loss
increases
by 1
dB
.
With
power meter
calibration controlling
the
RF
drive
level,
and
the
receiver
calibrated
to
measure
output power
,you
can
make
accurate
measurements
of
the
output
power
at
the
1
dB
compression
point.
Phase Measurements
When you are making linear measurements
, provide a reference for determining phase by
splitting the RF source power and send part of the signal into the reference channel. (This does
not work for frequency oset measurements
, since the source and receiver are functioning at
dierent frequencies
.)
To provide a reference signal for the phase measurement, you need a second mixer
. This mixer
is driven by the same RF
and LO signals that are used to drive the mixer under test. The IF
output from the reference mixer is applied to the reference (R) channel of the analyzer.
6-158 Application and Operation Concepts