Re: [LAD] [OT] 3ghz coax and soldering...

On Tuesday, January 18, 2011 09:40:50 pm fons@kokkinizita.net did opine:

Hello Fons ;-)
 
> 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.

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.

> > 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.

As long as the cable lengths were matched, shrug.
 
> 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.
>
> > 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.
 
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.

> 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
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