RE: [PSUBS-MAILIST] Passive sonar

["Sean T. Stevenson" <cast55@telus.net>]

Alec - I'm starting to suspect that noise is not your problem, but I have a few
comments:

1) I don't think you can find braid sheathing intended to be a signal shield
that you can fish a cable through.  You can run your cable through metallic
conduit (pipe, or steel / aluminium flex condiut - exposed or PVC coated) and
ground the conduit at one end.  For such a short run, I doubt that shielding is
the issue, though, unless you are dealing with low voltage digital transmissions
at high data rates.
2) The cable length doesn't do anything for you.  All it does is increase the
voltage drop over the cable (not significant over 10 feet, unless you are
dealing with very low voltage signals - check the equipment specs - sometimes
maximum cable runs are specified).
3) Coiling a cable can actually work against you, as by doing this you
essentially create an inductor, which is a first order lowpass filter.  Again, I
doubt that a ten foot run of cable is significant, but an inductor will
attenuate high frequency signal content.
4) Do not ground to the hull.  This is almost certain to create ground loop
issues.  I presume that your electrical system negative (common) is not tied to
the hull?  When you have shielded cables, tie the shield to ground or electrical
 negative at the device - at one end only of the cable.  The hull may be used as
a safety / chassis ground for components, but should not be considered a source
of constant potential to tie electrical commons to, including cable shields.

-Sean

PS Take everything I write as speculation - my background is mechanical, not
electrical.


Quoting "Smyth, Alec" <Alec.Smyth@compuware.com>:

> OK, I just went out to the sub and took a look. The cable has 9 conductors
> and the penetrator is a 10 conductor, so I do have one available for a
> shield. Currently I've got a foot of standard wires coming out on the inside,
> wired to the cable that came with the sonar. That cable is coiled up, so I'm
> not sure the exact length, but would guess I probably have 10 feet inside and
> about the same outside. It would be hard if not impossible, for space
> reasons, to solder the original sonar cable to the penetrator directly,
> although it would be possible to greatly shorten the leads. The run from
> penetrator to instrument could be about 5 feet if I cut the cable. There
> appears to be no grounding of the original cable at either the transducer or
> instrument end.
>  
> What if I find external braid shielding, wrap the whole inboard cable assmbly
> in that (including the one foot of plain wires), and ground to the hull?  
>  
>  
> thanks
> 
> Alec





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