HP 8901B Operation And Calibration Manual page 89

Tuning
Modes
2-3
Table
2-1.
Input Conditions That May
Require
Manual Tuning.
Input Condition
Multiple input signals
Level of 2nd or 3rd
harmonic is greater
than -10 dBc*
Input signals above
measurement
range (1300 MHr).
Low-rate, high-
deviation FM
Low-level carrier
with low-rate, high-
depth (percentage)
amplitude modulation
Description
For input signals whose
amplitudes are all between
-25 dBm and +30 dBm,
and are within approximately
15 dB of one another, the
HP 8901 B may auto tune
to the signal at the highest
frequency rather than the
signal with the highest level.
The HP 8901 B may interpret
these as fundamental signals
and tune to the signal at
the highest frequency.
Harmonics from the HP
8901B's LO may mix with
signals above 1300 MHz to
produce beat frequencies
within the IF passband.
While auto tuning, the
HP 8901 B may mistake
low rate
(c
20 Hz) high-
deviation (> 400 kHz)
FM for signal drift.
The AM may cause the
carrier level to move in
and out of the HP 8901B's
measurement range.
HP
8901B's
Response
Instrument will measure
and display characteristics
of input signal at the
highest frequency.
Instrument may measure
and display characteristics
of input signal at the
highest frequency.
Instrument may measure
and display characteristics
of signal above 1300 MHz.
Instrument may continuously
display four dashes.
Instrument may continuously
display two dashes or 0 MHz,
or
it
may alternately display
two dashes then four dashes.
Recommendation
If the desired signal is
not the highest-frequency
signal, manually tune to
the desired frequency or
place a bandpass filter at
the HP 8901
B's input to
block the unwanted signals.
If the desired signal is
not the highest-frequency
signal, manually tune to
the desired frequency.
Manually tune to the RF
signal below 1300 MHz, or
place a low pass filter at the
HP 8901 B's input to block thc
unwanted high frequencies.
Manually tune to the carrier
frequency or remove the
FM until the instrument has
auto tuned to the signal.
Manually tune to the
carrier frequency to
increase measurement
sensitivity. (Refer to
increasing Sensitivity in
this chapter to extend the
measurement range further.)
dBc
=
decibels referenced to the carrier level.