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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