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).
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.
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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