Display RFI

[Marcus D. Leech]

There was a discussion of RFI from displays (at 21cm specifically) on
the RTOP call.

Has anyone studied over what distances the problem exists, and what the
nature of the RFI is?  Like what its
  spectral footprint is?

[Facility 406]

RFI in general:

Having chased RFI from the 80's, to this day, and a family member in the
industry to locate, identify, and mitigate it, working from hobbyist, to
commercial, to DARPA, I can say...

It doesn't even matter.

RFI/EMI is not a definitive thing, it can exist wherever there is
energy, and exists in a unique manner comparable to all the grains of
sand on earth, and there is no "specifically" about any of it.

There is no one frequency, or type, other than that produced by each
individual source, although there may be some commonality, for example
power lines, which produce RFI, generally beginning at 60 Hz, and that
I've heard directly to 2.3 GHz, the 38,000th+ harmonic, with one single
source traced to a single connection, affecting 18 square miles.

If you experience it, address that RFI, from the source affecting you at
that moment (then expect to find up to several dozen more at your
location, it's that rampant). That will not, unfortunately, prevent one
from being affected by the several billion devices generating it
globally 24/7/365, any given instant, which, hopefully, won't affect
you, because, good luck getting the rest of the world to turn off.

That being said, however, not all RFI/EMI will affect a particular
installation, which makes things easier. If you aren't experiencing RFI
from arcing power lines, don't bother that find, and fix it. Same with
WiFi, any one of dozens of monitors, or displays, LED lights, switching
mode power supplies, et cetera.

Address RFI/EMI, per source, per device affected.

Kurt

[Marcus D. Leech]

On 06/03/2023 15:40, Facility 406 wrote:
> On 3/6/2023 12:18, Marcus D. Leech wrote:
>> There was a discussion of RFI from displays (at 21cm specifically) on
>> the RTOP call.
>> Has anyone studied over what distances the problem exists, and what
>> the nature of the RFI is?  Like what its spectral footprint is?
>
> RFI in general:
>
> Having chased RFI from the 80's, to this day, and a family member in
> the industry to locate, identify, and mitigate it, working from
> hobbyist, to commercial, to DARPA, I can say...
>
> It doesn't even matter.
>
> RFI/EMI is not a definitive thing, it can exist wherever there is
> energy, and exists in a unique manner comparable to all the grains of
> sand on earth, and there is no "specifically" about any of it.

Well, sure.  It's pervasive and invasive and all that .  Cannot disagree.

But for example, USB, owing to the way it is clocked and the average
characteristics of its data patterns will tend to have spikes in
  RFI at certain frequency ranges, and not others.    Ethernet is the
same thing--if you notice narrowband features at harmonics of
  25MHz, there's a good chance it's your Ethernet infrastructure.

Same with VGA and HDMI monitors -- YES, the electronics internals will
differ, but there's a lot of commonality--VGA operates in
  certain defined ways with certain defined clocks--similarly with
HDMI.   Displays are constrained to a small number of
  resolutions and refresh rates, that basically determine what clocks
MUST be used internal to displays operating in that
  parameter space--YES the internal implementations will differ
(sometimes remarkably), but there are constraints of
  external interoperability standards that will mean that some common
RFI complaints may emerge.

And that's what prompted my query.

[fasleitung3]

Hi Marcus,
Experience here is that displays can be a BIG noisance. The greatest
offender is a big screen which we use for demonstrations. This one has
not only be turned off but also have to be disconnected from mains when
we want to do something useful. Otherwise it is seen all over the
spectrum. It seems that here even the switching power supply in standby
generates significant RFI in L-band. Another example were two 24"
monitors which someone wanted to donate to us. We returned them to the
generous donator as they were observable all over our campus on all
telescopes (even though they carried a CE sign which is supposed to
demonstrate compliance with EMI regulations).
Other monitors are sort of ok if they are 10m or so away. So there is
no easy answer as there seems to be a varying degree of emission
depending on type and brand. We tend to be very careful when we set up
a new monitor.
Cheers,
Wolfgang

> --