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

[Kitski]

I'll second John's recommendations, particularly double-shielded coax, but
with a twist (pardon the pun).
There are various grades of 'double shielded' coax - some with loose braid
coverage and others with really tight weaves (loosely spec'd as 'coverage').
Use 95% or forget about it.

Next up (down ?) this rabbit hole is triax which by definition is double
shielded. More to the point, the braids are insulated from each other.
Another benefit is that BNC triax connectors are not that expensive.  As the
outer braid is typically at 'mains/safety earth' potential, the independent
inner braid can be used in a variety of hum-busting ways.
Then another rabbit hole to peer down is re-wiring your AC mains facility
with separate electrical earth and technical earths. Not for the
faint-hearted this one.

Mitigating leakage LF thru to UHF (and crosstalk) in and out of your
facility are part and parcel of MIL requirements in sensitive
establishments.  Measuring their effectiveness (BT-DT) is an interesting
past-time.  

My 10c worth (currently 67c US).

Kit
Canberra, Australia


-----Original Message-----

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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