[time-nuts] Close in phase noise of microwave VCOs

John Miles john at miles.io
Thu Jun 18 18:29:26 EDT 2015

> When you go very close in on something like a VCO, you get much higher phase
> noise
> than we normally worry about. Some of the “assumptions” that underly the
> measurements
> are no longer true. Small angle of modulation is one, but there are a few others.

That's a good point as well.  L(f) isn't defined in situations where the noise is strong enough to appear in higher-order Bessel sidebands  In these contexts it's better to think in terms of residual FM rather than phase noise.

See http://www.ke5fx.com/stellex.htm for one interesting case where a noisy X-band source is returning questionable results.  I measured it three different ways -- with a spectrum analyzer, an HP 3048A quadrature PLL, and a TimePod with a downconverter.  I'm still not sure why the 3048A gave different results in the 100 Hz - 1 kHz region relative to the other two analyzers.  I'd expect a small-angle violation to result in an artificially high noise plot but the opposite seems to be occurring.  I keep meaning to go back and re-run those tests with more careful attention paid to the reference signal level and a few other potential confounding factors.  Also need to run a discriminator measurement on the 3048A to see how it compares to the PLL result.

-- john, KE5FX
Miles Design LLC

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