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

[Tom Knox]

Where I have seen fiber is larger labs in which their primary standard such as a 5071A is fiber distributed to clean-up oscillators at each bench or rack. The systems I saw were made by Wenzel and used their ULN oscillators. This eliminates a number of potential issues such as ground loops. I think Johm Miles made one of the most important points, you could have the best ultra-low phase noise standards made, but without high-end cabling and proper grounding much of the performance could be lost. And sometimes grounding and cables routing can prove more art than science to achieve the full potential of an UNPN system.
Cheers;
Tom Knox
SR Test and Measurement Engineer
Phoenix Research
4870 Meredith Way Apt 102
Boulder, Co 80303
Formerly of:
357 Fox Lane
Superior Co 80027
303-554-0307
actast at hotmail.com

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________________________________
From: Dana Whitlow via time-nuts <time-nuts at lists.febo.com>
Sent: Friday, March 24, 2023 2:04 PM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Cc: Dana Whitlow <k8yumdoober at gmail.com>
Subject: [time-nuts] Re: 10 MHz Distribution Best Practices?

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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