HP 8901B Operation And Calibration Manual page 225

Model 8901B Operation
Frequency Agile Sources
For best results, use the manual-tune mode when working with frequency agile sources. With agile
frequency sources (such as frequency synthesizers), it
is
possible for the carrier or its harmonics
to
hop unexpectedly into the IF passband either directly (for frequencies below
2.5 MHz), by n o d
down-conversion, or by down-conversion of an image (the signal frequency minus the LO approximately
equals the
IF).
Depending on the frequency, amplitude, and timing of the signal source and the tuning
mode of the Modulation Analyzer, a tuning error may or may not be sensed.
If
the instrument does
not retune, frequency and level measurement results may not be as expected.
When Measuring Frequency Agile Sources
or
Multiple Signal Sources
Whenever tuning to signals less than
2.5 MHz, the input signal passes directly into the
IF.
The LO is
used only to turn on the mixer. The Modulation Analyzer sets the
LO to
101.5 MHz, a passband is
created from 99
to
104 MHz. If this frequency band causes a measurement problem, manually tune the
LO so the passband for the down converted signal falls in an unoccupied region of the input spectrum.
This method is not a preferred way to make measurements, but it might enable you to make some
desired measurements. Note that when using this method you should disable Error 01 (by selecting
8.1 SPCL) to prevent the Modulation Analyzer from trying to tune to a down-converted signal rather
than the signal that is less than 2.5 MHz.
Manual Tuning
When tuning manually, it is possible to accidentally adjust the LO so that the input signal falls in the
image passband; for example, when using the kHz (up arrow) and kHz (down arrow) keys. Frequency
measurements are inaccurate when the instrument is tuned to an image. (Refer
to R F Requency T h i n g
on page 3-115 for an illustration of the image passbands.)
Input/Output Status
When the
FREQ
key is selected, the MODULATION OUTPUT signal continues to be the last selected
modulation signal, filtered, and/or de-emphasized (except PRE DISPLAY) as previously selected (as
indicated by the MODULATION OUTPUT annunciator). Calibration of this output depends on the
IF
signal level and should be assumed only when
all
errors are enabled (Special F'unction 8.8).
RF Input Frequency
3- 113