HP 8901B Operation And Calibration Manual page 103

Extending
Operation 3-5
lncreasing Selectivity The selectivity of your modulation and audio measurements above 10 M H z
can be increased by selecting the 455
W z
IF. Key in 3.1 SPCL to select the
455 kHz IF with a 200 kHz bandwidth. (Note that the 455
kHz
IF should
only be selected for measuring modulation rates below 10 kHz.) To further
increase the selectivity of your measurement, select low-pass, high-pass,
or bandpass audio filtering using the Audio Filter keys on the front panel.
The HP 8901B's Auto Tuning mode is able to automatically find and
tune to the fundamental in a harmonically-rich spectrum as long as
the fundamental is at least 10 dB above the level of the second and
third harmonics. The HP 89018 will also auto tune to the carrier if the
carrier is at least 30 dB above
all
non-harmonically related RF signals. If,
however, your input signal does not meet these criteria, the HP 8901B
may have difficulty auto tuning to the carrier. If the HP 8901B tunes to
a signal other than the carrier, it
will
cause the instrument to display
modulation and audio readings for that signal rather than the carrier.
To check what RF frequency the
HP
8901B is actually tuned to, press
the FREQ key. If the instrument does not display the correct carrier
frequency, manually tune the
HP
8901B to the carrier frequency; key
in the frequency of the carrier and then press the M H t key. (Refer to
Chapter
2 for details on the Auto Tuning and Manual Tuning modes.)
Changing Frequency Although the
HP
8901B is not designed for swept measurement
applications, its Auto Tuning mode does enable it to automatically search
for the carrier signal when carrier frequency is changed. ( k r details on the
capabilities and limitations of Auto Tuning mode operation, refer to
Chapter
2.)
Noisy Carrier Excessive noise or low-rate (below 20 Hz), high-deviation (greater
than 400 kHz) Fh4 on the carrier signal can make it difficult for
the HP 8901B to auto tune to the carrier. If you know that your
carrier is noisy, or if the HP 8901B indicates it is having difficulty
setting up the measurement by continuously displaying four dashes,
manually tune the
HP
8901B to the carrier frequency. Simply key
in the frequency of the carrier and then press the
MHz
key. If the
HP 8901B does not display a measurement reading, press the TRACK
MODE key. (Refer to Chapter 2 if you are interested in details on how
the instrument operates in Manual Tuning mode and Track mode.)