19 Jan
2011
19 Jan
'11
4:07 a.m.
On Tuesday, January 18, 2011 09:40:50 pm fons(a)kokkinizita.net did opine:
Hello Fons ;-)
And as long as its not what we here in the US call a 'Cambridge' connector,
which is a solderless install, you special strip the coax and literally
screw the connector onto the end of the cable. But its a time bomb whose
fuse could be as little as a couple of months long when install on a soft
foam cored cable. The cable cold flows and collapses away from the
connector shell and you have a very poor shield connection and crosstalk
you can see at the other end of the cable when there are 40 such cables
bound in a truck going 10 meters to another rack.
As long as the cable lengths were matched, shrug.
On Tue, Jan 18, 2011 at 10:59:59PM +0100, J�rn
Nettingsmeier wrote:
> * source
> * 5m or so of suitable cable
> * bnc wall socket
> * 20m installation cable (-63dB/100m @ 3ghz)
> * a ghielmetti patchbay (which includes two canare contacts to the
> patch cord and two bnc on the rear, unfortunately)
> * another 20m installation cable
> * bnc wall socket
> * another 5m cable
> * sink
Which is several reflections and a lot of vswr in a 75 ohm video circuit.
Why? Unless special ordered, the bnc connector you can by in 20 pound
bags, is a 50 ohm connector, and every one encountered in a 75 ohm video
circuit sticks up like a pocket combs teeth on a TDR.
The down side is that while there are 75 ohm bnc available, there is no
provision to prevent someone from plugged a 50 ohm connector into that 75
ohm socket. The 50 pin is too big for the 75 ohm socket and breaks the
female socket, expanding it so that a proper 75 ohm connector will likely
not even make a connection when its locked on. Here in the states, after
encountering lots of that sort of damaged sockets, I did find a src of 75
ohm bnc's but then had to go round up the right sized numbered drill bit
and a pin vice to spin it with, and drill out the back end of the pin in
order to get my fav 75 ohm video cables center conductor into the pin for
crimping.
Needless to say that makes for about 4x the time in installing the
connector.
My fav cable is Susan Clarks (Clark Wire & Cable) type 7559, its quad
shielded with both a silver plated 90% or so braid over a metalized mylar
wrap on the foam core for s shielding effectiveness of about 110 db, and
with its foam core, its both low loss and very very flexible.
question 1:
any hopes for reliable hd-sdi?
Can't really answer this. But BNC seems to be the standard
connector for this, so it's probably OK. question 2:
how can it be that a kick-ass company like ghielmetti
does not offer video patchbays that allow direct connection of coax
installation cables, but require rear bnc connections instead?
This is probably just a question of being practical. Wiring up
a patch bay to things outside the rack (or the equipment it is
part of) without connectors leaves you with something that is more
or less cast in stone. It can also be quite difficult to install.
I recently wired up a 96 point audio patchbay without rear
connectors (i.e. soldered), with almost all of the connections
going outside. Luckily these cables had the other end free and
didn't go into some wall. But I'll avoid to do that again...
I doing another one these days (the Sala dei Concerti - you know
the system needs some rewiring :-) but this time the patchbay has
rear connectors - it would be near impossible get it wired up
otherwise.
question 2b: is there an alternative for direct
rear coax
connection, thereby cutting out two potentially disruptive contact
surfaces?
Haven't seen it. When I was working at Alcatel, all HF racks we
constructed had back panels with double through-panel BNCs for
external connections. No external cable would ever be attached
directly to any of the equipment inside the rack. And more often
than not, the same through-panel BNCs were used on the actual
equipment boxes themselves, with an internal BNC-SMA cable to
connect to the PCBs. So any signal going in or out would pass
via at least 4 BNCs, and 4 again if routed to another rack.
I often worried about this (since we mostly did equipment doing
very precise phase measurements etc.), but the HF 'old hands'
told me each time that I shouldn't. For really critical stuff at 2 GHz and above they
would prefer
SMA, and if things were really extreme the coax cables would
be replaced by rigid tubes, welded to the SMA connectors using
special HF induction welding equipment. SMA handles much higher
frequencies than BNC, but I don't think it matters for SDI.
The secret is achieving a 'gas-tight' crimp. Not having the correct
crimping tool to do that leaves you with a time bomb with a years long
fuse.
question 3: i'm thinking of getting neutrik
isolated bnc connectors
(the d-type ones that are semi-recessed and thus well protected from
clumsy passers-by). but their soldering lugs break the coaxial
structure - cause for concern?
That could well be cause for concern. Is there really any advantage
to recessed BNCs ? The connector itself is usually solid enough,
and once a cable is attached to it the Neutriks are as vulnerable
as any other.
question 4: do i really want to solder hf stuff
(even though the
voltages are not too high), or will it unsolder itself eventually?
any recommendations as to procedures and tin?
Unsolder itself ?
Most BNCs today are crimped rather than soldered. If done using
the correct tools and procedure this should actually be better. Ciao,
--
Cheers, Gene
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
She often gave herself very good advice (though she very seldom followed
it).
-- Lewis Carroll