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Selecting a Reference
Using a Similar
Device
Document Part No. E5500-90024Ed. 1.0
Selecting an appropriate reference source is critical when you are
making a phase noise measurement using the phase lock loop
technique. The key to selecting a reference source is to compare the
noise level of the reference with the expected noise level of the
unit-under-test (UUT). In general, the lower the reference source's
noise level is below the expected noise level of the UUT the better.
(Keep in mind that you only need to be concerned about the
reference source's noise level within the frequency offset range over
which you plan to measure the UUT.)
As shown by the graph in
source's noise level is below the noise level of the UUT, the less the
reference source's noise will contribute to the measurement results.
Figure 6-6 Increase in Measured Noise As UUT Noise Approaches Reference
Noise
The test system performs best when you are able to use a device
similar to the UUT as the reference source for your PLL
measurement. Of course one of the devices must be capable of being
voltage tuned by the system to do this.
To select a similar device for use as the reference source, you must
establish that the noise level of the reference source device is
adequate to measure your UUT. The Three Source Comparison
technique enables you to establish the actual noise levels of three
comparable devices when two devices are available in addition to the
UUT.
E5500 Phase Noise Measurement System Version A.02.00
Absolute Measurement Fundamentals
Selecting a Reference
Figure
6-6, the further the reference
6
6-9
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