HP 8901B Operation And Calibration Manual page 230

Model 8901B Operation
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.
Frequencies Greater Than 10 MHz
When making measurements on inputs with frequencies greater than 10 MHz, signals present
at
the
input that are less than 2.5 MHz pass directly into the IF. These low-frequency signals can be removed
by inserting the
RF input high-pass filter. (Refer to Filters, RF and IF on page 3-229).
Searching for a Signal
When manually tuning, often the exact input frequency is unknown. If during tuning, Error Ol*(signal
out of
IF
range) is displayed, select the MHz key alone to center the signal in the IF. Also, if searching for
a signal using the kHz keys, it is best to search down from above the signal frequency while monitoring
IF LEVEL. (Refer to IF Level on page 3-93.) When the IF
level rises significantly, switch to FREQ
ERROR and enter the displayed value as a frequency step and complete tuning using the kHz keys.
Default Conditions
When the Instrument Preset key is pressed, or on power-up, the Modulation Analyzer selects the
automatic tuning, low-noise LO mode.
Responding to Frequency Changes
Using the automatic tune mode, the Modulation Analyzer might not retune when the frequency
changes, but its spectrum still has sufficient power within the tuned passband of the Modulation
Analyzer's RF INPUT.
It will appear that the carrier simply changed its level. If the instrument does
not retune, frequency and level measurement results might not be as expected. Use a manual tune
mode, if possible, in the following situations:
0
When working with frequency agile sources (such as frequency synthesizers) that have a frequency
0
When changing frequency in harmonic steps.
hopping function.
Extending Tuned Measurements Below 2.5 MHz
Use the following procedure to extend the range of tuned measurements below 2.5 MHz.
NOTE
Only the 455 kHz IF can be selected. The minimum input frequency
is
795
kHz. Measurement accuracy may be degraded. IF signal may appear at the
modulation monitoring connectors.
1.
2.
3.
4.
5.
6.
Select INSTR
PRESET
(Blue Key, AUTOMATIC OPERATION).
Set the signal source to a carrier frequency between 3 and 9 MHz and
to
the level of the intended
measurement.
Connect the signal source to the Modulation Analyzer's INPUT (or
to
a properly connected
sensor module). Allow the Modulation Analyzer
to
tune to the input signal.
Select RANGE HOLD. Select DISABLE ERROR (Blue Key, RANGE HOLD).
Enter the frequency value of the input signal and retune the signal source to that frequency.
Select the desired measurement mode. (RANGE HOLD and DISABLE ERROR can be released
at this time.)
RF Frequency Tuning
3-119