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Phase Noise in Signal Generators

Phase Noise in Signal Generators


Phase noise serves as an important oscillator parameter and some amateur radio enthusiasts seem confused about what it is, how it’s measured, and most importantly, how it might affect what they’re working on.
For the amateur making modest-quality, homebrew receivers, it may not matter. In a VHF PLL chain, or a contest grade receiver, a low phase noise oscillator might rank as important to the builder.
Aside from your demands and goals, it’s still important to understand how phase noise arises and effects radio circuity — and to understand and empathize why some feel concerned, while others don’t care about it. On any radio topic, open-questioning, honesty, plus reliance on measurement & evidence serves us much better than dogma and/or emotional-based ranting.
A large body of science-informed publications covers phase noise measurement. The procedure isn’t particularly difficult, but requires expensive, calibrated equipment. Most of us will depend on datasheets and measures from others fortunate enough to posses this equipment.

Regarding what oscillator phase noise is acceptable for a given project: context is everything. I think each builder needs to decide for themselves based upon some critical thinking. At some level — our Western world is suffering from the death of reason, and perhaps even common sense.

Partisan arguing, commission of logical fallacies, plus us–versus-them stances hardly lifts up, or unites our increasing less relevant amateur radio community.
How will close-in phase noise impact the intended use(s) of my receiver? What can I afford? How will I benefit from 1 choice when compared to another design choice? What are the pros versus cons? How can I test my choice objectively? What data exists to inform my decision? Who can I ask for help?
Applying logic, reasoning, and better yet, applying experimental methods using peer-reviewed standards of measurement & incorporating evidence (as possible) seem the most consistently reliable way to understand radio electronics and help us choose our designs.
Paul Holes, an RF and Microwave field applications engineer with Anritsu wrote a stellar piece called The truth about phase noise in signal generators
Although aimed at microwave signal generators, he states ‘the right tool for the job’ as the most important truth to consider.
“Manufacturers of signal generators set great store by the specification of phase noise, such that, regardless of the application, phase noise is frequently taken as a proxy of the instrument’s overall performance. However, phase noise performance might have little or no impact in some applications.”
Sadly,  this link often fails, or loads slowly, but it’s well worth the trouble.
KI6WX — Rob’s page linked below well describes and graphically shows some of the implications of phase noise to reception. Also, The ARRL Lab frequently comments about reciprocal mixing issues related to LO phase noise.  Again, a builder might not care — and that’s OK. However, this is core information to boost your knowledge as a radio maker or enthusiast. An informed decision based on clear thinking trumps putative adoption every time!
For a modest quality, homebrew HF receiver, I think a low cost digital chip oscillator like the SI5135A seems a reasonable choice. Amplifying it to 50- 100 Watts power on Field Day, or during a multi-op contest might be another story. On my last Field Day experience, a couple of guys with DDS-based LO homebrew rigs driving bricks pretty much wrecked weak signal reception on 20 and 10 Meters for us.
A Field Day related-link with good discussion and measures
It’s good to think about, discuss and share information in the highest moral spirit of Ham Radio.

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