56 Chapter 1
Making Basic Measurements
Making Noise Measurements
Noise Measurement Example 2:
The Normal marker can also be used to make a single frequency
measurement as described in the previous example, again using video
filtering or averaging to obtain a re a sonably stable measurement.
While video averaging auto m atically selects the sample di splay
detection mode, video filtering does not. With sufficient filtering that
results in a smooth trace, there is no difference between the sample and
peak modes because the filtering takes place before the signal is
digitized.
Be sure to account for the fact that the averaged noise is displayed
approximately 2 dB too low for a noise bandwidth equal to the
resolution bandwidth. Therefore, you must add 2dB to the marker
reading. For example, if the marker indicates –100dBm, the actua l
noise level is –98 dBm.
Noise Measurement Example 3:
You may use adjustable markers to set the frequency span over which
power is measured. The mar kers allow you to easily and con v e niently
select any arbitrary portion of the displ ay e d sig nal for measurement.
However, while the analyzer does select the average (video/rms) display
detection mode, you must set all of the othe r pa rameters.
1. Reset the analyzer by pressing Preset, Factory Preset (if present).
2. Tune the analyzer to the frequency of 50MHz. In this example we
are using the amplitude reference signal. Press FREQUENCY, 50,
MHz.
3. Set the span to 1 0 0 kHz by pres s i n g SPAN, 100, kHz.
4. Set the Y-Axis Units to dBm by pressing AMPLITUDE, More,
Y-AxisUnits, dBm .
5. Set the resolution bandwidth to 1 kHz by pressing BW/Avg,
Resolution BW, 1, kHz.
6. Set the reference level to −20dB m by pressing AMPLITUDE, Ref Level,
–20, dBm.
7. Set the input attenuator to 40dB by pressing Attenuation, 40, dB.
8. Set the marker span to 40kHz by pressing Marker, Sp a n P air (Span),
40, kHz.
The resolution bandwidth should be about 1 to 3% of the
measurement (marker) span, 40 kHz in this example. The 1kHz
resolution bandwidth that the analyzer has chosen is fine. The video
bandwidth should be ten times wider.
9. Set the video bandwidth to 10kHz by pressing BW/Avg,
Video BW (Man), 10, kHz.