[time-nuts] sub cables...and GBR
didier at cox.net
Thu May 1 22:32:58 EDT 2008
Well, I am not sure I can tell much about how modern repeater power supplies
are made because I do not believe the spec I got is public domain, but I can
tell you it's a little more elaborate than a power resistor in series with a
shunt regulator, unfortunately.
They are apparently running more than a few watts of electronics at each
repeater. Each repeater operates from about 200V input voltage nominal, so
10,000 volts can drive about 50 repeaters in that case. I have not read the
spec in great details (we won't bid on it as it is commercial) but I will,
to try to figure out how they keep the voltages balanced. That's about 200W
of power for each node.
Also, and this is absolutely not clear to me, but they want the full
isolation (10,000 volts) between input side and output side of the DC/DC
converters. If the repeater is composed of optical devices, even if they
convert optical to electrical and back to optical, I do not see the need to
electrically isolate the electronics from the power cable, as there should
be no other return for ground currents.
10,000 volts isolation with 2,000,000 hours MTBF in a 0.6" high package is
no piece of cake...
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Alan Melia
> Sent: Thursday, May 01, 2008 5:19 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] sub cables...and GBR
> 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
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