[time-nuts] BNC Question

[Oz-in-DFW]

On 4/16/2011 2:59 PM, shalimr9 at gmail.com wrote:
> As pointed out earlier (by Bruce and others), there is a vast quantity of 75 ohm BNC connectors which mate perfectly with 50 ohm BNC sockets (save for impedance mismatch). I have a set of 10 cables bought off the *bay with such connectors. These cables are 75 ohm, so there will be a mismatch in a 50 ohm system no matter what the connector is.
No doubt, but I think you'll also find these are the Amphenol
proprietary parts - which will damage the MIL-STD 75 ohm parts that HP
and a lot of other vendors used for a long time, just damaged less
frequently. 
> I am not sure what would make those less *true* that anything else.
Strictly speaking, BNC, TNC, and N are rooted in MIL-STDs from the
fourties, the other stuff intermates and has improvements of varying
nature.  This gets into detailed semantics.  Are the cheapie import
connectors that can't sweep past 100 Mhz Ns or something else? They
intermate, but they don't work so well.
>  I am sure the mismatch of the connectors themselves, within the frequency range they are intended for will be well below the level where most would care.
Right, but the issue was intermating and damaging one type with another
and poor mating reliability.   The fact is that there ARE BNC (and N and
TNC) variants that will have this problem.  There are a number of folks
here who have flatly stated that this is not the case, but it really is. 
> If you try to use BNC for precision RF measurements, be prepared to be surprised.
BNCs can be as good as TNCs when properly applied, but the bayonet
mechanism allows too much mechanical alignment play for reasonable
reliability past a GHz or so.  If they are properly installed and the
cable is not allowed to put a radial or significant tensile load they
perform as well as a TNC and close to an N.  For all that, I won't use
them in designs above 100 Mhz without a compelling reason.  F's are
cheaper, and SMAs are more reliable.  If it's hard coded at 75 ohms and
related to measuremnts - I use Ns.
> Unless the argument is about what they "should" be, what the standard says is irrelevant when you have a box of parts and wonder if you can used them.
I think we're in 'violent agreement' here.  My point is that you need to
be aware of the many variations of connectors that have appeared in the
last 50+ years and use care lest you end up with an undesired repair. 
> Don't assume anything, just check before you plug.
And this is the point.  There *are* connectors that will not reliably
intermate and will be damaged.  This is true in BNC as well as N and TNC. 
> Didier KO4BB
>  

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