[time-nuts] Power supply for time source concerns

[jimlux]

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.