[time-nuts] sub cables...and GBR

Alan Melia alan.melia at btinternet.com
Thu May 1 18:19:00 EDT 2008

Oh Dear I think I started something......
I am not up to date with the latest technology, but sub cable engineers are
very (very) conservative. Yes there is a power resistor in series and of
course that means the electronics is floating wrt the case. Interestingly
there is a discharge tube and in the semiconductor FDM systems a massive
"zener diode". I have a feeling the junction of this is about an inch in
diameter and the avalanche breakdown at about 70 volts. With the power
filter this removed most of the surge.

I am not sure about the optical cables I had moved out of the area by then
but i doubt that repeaters were laid any closer than they need, it slows
laying down a lot and is very "expensive". The last of the analogue cables
was bad enough with repeaters every 7 miles or so. Each repeater is about a
foot in diameter and around 8 foot long. The steel casing is quite thick to
withstand pressures in the deepest oceanic trenches....not to be thrown
overboad lightly !!

Another interesting snippet is that the repeater can become full of hydrogen
after a few years service, due to electrolysis. This was definitely not good
for the tubes, because this attacks the metal-oxide-glass pin seals reducing
the oxide and leaving a path for the tiny atom of hydrogen into the tube
vacuum. My group did a failure analysis on one tube in the 1970s that was so
full of hydrogen (significant fractions of a bar) that we could not get the
heater hot enought for the cathode to emit any electrons at all. Hydrogen is
a very good heat carrying "fluid". When we cracked it in the mass
spectrometer even the pirani gauge told the story....I think it was off
scale on the mass spec.

Not a lot to do with time....but I do remember some of the goings on when
Rugby GBR was first stabilised and used to compare UK and US frequency
standards. Some of the engineers who shared my office had a hand in that,
and tea breaks we fascinating times (yes we were part of the UK Civil
Service then and tea breaks are an "institution", but gave a valuable
informal cross-boundary view of projects).

Alan G3NYK

----- Original Message -----
From: "Neon John" <jgd at johngsbbq.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts at febo.com>
Sent: Thursday, May 01, 2008 9:46 PM
Subject: Re: [time-nuts] sub cables

> On Thu, 1 May 2008 11:13:48 +0100, "Alan Melia"
<alan.melia at btinternet.com>
> wrote:
> >At least with the optical cables it is only the powerfeed circuit that
> >the surge not the amplifier front end as it was in the FDM systems.
> >
> >I always thought the terminal equipment was a slighly lower reliability
> >requirement than the submerged parts but these days the loss of revenue
> >with an easily accessed terminal is very high.
> Now I'm curious how power is tapped off at each repeater point.  I
> the daisychain architecture but I'm wondering about the details, what
> components can withstand the intial high voltage surge as the cable is
> charging and how the voltage drop at each repeater is maintained
> constant, even if the repeater fails and quits consuming power.
> Also, is it true that an optical cable will keep working with a single
> repeater failure, that the laser light will pass through unamplified?
> like I read that somewhere but I'm not sure.
> John
> --
> John De Armond
> See my website for my current email address
> http://www.neon-john.com
> http://www.johndearmond.com <-- best little blog on the net!
> Tellico Plains, Occupied TN
> The secret to creativity is knowing how to hide your sources -Albert
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