Re: Signal Generator Source Impedance

In article <pMlnTlGzzQMmFwm9@brattleho.plus.com>,
ianREMOVETHISjackson@g3ohx.co.uk says...

 

A much bigger problem is that 50 and 75 ohm BNC connectors use
different center pin sizes.
<https://www.vintage-radio.net/forum/attachment.php?attachmentid=30120&d
=1258015846>
I wrecked the 75 ohm receptacle on a CATV sweep generator by force
fitting a 50 ohm BNC plug.  Color coding my cables have largely
prevent a recurrence.  Green tape or nail polish for 50 ohms and
violet for 75 ohms.
>

Despite over 40 years in the CATV industry, I've never seen any 50 or 75
ohm BNCs that weren't mechanically mateable. The mating parts of the
metalwork are usually pretty-well identical.
 
OK, with some 50 versions the point of the male pin might be just a
little bit blunter, but there's little chance of it splaying the female
receptacle. Otherwise, the diameter of the pin is the same. But note
that with the 75 ohm the hole at the rear end of the pin and receptacle
might be a rather tight fit for the inner of 50 ohm coax.
 
What makes one connector 50 ohms and the other 75 is that the amount of
the PTFE dielectric is considerably cut back in the 75 (making it as
air-spaced as possible). IIRC, the 50 is the more constant-impedance,
which makes it more-usable to higher frequencies).
 
On the other hand, the N-connector is a very different beast. The pin of
the 50 ohm has a much greater diameter than the 75 (which may be what
the photo shows). If you insert a 50 ohm male into a 75 ohm female, you
will almost certainly cause irreparable damage to the female (in some
companies I believe it's still a hanging offence), while a 75 male into
a 50 ohm female won't male contact.
 
Finally, the inner parts of a female BNC and an N are very similar, and
you can usually mate either with a 75 ohm N (but not with a 50, as the
pin diameter is too large).

>

 
 

That is the way I have seen it in the US.  Ok to mate the 50 and 75 ohm
BNCs but not the 50 and 75 ohm N connectors.

I do not recall which way it is if you stick a BNC to a N connector.
Just that it can be done if the pin size matches up.