HP 8753D - Page 413

The
second/third harmonic
response can
be displayed
directly
in
dBc
,
or
dB
below
the
fundamental
or carrier
(see
Figure
6-83
).
The
ability
to
display
harmonic
level
versus
frequency
or
RF power
allows
\real-time"
tuning
of
harmonic
distortion.
Further
,
this
swept
harmonic
measurement,
as well
as
all
of
the
traditional
linear
amplier
measurements
can
be
made
without
reconnecting the
test
device
to
a
dierent
test
conguration.
Figure
6-83.
Swept
Frequency
Amplier
Measurement
of
2nd
and
3rd
Harmonic
Distortion
(dBc)
Gain
Compression
V
ector
network
analyzers
are
commonly
used
to
characterize
amplier
gain
compression
versus
frequency and
power
level.
This
is
essentially
linear
characterization
since
only
the
relative
level of
the
fundamental
input
to
the
fundamental
output
is
measured.
The
narrowband
receiver
is
tuned
to
a
precise
frequency
and,
as
a
result,
is
immune
from harmonic
distortion.
Y
ou
may
want
to
quantify
the
harmonic
distortion
itself.
Gain
compression
occurs
when
the
input
power
of
an
amplier
is
increased to
a level
that
reduces
the
gain
of
the
amplier
and
causes
a
nonlinear
increase in
output
power
.
The
point
at
which the gain is reduced by 1 dB is called the 1 dB compression point. The
gain compression
will
vary with frequency
, so it is necessary to nd the worst case point of gain compression in
the frequency
band.
Once that point
is identied, you can perform a power sweep of that CW frequency to measure
the input power at
which the 1 dB compression occurs and the absolute power out (in dBm) at
compression.
6-146 Application and Operation Concepts