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 nod
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
RF
Requency
Thing
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
To Next Page
Loading...
