HP 8901B Operation And Calibration Manual page 229

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. ( k e d
RF
Level does not apply.)
/
\ /
455 kHz IF 1.5 MHz IF
\
( L O 4 5 5
k H z l LO
(LO '455 kHzI FREQUENCY (LO
-
1 5 MHzl LO
ILO
+ I 5 M H z l
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 T h i n g