Vector network analysis question

[Mike Monett]

For sure. It looks like it's not safe to mate a 75 ohm connector with
anything, even another 75 ohm connector, without checking pin diameter.

Roy Lewallen, W7EL

Or even 50 ohms, for that matter. But how do you check the pin diameter on
an existing connector? A caliper won't go in there. Also, the diameter
could be correct but the pin could be assembled incorrectly and stick out
too far. This will damage the female pin.

Or maybe you know of some other non-destructive method to tell if you can
mate an existing male and female coax connector without damage?

The vulnerable part is the female pin. Once those legs get splayed there's
no way to put them back. All it takes is one event to effectively ruin a
brand-new scope.

Regards,

Mike Monett

 

[Roy Lewallen]

Or even 50 ohms, for that matter. But how do you check the pin diameter on
an existing connector? A caliper won't go in there. Also, the diameter
could be correct but the pin could be assembled incorrectly and stick out
too far. This will damage the female pin.

Or maybe you know of some other non-destructive method to tell if you can
mate an existing male and female coax connector without damage?

The vulnerable part is the female pin. Once those legs get splayed there's
no way to put them back. All it takes is one event to effectively ruin a
brand-new scope.

Well, a scope will have a 50 ohm female connector, which should tolerate
any kind of male without damage. The only vulnerable connector should be
a 75 ohm female connector of the sort which has a smaller pin diameter.
I don't believe I've ever seen one of those, and you'll certainly never
see one on any commercial equipment except only possibly some
video-related units.

Roy Lewallen, W7EL

 

[Mike Monett]

Well, a scope will have a 50 ohm female connector, which should
tolerate any kind of male without damage.

That's what I was talking about earlier. I ruined the BNC inputs on
some of my older TEK scopes by using off-brand 50 ohm terminators
that were difficult to install. Apparently the male pin extended too
far into the female and spread the pins. Some cheap commercial coax
cable had the same problem. The scope bnc connectors are special and
had the multiplier contact for 10X probes, and were too difficult
and expensive to get and install.

I would often waste time debugging strange waveforms when it turned
out the problem was in the scope connector. Squeezing the pins
gently would make it work for a while, then it would come right
back.

You often see the problem on used equipment like spectrum analyzers,
sig generators and counters. Evidently someone went in with
needlenose pliers to try to tighten the legs, but instead squashed
them into a square instead of a circle. They don't make reliable
connections after that.

After discussing this, I'm inclined to get a handful of female
connectors to test the male coax hardware. If it ruins the connector
and cannot be fixed, toss it in the garbage where it belongs.

The only vulnerable connector should be a 75 ohm female connector
of the sort which has a smaller pin diameter.

I don't believe I've ever seen one of those, and you'll certainly
never see one on any commercial equipment except only possibly
some video-related units.

That's good - I don't think I've ever used or seen any 75 ohm
hardware, but maybe someone working on TV might run into the
problem.

Roy Lewallen, W7EL

Regards,

Mike Monett

 

[Rene Tschaggelar]

Yes, thanks, Rene. But for something as basic as short, open and
matched terminations, I'd assumed the spot displayed should just hop to
each appropriate place along the horizontal center line. I'm not even
doing any transmission tests at present; this is so far just
reflections only...

Naa. That is exactly what the calibrtion is for.
To make open, short and 50 Ohms at the right spot,
over the bandwidth and amplitude range.

Rene

 

[K7ITM]

We've built some equipment with 75 ohm connectors. In those network
analyzers that use a separate S-parameter test set, you can often get
the test set in a 75 ohm version as well as the more common 50 ohm.
But the 75 ohm units are (as far as I know, _always_) labeled with a
caution about using the right impedance connectors.

Cheers,
Tom

 

[Harold E. Johnson]

We've built some equipment with 75 ohm connectors. In those network
analyzers that use a separate S-parameter test set, you can often get
the test set in a 75 ohm version as well as the more common 50 ohm.
But the 75 ohm units are (as far as I know, _always_) labeled with a
caution about using the right impedance connectors.

Cheers,
Tom

You've built a hell of a lot of it. All the stuff for the Telco's. like my
backup HP-313 that I mod'd back to 50 Ohms.

Regards
W4ZCB

 

[K7ITM]

Well, yeah, I guess we have. In fact, not long ago I was cleaning up a
lab bench and found some of the old Telco style connectors, all gold
plated and pretty. I couldn't bear to toss them, even though the
chances of us ever using them again are somewhere between slim and
none, toward the none end of that scale. I suppose that any
special-impedance market that was large enough, we built for. Of
course the older audio generators (200CD et al) were 600 ohm.

Cheers,
Tom