HP 8753D - Page 132

Conversion Loss
Using the
Frequency Oset
Mode
Conversion
loss
is
the
measure of
eciency of
a mixer
.It
is the
ratio of
side-band IF
power
to
RF
signal
power
,
and
is usually
expressed in
dB.
(Express ratio
values in
dB amounts
to
a
subtraction
of
the
dB
power
in
the denominator
from the
dB power
in the
numerator.)
The
mixer
translates
the
incoming
signal,
(RF),
to a
replica, (IF),
displaced in
frequency by
the
local
oscillator
,
(LO).
Frequency translation
is characterized
by a
loss in
signal
amplitude
and
the
generation
of
additional
sidebands
.
For
a given
translation, two
equal
output
signals
are
expected,
a
lower
sideband
and
an
upper sideband.
Figure
3-4.
An
Example
Spectrum
of
RF
,
LO
,
and
IF
Signals
Present
in
a
Conversion
Loss
Measurement
The
analyzer
allows
you
to
make
a
swept
RF/IF
conversion
loss
measurement
holding
the LO
frequency
xed.
Y
ou
can
make this
measurement
by
using
the
analyzer's
frequency
oset
measurement
mode
.
This
mode
of operation
allows you
to
oset
the
analyzer's
source
by
a
xed
value
,
above
or
below the
analyzer's receiver
.
That
is
,
this
allows
you
to
use
a
device
input
frequency
range
that
is
dierent from
the receiver
input
frequency
range
.
The
following
procedure
describes the
swept IF
frequency conversion
loss measurement
of
a
broadband
component mixer:
1.
Set the
LO source
to
the
desired
CW
frequency
and
power
level.
CW
frequency =
1000 MHz
P
ower =
13 dBm
2. Set the desired source power to the value which will provide
0 10 dBm or less to the
R-channel input. Press:
4
MENU
5
NNNNN
NNNNNNNNNNNN
POWER
NNNNN
NNNNNNNNNNNNNNNNNNNNNNNNN
NNNNNNNNNNN
PWR RANGE MAN
4
0
54
x1
5
3. Calibrate and zero the power meter
.
4. Connect the measurement equipment as shown in Figure 3-5
.
The low pass lter is required to limit the range of frequencies passed into the R-channel
input port. The lter is selected to pass the IF frequencies for the measurement but
prevent the LO feedthrough and unwanted mixer products from confusing the phase lock
loop operation.
A pad is used to isolate the lter and improve the IF port match for the mixer.
The attenuation of the power splitter is used to improve the RF port match for the mixer.
Making Mixer Measurements 3-7