[time-nuts] Re: 10 MHz Distribution Best Practices?

[David Bengtson]

Yes, I've measured the existing distribution system with an FSWP,
which is in a single rack, and there's ~ 10 to 20 dB of noise present
in the system, so there's for sure some mitigation needed even now.

Dave

On Fri, Mar 24, 2023 at 10:36 AM John Vendely via time-nuts
<time-nuts at lists.febo.com> wrote:
>
> Bob makes a good point about the devices terminating the ends of
> reference distribution cables.  Many frequency counters have little
> shielding, and can be a significant source of 10 MHz radiation,
> regardless of the shielding effectiveness of the reference cable runs
> feeding them.  Counters such as the HP 53132 and 5335 are good examples
> of this.  Even some signal generators and spectrum analyzers can exhibit
> this problem.  Despite good shielding in their RF sections, sometimes
> inadequate attention was paid to shielding of the external reference inputs.
>
> I encountered this problem when building a reference distribution system
> for my low phase noise house standard.  The distribution amp was
> meticulously shielded and double shielded cable was used throughout,
> with runs up to 60'.  When individually terminated, the cables showed
> very low leakage.  However, significant 10 MHz radiation resulted when
> connected to numerous test instruments...
>
> 73,
>
> John K9WT
>
>
> On 3/24/2023 8:26 AM, Bob Camp via time-nuts wrote:
> > Hi
> >
> > There are indeed multi year sort of courses (actually sets of courses)
> > that various folks put on about the EMI side of this. Unfortunately it does
> > quickly get pretty deep in theory and thus the long slog to get to this or
> > that level of the knowledge base.
> >
> > How big is “work” and how big is “lab”? I’ve seen places that measured
> > the answers in kilometers. I’ve also seen places that had everything within
> > 20 meters of everything else. What you do in one is profoundly different
> > than another.
> >
> > How many “destinations” do you have that need this or that? Again, some
> > will have an answer in dozens, for others it will be in the thousands or even
> > tens of thousands.
> >
> > What connects to those end points? If the answer (as noted earlier) is
> > a bunch of ADEV / phase noise gear, that’s very different than more
> > conventional test gear. Keeping the 10 MHz away from the DUT can be
> > a *very* big deal.
> >
> > Like it or not, much of this has been decided for you. That piece of test
> > gear has a very normal BNC connector on the back of it. They are a
> > terrible thing for this, but that’s what you get. The cable you use is
> > going to mate with them. If you use off the shelf gear for the distribution,
> > it also has BNC’s on it.
> >
> > Cable needs to be run from here to there. Loss is rarely an issue. Skin
> > depth vs the shield is very much an issue at 10 MHz. Foil as a shield is
> > useless in this case. Big thick multiple layers of braid is needed. That
> > isn’t just a “get RG-400” sort of thing. It’s also who you get it from. The
> > expensive source may be the one ….
> >
> > There is a tendency to overdo this kind of thing. Think through how
> > many endpoints you need. Cable up the reasonable minimum and
> > see how it goes. Plan on a re-visit in 6 months and maybe again in
> > 12 months. Having a lot of cable to nowhere is a problem in a number
> > of ways ….
> >
> > Fun
> >
> > Bob
> >
> >
> >
> >> On Mar 23, 2023, at 8:45 PM, David Bengtson via time-nuts <time-nuts at lists.febo.com> wrote:
> >>
> >> Has anyone run across any publications on best practices or examples
> >> of 10MHz Lab wide distribution networks? I'm looking for a discussion
> >> on how to physically locate oscillators/distribution amplifiers, cable
> >> types and runs, RFI mitigation etc. I haven't come across any, and I'm
> >> starting to build one at work. We've got a Cs oscillator and I'd like
> >> to make sure we deliver that performance across our systems lab.
> >>
> >> Thanks
> >>
> >> Dave
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