[time-nuts] BNC Question

Chuck Harris cfharris at erols.com
Sun Apr 17 00:38:37 UTC 2011


HP made a point of telling users that the 75 ohm female BNC connectors used
on HP equipment were fully and safely mate-able with standard male 50 ohm
parts.  This has been so certainly from the 1970's.  Casual inspection of
the part shows that it uses the identical center pin set as the 50 ohm
parts.

-Chuck Harris

Oz-in-DFW wrote:
>
>
> 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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