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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 reasonably stable measurement.
While video averaging automatically selects the sample display
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 2 dB to the marker
reading. For example, if the marker indicates –100 dBm, the actual
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 markers allow you to easily and conveniently
select any arbitrary portion of the displayed signal for measurement.
However, while the analyzer does select the average (video/rms) display
detection mode, you must set all of the other parameters.
1. Reset the analyzer by pressing Preset, Factory Preset (if present).
2. Tune the analyzer to the frequency of 50 MHz. In this example we
are using the amplitude reference signal. Press
FREQUENCY, 50,
MHz.
3. Set the span to 100 kHz by pressing SPAN, 100, kHz.
4. Set the Y-Axis Units to dBm by pressing AMPLITUDE, More,
Y-Axis Units,
dBm.
5. Set the resolution bandwidth to 1 kHz by pressing BW/Avg,
Resolution BW, 1, kHz.
6. Set the reference level to −20 dBm by pressing AMPLITUDE, Ref Level,
–20, dBm.
7. Set the input attenuator to 40 dB by pressing Attenuation, 40, dB.
8. Set the marker span to 40 kHz by pressing Marker, Span Pair (Span),
40, kHz.
The resolution bandwidth should be about 1 to 3% of the
measurement (marker) span, 40 kHz in this example. The 1 kHz
resolution bandwidth that the analyzer has chosen is fine. The video
bandwidth should be ten times wider.
9. Set the video bandwidth to 10 kHz by pressing BW/Avg,
Video BW (Man), 10, kHz.