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

[John Miles]

Also see http://www.ke5fx.com/gunnpll.html , a quick and dirty but successful attempt at locking a Gunnplexer in a relatively low (1 kHz) loop bandwidth.  The inband noise is likely too high for good performance in a radar application, but the basic idea is workable enough.  Unfortunately I tried this experiment before I had any way to generate log plots. :(

I can't think of many noise-critical applications where a microwave VCO is used without some form of phase locking.  I'd think that a homodyne architecture would still need a synthesized source, just because the waveform being received is delayed relative to the one that was sent.  Without a clean source, I'd imagine that you'd have to do autocorrelation between the outgoing and incoming channels rather than simple/cheap baseband mixing.  An obvious question would be whether it's cheaper to add another digitizer and correlator to your pipeline than it would be to clean up your source...

-- john, KE5FX
Miles Design LLC




> -----Original Message-----
> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Magnus
> Danielson
> Sent: Wednesday, June 17, 2015 1:08 PM
> To: time-nuts at febo.com
> Cc: magnus at rubidium.se
> Subject: Re: [time-nuts] Close in phase noise of microwave VCOs
> 
> Jim,
> 
> John Miles have been a bit active:
> http://www.ke5fx.com/brick/brick.htm
> 
> Just to give you a start-sample.
> 
> Cheers,
> Magnus
> 
> On 06/17/2015 05:22 PM, 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,
> > but just bare: with a DC voltage on the tuning input. I'm particularly
> > interested in data closer than 100 Hz.
> >
> > Most of the data sheets (e.g. from Minicircuits, ROS-3710; crystek
> > CVCO33 series) show noise from 1 kHz or 10kHz out, because most of these
> > parts are intended for use in a PLL, and the "close in" will be
> > determined by the loop.
> >
> > Before I go out and hook one of these up and measure it, I figured I'd
> > ask if someone out there has done it, or if there's a data sheet.
> >
> > I'm not looking for any particular part or frequency or even exact
> > numbers: more "representative, typical" kind of performance one might
> > get from one of the plethora of $20-50 VCOs out there.
> >
> > Something like the ROS-3710 looks like it's about -30dB/decade trending
> > to 20 dB/decade.
> > (-70 @ 1 kHz, -96 at 10kHz, -118 at 100kHz, -138 at 1MHz)
> >
> > A paper I found on 77 GHz sources cite a 30dB/decade (actually they give
> > it as f^-3.05).
> >
> >
> >
> >
> > background:
> >
> > I've got a homodyne radar at work we use for detecting heartbeats of
> > buried earthquake victims. I've also got a variety of gunn oscillator
> > "doppler radars" of one sort or another.
> > There's 10GHz homodyne radars available for $5 from China (the selling
> > prices range from $1 to $20, with corresponding inverse costs in
> > shipping.. ).  They're designed for intrusion detectors and automatic
> > door openers.
> >
> > There's all kinds of cheap 2.45 GHz sources around: one might be able to
> > repurpose an old 802.11b WiFi interface, for instance, although I think
> > those are all synthesized PLL designs.
> >
> > And my car has a 77 GHz radar in it for adaptive cruise
> > control/automatic braking.
> >
> > There's also Greg Charvat's "build a SAR with coffee cans and a laptop"
> > mini-class/dissertation project.
> >
> >   RF wise these radars are simple device, and I was asked to give a
> > presentation to the JPL Amateur Radio Club on the principles and
> > limitations on performance.  Most of the members of the JPLARC (like me)
> > actually know quite a lot about RF design, so they'll be asking about
> > "what about the phase noise of the Tx".  We all know, qualitatively,
> > that gunns are noisy close in (to the bane of hams who want to do narrow
> > band stuff with the old MaCom gunnplexers), although, like with the
> > minicircuits VCOs, there's no published data on their 1-100 Hz phase noise.
> >
> > So I'm writing up a set of notes on the various factors, and the self
> > noise of the oscillator is particularly important when looking for low
> > frequency modulations (like heartbeats at 1 Hz, or people walking).
> >
> > I've got empirical "as measured in the system" data from my 3GHz
> > homodyne radars, but I was looking for some component data as an example.
> >
> >
> > Actually, if someone has some close in data from a 10.525 GHz Gunn (or
> > the newer motion detectors), I'd love to see that too.
> >
> >
> >
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