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

[Magnus Danielson]

Hi,

On that note I noticed that the HP5370A has a severe 5 MHz pulse 
polution coming from a 10 MHz external signal detector which 
implementation in ECL spews out a lot of noise. All to provide a 
diagnostic LED only vissible if you lift the lid. A strategic short 
killed the 5 MHz loop and removed the noise, without loosing any 
significan function other than this signal present.

I did not make any attempt to measure to which degree this affected 
actual measurements on the instruments, which I naturally should have done.

So yes, at times one find that counters are not as clean as you wish 
them to be.

Cheers,
Magnus

On 2023-03-24 15:15, John Vendely via time-nuts 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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