What's wrong with fiber optical distribution?

Dana

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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Bob makes a good point about the devices terminating the ends of
reference distribution cables.  Many frequency counters have little
shielding, and can be a significant source of 10 MHz radiation,
regardless of the shielding effectiveness of the reference cable runs
feeding them.  Counters such as the HP 53132 and 5335 are good examples
of this.  Even some signal generators and spectrum analyzers can exhibit
this problem.  Despite good shielding in their RF sections, sometimes
inadequate attention was paid to shielding of the external reference inputs.

I encountered this problem when building a reference distribution system
for my low phase noise house standard.  The distribution amp was
meticulously shielded and double shielded cable was used throughout,
with runs up to 60'.  When individually terminated, the cables showed
very low leakage.  However, significant 10 MHz radiation resulted when
connected to numerous test instruments...

73,

John K9WT

On 3/24/2023 8:26 AM, Bob Camp via time-nuts wrote:

Hi

There are indeed multi year sort of courses (actually sets of courses)
that various folks put on about the EMI side of this. Unfortunately it does
quickly get pretty deep in theory and thus the long slog to get to this or
that level of the knowledge base.

How big is “work” and how big is “lab”? I’ve seen places that measured
the answers in kilometers. I’ve also seen places that had everything within
20 meters of everything else. What you do in one is profoundly different
than another.

How many “destinations” do you have that need this or that? Again, some
will have an answer in dozens, for others it will be in the thousands or even
tens of thousands.

What connects to those end points? If the answer (as noted earlier) is
a bunch of ADEV / phase noise gear, that’s very different than more
conventional test gear. Keeping the 10 MHz away from the DUT can be
a very big deal.

Like it or not, much of this has been decided for you. That piece of test
gear has a very normal BNC connector on the back of it. They are a
terrible thing for this, but that’s what you get. The cable you use is
going to mate with them. If you use off the shelf gear for the distribution,
it also has BNC’s on it.

Cable needs to be run from here to there. Loss is rarely an issue. Skin
depth vs the shield is very much an issue at 10 MHz. Foil as a shield is
useless in this case. Big thick multiple layers of braid is needed. That
isn’t just a “get RG-400” sort of thing. It’s also who you get it from. The
expensive source may be the one ….

There is a tendency to overdo this kind of thing. Think through how
many endpoints you need. Cable up the reasonable minimum and
see how it goes. Plan on a re-visit in 6 months and maybe again in
12 months. Having a lot of cable to nowhere is a problem in a number
of ways ….

Fun

Bob

On Mar 23, 2023, at 8:45 PM, David Bengtson via time-nuts time-nuts@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.

Thanks

Dave

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What's wrong with fiber optical distribution?

Dana

On Thu, Mar 23, 2023 at 9:54 PM John Miles via time-nuts <
time-nuts@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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