Operation Model
8901B
PROCEDURE
To use LO OUTPUT
as
a
signal
source:
1.
Select an IF frequency of either
1.5
MHz or
455
kHz using Special Function
3.
(If
the
IF
frequency
is
not specified, the Modulation Analyzer will assume the
1.5
MHz
IF
for keyboard-entered
frequencies greater than
10
MHz and the
455
kHz
IF
for frequencies less than
10
MHz but
greater than
2.5
MHz.)
2.
Subtract the
IF
frequency from the frequency desired at
LO
OUTPUT, and enter the result, in
MHz,
via
the numeric keys; then press the
MHz
key. (To tune the
LO
below
2.96
MHz, use the
procedure as described, but select the
455
kHz
IF
using Special Function code
3.1,
and disable
Error
01
using Special Function code
8.1.)
3.
Connect the LO OUTPUT
to
the desired input.
The
LO
frequency can be monitored using Special Function
33.
(Refer
to
u)
Requency
on page
3-127.)
To
use an external source
as
the LO INPUT:
1.
Select an IF frequency
(1.5
MHz
or
455
kHz)
using Special Function
3.
2.
Press the MHz key
to
enter manual tune mode
(to
prevent the internal
LO
from continuously
3.
Add the IF frequency
to
the frequency
to
which the Modulation Analyzer
is
to
be tuned, and set
4.
Adjust the external LO
to
0
dBm and connect
it
to
the LO INPUT.
tuning).
the external
LO
to
that
frequency.
COMMENTS
Using the
LO
OUTPUT
LO tuning is not exact. The instrument tunes the
LO
only close enough
to
make calibrated
measurements.
Using the
LO
INPUT
To tune
to
inputs below
2.5
MHz with an external
LO,
select the
1.5
MHz
IF,
but set the
LO
to
approximately
100
MHz. LO signal
is
required for the Input Mixer
to
function even when
down-conversion
is
not desired.
Invalid Frequency Measurements (RF and
LO)
When an external
LO
is
used, measurements made using the FREQ
or
FREQ ERROR keys are invalid.
To determine input frequency, subtract the
IF
frequency
(34.0
SPCL) from the
LO
frequency used.
The LO frequency function
(33.0
SPCL) continues
to
measure the internal
LO.
FM Transients Using
an
External
LO
When using an external
LO,
rapid changes in
LO
frequency within the
IF
passband cause FM transients.
Several seconds are then required before accurate measurements are possible. To avoid the problem,
turn
off
the external LO when switching
its
frequency.
3-130
LO
Input and LO Output (Option
003)