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

[John Vendely]

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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