[time-nuts] Framework for simulation of oscillators

Magnus Danielson magnus at rubidium.se
Mon Mar 21 03:20:04 EDT 2016


Hi Ulrich,

Interesting article. Did you see Craig Nelsons article on building a 
mixer out of 2N2222A transistors?

Cheers,
Magnus

On 03/21/2016 12:45 AM, KA2WEU at aol.com wrote:
> http://joerg-berkner.de/Fachartikel/pdf/2000_AKB_Berkner_1f_noise.pdf
> 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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