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

[Dana Whitlow]

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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