User`s guide
10. To confirm that the resolution bandwidth and video bandwidth are correct for measuring
the modulation amplitude, use the following procedure:
a. Press
(MKRI
and use the knob to move the marker to the desired modulation signal. In
this example, place the marker on the 60 Hz fundamental modulation signal.
Note
For HP 8590L with Option 713 or HP 8592L with Option 713 the resolution
bandwidth must be left at about 100 kHz to accommodate frequency drift of
the spectrum analyzer. If you are using an HP 8590L with Option 713, or an
HP 8592L with Option 713, do not do step b.
b. Press
CBW)
and decrease the resolution bandwidth using the
@J
key, until measured
signal amplitude drops. Then press
a
to increase the bandwidth until the signal
amplitude stops increasing and stays the same, or until the maximum resolution
bandwidth is reached. Use the narrowest bandwidth that does not cause a change in the
signal amplitude.
Note
As the resolution bandwidth is stepped down, the modulated signal must be
re-centered in the filter bandwidth. This is a zero span display. To center the
signal, select
[FREQUENCY)
and adjust the center frequency to maximize the
amplitude of the trace. If this is not done, the signal amplitude can decrease
due to off tuning of the spectrum analyzer and not because of the resolution
bandwidth chosen.
For the best amplitude accuracy, the resolution bandwidth should be about 10 times
greater than the highest modulation frequency of interest.
For the 60 Hz fundamental, a 1 kHz resolution bandwidth works well. (For the
HP 85913, HP
8593E,
HP 85943, HP 85953, and HP
8596E,
if harmonics are not a
concern, a 100 kHz resolution bandwidth can be used and it will provide a faster update
rate.)
C.
Press
IBW),
VID BW AUTO MAN (MAN) and use the step keys to decrease the video
bandwidth until the amplitude of the measured signal drops. Then step the bandwidth
up until the signal amplitude stops increasing, or until the maximum video bandwidth is
reached. Use the narrowest video bandwidth that does not cause a change in the signal
amplitude.
For the best amplitude accuracy, the video bandwidth should be about 10 times greater
than the highest modulation frequency of interest.
For the 60 Hz fundamental, a 1 kHz video bandwidth works well.
11.
Press
[MEAS/USERJ
and
%
AM ON OFF so that ON is underlined. The spectrum analyzer
reads out the percent AM of the largest modulation frequency. An arrow indicates the
signal being measured (see Figure 4-l). This measurement does not include all of the
harmonics of the modulating signal.
Note
The percent AM function will not run if the SIGNAL CLIPPED error message
is being displayed. Increase the reference level until the error message goes
away.
4-4 Making Measurements