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

Richard (Rick) Karlquist richard at karlquist.com
Wed Jun 17 20:08:04 UTC 2015

On 6/17/2015 8:22 AM, Jim Lux wrote:
> I'm looking for some representative data for inexpensive microwave VCOs
> (in the 2.5-6 GHz range, in general).  Not in a locked loop situation,

If the phase noise data you have goes to a low enough frequency to
get below the 1/f corner (which is the case for the example you cited)
then it is a very safe bet that the noise will go up by 30 dB/decade
below that.

Having said that, if an ordinary engineer had asked me this question,
I would think that he needed some coaching on how to clean up the
VCO with a synthesizer of sufficiently wide loop bandwidth.  However,
you are very knowledgeable, so I will assume you are going to do
that and just want to predict the phase noise after clean up.  The
trick (as most time nuts know) is to use a small enough capacitor
in the loop filter so that you get clean up at a 40 dB/decade rate
so you can actually make some headway against the 30 dB/decade
1/f slope.

I have been through this exercise innumerable times and also taught
it to many others, and it seems to be very predictable.

In the unlikely event you use the VCO open loop, you'll have lots
of problems with microphonics, power supply noise, and even magnetic
fields from power transformers, as well as load pulling and thermal
drift.  Making microwave oscillators that can be used open loop 
(especially inexpensive ones) is definitely a lost art.  It died with 
the HP8640 sig gen.

Rick Karlquist N6RK

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