HP 8901B Operation And Calibration Manual page 239

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 . 9 6 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)