[time-nuts] Power supply for time source concerns

[Taka Kamiya]

I think the conversation is going into an area way beyond my initial intent.
My goal is to have a "good enough" source for 24V to run multiple time sources.  Yesterday, I measured the output of switcher I've been using for testing purpose only.  Holy cow it's dirty!  120mV peak-to-peak wide band noise.  Put one ferrite bead on plus side and it had an amazing result.  Broadband noise disappearance and periodic pulse (still 120mV p-p)
My goal is to have about 5mV p-p or lower capable of delivering 20Amp in switching arrangement.  It's easy to do with linear regulators but it makes the room awfully warm.  Why 5mV?  Because it is good enough.  If more is necessary, it will have to be dealt with locally inside the time source itself.  Cabling can pick up that much easily.

--------------------------------------- 
(Mr.) Taka Kamiya
KB4EMF / ex JF2DKG
 

    On Monday, December 23, 2019, 1:09:58 PM EST, jimlux <jimlux at earthlink.net> wrote:  
 
 On 12/23/19 5:42 AM, Bob kb8tq wrote:
> Hi
> 
> If you dig into the app notes on the LT304x parts after a lot of extolling the
> wonders and virtues, they eventually get into magnetic coupling between the
> “upstream” and “downstream” bypass components. Taking care of that with
> proper layout is possible, but non-trivial.

And not entirely predictable by modeling, even with the fancy 
multiphysics signal integrity modeling.

It is one of those things where you will probably need a few iterations.

> 
> Is milivolts of noise “ok”? Maybe it is. There are a lot of devices out there that
> run with 10’s of milivolts of noise on the supply line. Do you need <100uV p-p
> over 10KHz to 100 MHz? There are devices that do. If that’s what you need,
> it’s going to be a struggle. There is no one single “good enough” number.
> 
> The power supply world still stops at pretty low frequencies. Very common IC’s
> these days are quite happy to produce crud at the “many GHz” level. There are
> a few folks who carry around cell phones that put out signals up there as well.
> Regulator IC’s will only do part of the job, filtering in one form or the other (likely
> several forms) is still needed.

Yes, I think, though, that filtering (even over multiple decades) is 
easier at higher frequencies - Assuming you can tolerate the IR voltage 
drop through the filters.


> 
> Past that system layout begins to get into the act. You can spend a couple of fun
> weeks in intro level classes on this sort of stuff. You can be the prof presenting the
> class and pretty quickly get to “that would require some research” as the answer
> to this or that seemingly basic question. It’s not in any way a trivial topic.
> 
> So what to do?
> 
> Bottom line is still that test equipment is your friend. Spectrum analyzers that cover
> a wide range of frequencies (and have good sensitivity) *are* part of the “kit” that
> allows you to keep things quiet. You can only go just so far ( = not very far) before
> you are into the “verify” stage of things. Indeed sound cards and SDR’s can collect
> some of this data so there *are* a range of tools you can use.
> 
> Checking cabling / grounding / cross talk is very much part of setting up any RF
> bench. It always has been. Tracking down that stupid whatever on the other side
> of the room that is putting out a ton of noise *is* part of the drill. Repeating the
> process from time to time also is part of the fun. Looking at the data you collect to
> spot “strange” stuff *is* part of the deal …… There is work involved and you need
> to understand what you are doing.
> 


I think understanding how the effect of unwanted interference (by 
whatever path) manifests itself in the desired output is part of the 
challenge - In many cases, the most sensitive detector of interference 
is the unit under test (certainly this is the case for a low noise 
receiver).

One thing that can help evaluate a design is it artificially introduce 
interference or noise into the power supply - if you can't see a change 
in the output of the UUT, then the filtering/isolation scheme must be 
working.  If the artificial interference is at a level that is >> the 
level you'd see in expected use, then you're in great shape.





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