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

[John Miles]

> 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