HP 8753D - Page 424

RF
Feedthru
RF
feedthru, or
RF-to-IF
isolation,
is
the
amount
the
RF
power
that
is
attenuated
when it
reaches
the IF
port.
This
value
is
low
in
double
balanced
mixers
.
RF
feedthru is
usually less
of
a
problem than
the
LO
isolation
terms
because
the
LO
power
level
is
signicantly
higher
than
the
RF power
drive
.
Y
ou
can
make
an
RF
feedthru
measurement
using
the
same
instruments
and
setup
that you
use
to
measure
conversion
loss
.
Because
the
source
and
receiver
frequencies
are
the
same
,the
analyzer
can
use
narrowband
(tuned
receiver)
detection
to
make
the
measurement.
The
only
dierence
that
you
need
in
the
hardware
conguration
is
that
the
IF
lter
needs
to
be
removed
so
the
RF
feedthru
will
not
be
ltered
out.
The
RF
signal
is
applied
to
the
RF
port
of
the
mixer
and the
feedthru is
measured at
the IF
port.
The
RF
feedthru
level
is
very
dependent
on
the
LO
signal
that
is applied.
For
this reason,
you
should
make
the
measurement
with
the
LO
signal
present
at
its
normal
operating level.
Y
ou
should
perform
a
frequency
response
calibration
to
improve
accuracy.
SWR
/
Return
Loss
Reection
coecient
(0)
is
dened
as
the
ratio
between
the
reected voltage
(Vr) and
incident
voltage
(Vi).
Standing
wave
ratio
(SWR)
is
dened
as
the
ratio of
maximum standing
wave
voltage
to
the
minimum
standing
wave
voltage
and
can
be
derived
from
the reection
coecient
(0)
using
the
equation
shown
below
.
Return
loss
can
be
derived from
the reection
coecient
as
well.
0
=
V
r
V
i
S
W
R
=
1
+
j
0
j
1
0
j
0
j
R
etur
n
l
oss
=
0
20
l
og
j
0
j
Note
Mixers
are
three-port
devices
,
and
the
reection
from
any one
port
depends
on
the
conditions
of
the
other
two
ports
.
Y
ou should
replicate the
operating
conditions
the
mixer will
experience as
closely as
possible
for
the
measurement.
When you
measure the
RF port
SWR, you
should have
the
LO
drive
level
present
and
set
to
the
expected frequency
and
power
levels
.
The
IF
port
should
be
terminated
in a
condition
as
close
to its operating state as possible
.
The measurements of LO port SWR and IF port SWR are very similar
.F
or IF port SWR, you
should terminate the RF port in a matched condition and apply the LO signal at its normal
operating level. F
or the LO port
SWR, the RF and IF ports should both be terminated in
conditions similar to what will be present during
normal operation.
Application and Operation Concepts 6-157