Keysight Technologies N5511A, Руководство пользователя

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148 N5511A Phase Noise Test System User’s Guide
Absolute Measurement Examples
Dual Channel (DCFM)
Checking the Beat Note
Use an oscilloscope (connected to the monitor port on the test set) or a
counter to check the beat note being created between the reference source
and your DUT in each channel. The objective of checking the beat note is to
ensure that the center frequencies of the two references are close enough in
frequency with the DUT to create a beatnote that is within the capture range of
the system.
The phase lock loop (PLL) capture range is 5% of the peak tuning range of the
VCO source you are using. (The peak tuning range for your VCO can be
estimated by multiplying the VCO tuning constant by the tune range of VCO.
Refer to Chapter 14, “Evaluating Your Measurement Results” if you are not
familiar with the relationship between the PLL capture range and the peak
tuning range of the VCO. The beat note frequency is set by the relative
frequency difference between the two sources. If you have two very accurate
sources set at the same frequency, the resulting beat note will be very close to
0 Hz. Searching for the beat note will require that you adjust the center
frequency of one of the sources above and below the frequency of the other
source until the beat note appears on the oscilloscope's display. If
incrementing the frequency of one of the sources does not produce a beat
note, you will need to verify the presence of an output signal from each source
before proceeding.
Figure 6-30 shows the Manual Tune Pause Point, which is triggered during
calibration if the beat note of one of the references is out of the desired range.
The window will specify which channel needs to be tuned.
Figure 6-30 Manual Tune Pause Point
If the center frequencies of the sources are not close enough to create a beat note within the
capture range, the system will not be able to complete its measurement.