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

Jim Lux jimlux at earthlink.net
Wed Jun 17 11:22:53 EDT 2015

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).


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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