[time-nuts] Framework for simulation of oscillators

KA2WEU at aol.com KA2WEU at aol.com
Sun Mar 20 19:46:41 EDT 2016


http://scholarcommons.usf.edu/cgi/viewcontent.cgi?article=2270&context=etd
 
 
 
 
 
 
 
In a message dated 3/20/2016 5:33:21 P.M. Eastern Daylight Time,  
magnus at rubidium.se writes:

Ulrich  and Attila,

As you read the appendixes of ITU-T Rec. G.823, G.824 and  G.825 they 
will not give very detailed information, but hints. The flicker  noise 
model comes from Jim Barnes and Chuck Greenhalls PTTI 19 article  "Large 
Sample Simulation of Flicker Noise". Be aware of Chuck's follow-up  
correction. Further, they model the amount of noise and add into the  
loop in place of the oscillator, which then has a normal PI-loop. Such a  
simulation can be done fairly efficiently considering that the  
oscillator and loop is very simple linear models of phase, not too  
different to what I proposed. For the stuff that Attila needs to  
simulate, some additional thought needs to go into how to simulate the  
effect he is seeing, but a fairly simple approach should be interesting  
to try out initially.

The Barnes&Greenhall flicker generator  builds on a filter-bank where the 
poles and nulls is placed such that they  approximate the flicker noise 
slope of 1/tau. This is a generalized  variant of Jim Barnes PhD work 
where he had fixed relations and where  Chuck Greenhall have contributed 
significantly by providing means to setup  the state of the filter such 
that the filter will act as a filter in  equilibrium from start, rather 
than taking much time to converge,  something which may introduce a bias 
into the measurement results. I have  re-implemented their BASIC-code 
into C and run Chuck's original code  along-side to verify (just to find 
where I did my mistake in converting  it).

If this simulation approach is sufficient for either of your  efforts, or 
not, depends on what you try to capture. For instance, the  oscillators 
performance have been idealized in assuming fully linear EFC,  fully 
linear integrator of the crystal, assuming noise profile etc. This  may 
or may not be sufficient. Inherent lowpass filtering may be important  or 
not.

I've done PLL simulations many times, in fixed integer, in  floating 
point and in VHDL. It's always a challenge to model it right to  the needs.

Let me also have reader of this thread reminded of TvB's  simulator for a 
GPSDO, which is interesting as it adds real GPS PPS data  and real open 
loop oscillator data with a simple PLL oscillator core you  can then 
tweak. Great fun in all it's simplicity and nice way to do  reality 
check. I've done similar things with about the same code amount  that 
have proved very useful.

However, recall that whenever you  make a model, you do it with 
assumptions for your particular problem, so  some stuff will be left out 
and some will be particular to your problem.  One guys model may be crap 
to another ones problem. There is a few tricks  to be learned and a few 
things to recall to  include.

Cheers,
Magnus

On 03/20/2016 09:19 PM,  KA2WEU at aol.com wrote:
> I am interested in this topic too, thanks,  Ulrich
> In a message dated 3/20/2016 4:10:12 P.M. Eastern Daylight  Time,
> magnus at rubidium.dyndns.org writes:
>
>   Attila,
>
>     On 03/17/2016 10:56  AM, Attila Kinali wrote:
>      > Moin,
>   >
>      > Measurement we recently  did showed some quite unexpected behaviour
>      >  and I am trying to figure out where this comes from. For this
>   > I would like to simulate our system, which consists of  multiple
>      > crystal oscillators that are coupled  in a non-linear way (kind of
>      > a vector-PLL  with a step transfer function) with a "loop 
bandwidth"
>     > of a few 10kHz.
>      >
>   > My goal is to simulate the noise properties of the  crystal
>     oscillators
>       > both short term (in the 10us range) and long term (several  1000
>     seconds)
>      > in a  way that models reality closely (ie short term instability
>   is uncorrelated
>      > while long term  instability is correlated through 
temp/humidity/...)
>     >
>      > As I am pretty sure not the first  one to attempt something like 
this,
>      > I would  like to ask whether someone has already some software
>   framework
>      > around for this kind  of simulation?
>      >
>       > If not, does someone have pointers how to write realistic
>   oscillator models
>      > for this kind  of short and long term simulation?
>
>     It is a  large field that you tries to cover. What you need to do is
>   actually find the model that models the behavior of your physical  
setup.
>
>     You need to have white and flicker  noises, there is a few ways to get
>     the flicker  coloring. I did some hacking of the setup, and ran tests
>   against Chuck Greenhalls original BASIC  code.
>
>     You probably want a systematic effect  model of phase, frequency and
>     drift. Also a cubic  frequency vs. temperature. All the properties 
needs
>      to be different for each instance. Similarly, the flicker filter  
needs
>     to be independent for each  oscillator.
>
>     Similar enough things have been  tried when simulating the jitter and
>     wander in the  G.823-825 specs.
>
>     An aspect you need to  include is the filtering properties of the EFC
>      input, it acts like a low-pass filter, and the Q of the resonator  is
>     another catch-point.
>
>   I wonder how complex model you need to build before you have  catched
>     the
>      characteristics you are after.
>
>     The EFC  measures you have done so far indicate that your steering
>   essentially operates as if you do where doing something similar  to
>     charge-pump operation.
>
>   Cheers,
>     Magnus
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