5120A/5115A Operations and Maintenance Manual 91
In order to produce a useful PSD estimate, the spectrum estimation process is broken into a series
of spans that have appropriate frequency resolution (DFT length). The process as performed by the
5120A and 5120A-01 with cross-correlation is shown in Figure B-8. A similar process is used in
the 5115A. This process allows more rapid estimation of the higher frequency portions of the
spectrum. Data for each lower frequency span is obtained by low-pass (anti-alias) filtering the
higher sample rate data, then decimating it appropriately.
Figure B-8: 5120A: Successive Filter-decimate-transform Signal Processing Stages
The spectrum estimation process for all unit types is broken into a series of spans that have
appropriate frequency resolution (DFT length). The process, as performed in the 5120A and
5120A-01 with cross-correlation, is shown in Figure B-8. The process allows more rapid
estimation of the higher frequency portions of the spectrum. Data for each lower frequency span is
obtained by low-pass (anitalias) filtering the higher sample rate data, then decimating it
appropriately.
B.3 Measurement Methodology
All phase noise measurements are referenced to the frequency of the Input port. This has an effect
on how you connect the instrument for different measurements. One example is listed below.
Example: You have a noisy synthesizer tuned to 12 MHz and a very clean 5 MHz reference. You
want to know the phase noise of the synthesizer.
Test Method 1—You connect the 12 MHz signal to Input and the 5 MHz signal to
Reference. The phase noise of the synthesizer under test will be measured and displayed
relative to 12 MHz.
Test Method 2—You connect the 5 MHz signal to Input and the 12 MHz signal to
Reference. The phase noise of the synthesizer will be measured and displayed referenced
to the 5 MHz signal. To make the phase noise measurements referenced to 12 MHz, you
will need to add a frequency scaling term of 20*log10(f(INPUT)/f(REFERENCE) =
20*log10(12MHz/5MHz) = 7.60 dB to all phase noise measurements displayed. You will
do this scaling with a separate tool.
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