Operation Model
8901B
Tuning to Low-Level Signals
Manual tuning, low-noise mode should be used with signals that may drop below the minimum threshold
for automatic tuning. Manual tuning makes
it
possible to make tuned modulation and frequency
measurements on very low-level signals.
Measuring Drift
Manual tuning, track mode is useful for measuring
drift
on low-level signals in the presence of
higher-level signals.
Determine Tuning Accuracy
When manually tuning or frequency stepping, use the frequency error measurement
to
determine tuning
accuracy.
Image Frequencies
When the Modulation Analyzer tunes
to
an input signal greater
than
2.5
MHz, the
IF
frequency
positions the internal LO above the nominal tuning. The Modulation Analyzer, however, will respond
equally well
to
a
signal
at
the image frequency, that
is,
when the signal frequency minus the LO
is
within the
IF
passband. (For example,
if
the LO
is
at
101.5 MHz, signals
at
either 100 MHz or 103
MHz (the image) generate a
1.5
MHz
IF.)
When the instrument is tuned
to
an
image, measurement
errors may result with no Error Message displayed. (The input frequency measurement will always be in
error. For FM and @M measurements, the Peak+
and
Peak- functions are reversed. One way
to
solve
this problem
is
to
step the tuning down by twice the
IF
frequency, or let the instrument automatically
tune. The following
IF
response plot illustrates the image passbands for each
IF
frequency. (ked
RF
Level does not apply.)
/
\/
455
kHz
IF
1.5
MHz IF
\
(LO
455
kHzl LO
(LO
'455
kHzI
FREQUENCY (LO
-
1
5
MHzl LO
ILO
+I
5
MHzl
NOMINAL
SIGNAL
NOMINAL SIGNAL
TUNED FREDUENCY IMAGE TUNED FREQUENCY IMAGE
455
kHz
and
1.5
MHz
IF
Passband
Responses
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. However, since the Modulation Analyzer arbitrarily 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
3-118 RF Frequency Thing
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