Vector network analysis question

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

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

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.

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.

Regards,

Mike Monett

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

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

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