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

[Bob Camp]

Hi

Nothing is wrong with it. If you are doing runs measured in kilometers, its a 
pretty good idea. Back a few decades, it wasn’t an option and you used coax
even for those runs. 

There are very few instruments that will accept a 10 MHz signal via fiber …. :)

Bob

> On Mar 24, 2023, at 4:04 PM, Dana Whitlow via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> What's wrong with fiber optical distribution?
> 
> Dana
> 
> 
> On Thu, Mar 23, 2023 at 9:54 PM John Miles 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.
>> 
>> A lot of things can be said in favor of low-noise distribution amps with
>> good VSWR, good channel isolation, and good PSRR (not so fast, HP 5087A.)
>> But over the years, I've learned that the three most important factors when
>> it comes to piping 10 MHz signals around are:
>> 
>> 1) Shield resistance
>> 2) Shield resistance
>> 3) Shield resistance
>> 
>> I've spent a lot of time recommending double-shielded coax in the TimeLab
>> manual and elsewhere, and I still stand by that advice, but what I've come
>> to realize is that this is really just a proxy for low shield resistance.
>> Good grades of single-shielded cable are basically as effective at HF as
>> double-shielded cable.  To the extent your cable ground shield exhibits
>> resistance, it's not a shield, it's a resistor.
>> 
>> Avoiding ground loops is on the list too, but further down.  Never lift a
>> ground to avoid a ground loop.  Use coax-to-coax baluns only when you can
>> see a beneficial effect.  Focus instead on providing a shared
>> low-resistance
>> common ground  to your entire network -- ideally not the ground all the way
>> back at the service entrance -- and rely on low shield resistance on the RF
>> side to do the rest.
>> 
>> Every installation is different and your mileage will most certainly vary,
>> but this is my take on it.
>> 
>> -- john
>> 
>> 
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