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

Jim Lux jimlux at earthlink.net
Wed Jun 17 23:19:28 EDT 2015

On 6/17/15 1:08 PM, Richard (Rick) Karlquist wrote:
> 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.

That's sort of what I was thinking..  But a couple good measurements is 
worth hours of dialectic.

> 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.

Actually, in a homodyne FMCW radar, there's no loop, so you have just 
the bare noise of the oscillator.  Actually, it looks a lot like a 
"delay line" type phase noise test set, where the length of the delay 
line is some tens of ns (e.g. free space propagation).

> 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.

In a homodyne radar, you've got to deal with all those things. 
Actually, magnetic fields aren't nearly as big a problem as the 120 Hz 
signal you get from all the fluorescent lights, which are basically 
radar reflectors that turn on and off every half cycle.

If you're doing a "door opener motion detector" at 10.525 GHz (ISM Part 
15) the doppler is 60 Hz/ m/s (roughly).  Walking speed is 1-2 m/s

> Rick Karlquist N6RK
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