Look at this 50 ohm terminator. This isn't right is it?

[OilsFan]

I bought this 50 ohm terminator today and noticed it looked awfully weird when I got home. Is this thing missing the internal insulation piece and center conductor?

[kise]

ohh, yes that is broken...
sad... ;(

[dentaku]

Is that an old MPC in the background of the photo?

[Richard Crowley]

Yes, the center pin and the insulator that holds it are both missing here.

Now if it were a cheap "F-connector" terminator, you might see the bare lead of the resistor used a the center "pin".
But certainly not for a proper connector like BNC or N, etc.

[OilsFan]

Is that an old MPC in the background of the photo?

Yep. MPC-2000.   

I guess I'll have to go back by the place and exchange the terminator for a good one...

[SL4P]

It also looks like it may have been a 75-ohm termination.

From my experience - 'true' 75-ohm BNC do NOT have the nylon sleeve extending forward into the barrel of the connector body, usually seen around the outer edge of the conductor (it stops flat back at the rear of the male body).

50-ohm connectors - often used incorrectly in crap video leads - DO have the nylon sleeve extending forward.

[Richard Crowley]

From my experience - 'true' 75-ohm BNC do NOT have the nylon sleeve extending forward into the barrel of the connector body, usually seen around the outer edge of the conductor (it stops flat back at the rear of the male body).

Right. I am careful to use true 75 ohm BNC hardware for my HD-SDI digital video cameras.

Note that the 75 ohm connector has the same center pin as the 50 ohm version, however.

The photo of the terminator in question appears to show not only the bare wire from the resistor, but also the resistor body itself (turquoise blue color). So the dielectric insulation is missing, also

[Electro Fan]

It also looks like it may have been a 75-ohm termination.

From my experience - 'true' 75-ohm BNC do NOT have the nylon sleeve extending forward into the barrel of the connector body, usually seen around the outer edge of the conductor (it stops flat back at the rear of the male body).

50-ohm connectors - often used incorrectly in crap video leads - DO have the nylon sleeve extending forward.

Don't know if Tektronix 012-0117-00 cables and connectors are "crappy" or not but they seem to have the nylon sleeve extending forward into the barrel of the connector body.

[miguelvp]

That's why I prefer mine color coded.

Purple 75 Ohms, Green 50 Ohms.

[Richard Crowley]

Don't know if Tektronix 012-0117-00 cables and connectors are "crappy" or not but they seem to have the nylon sleeve extending forward into the barrel of the connector body.

Yes, that is a normal 50-ohm BNC connector.  Exactly what we would expect to see on a 50-ohm cable.

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Tektronix didn't make much that could be properly characterized as "crap".

[Electro Fan]

Don't know if Tektronix 012-0117-00 cables and connectors are "crappy" or not but they seem to have the nylon sleeve extending forward into the barrel of the connector body.

Yes, that is a normal 50-ohm BNC connector.  Exactly what we would expect to see on a 50-ohm cable.

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Tektronix didn't make much that could be properly characterized as "crap".

This is what I was thinking  (didn't seem like if Tektronix put the nylon sleeve extending forward into the barrel that this attribute alone could cause a connector or cable to be considered crap).  Personally, I am a huge Tek fan.

[SL4P]

ElectroFan, I think you misread my original quote.

Crappy 75-ohm video cables are often sold with 50-ohm connectors (not the correct 75-ohm variant).

That doesn't make the specific cable or connector crappy - just the manufacturer that decided to mismatch the components - which causes termination, signal reflection, and standing wave problems at higher frequencies.

Cheers

[Richard Crowley]

Back in the analog age of baseband video, the mismatch caused by a 50-ohm connector on a 75-ohm cable was minimal to inconsequential.
But now in the digital age of HD-SDI digital video, it is much more critical to use proper 75-ohm connectors.
That is my first-hand experience dealing with analog and now digital video and RF for several decades.

[SL4P]

Richard  - Sounds like we come from the same workspace!

[tggzzz]

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Just so.

One of the longest and most expensive coax cables in history had an impedance of 61.8ohms, because practically that was the only possible value. I really wish I had kept a small section of the cable.

I worked for the company that made the 61.8ohm to 50ohm connectors

[SeanB]

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Just so.

One of the longest and most expensive coax cables in history had an impedance of 61.8ohms, because practically that was the only possible value. I really wish I had kept a small section of the cable.

I worked for the company that made the 61.8ohm to 50ohm connectors

OK, more info on this submarine cable then.

[tggzzz]

There is nothing "crap" about 50-ohm cable. Any more than about coaxial cable of any other impedance.
The very notion seems preposterous.

Just so.

One of the longest and most expensive coax cables in history had an impedance of 61.8ohms, because practically that was the only possible value. I really wish I had kept a small section of the cable.

I worked for the company that made the 61.8ohm to 50ohm connectors

OK, more info on this submarine cable then.

Thought you'd never ask http://en.wikipedia.org/wiki/TAT-7

Reason the impedance was fixed:

• polythene dielectric => fixed epsilon-sub-r, 2.25
• steel core strong enough to support cable on the 4km trip to the sea bed => fixed core diameter, ~10mm from memory
• cable vessel's hold has a certain volume, sufficient cable length required => fixed outer diameter, ~50mm from memory

There are no degrees of freedom left, and 61.8 ohms is what you get. (those figures give 64.4ohms, so they are in the right ballpark)

[SeanB]

ATT Techtrack on YT had a little series on those coax cables recently. It was very interesting on how they were made and shipped, along with the repeater design as well.

[Electro Fan]

ElectroFan, I think you misread my original quote.

Crappy 75-ohm video cables are often sold with 50-ohm connectors (not the correct 75-ohm variant).

That doesn't make the specific cable or connector crappy - just the manufacturer that decided to mismatch the components - which causes termination, signal reflection, and standing wave problems at higher frequencies.

Cheers

Hi SL4P, sorry - I should have been clearer in my post - I was just trying to show that a high quality provider like Tektronix used the particular cable/connector assembly technique under discussion without getting into a debate about what constitutes "crappy" (I tried to offset the word "crappy" with quote marks to help distinguish/differentiate it from Tektronix but I could have done a much better job of that.) Sorry again.  EF

[krivx]

ElectroFan, I think you misread my original quote.

Crappy 75-ohm video cables are often sold with 50-ohm connectors (not the correct 75-ohm variant).

That doesn't make the specific cable or connector crappy - just the manufacturer that decided to mismatch the components - which causes termination, signal reflection, and standing wave problems at higher frequencies.

Cheers

Hi SL4P, sorry - I should have been clearer in my post - I was just trying to show that a high quality provider like Tektronix used the particular cable/connector assembly technique under discussion.  EF

The use that assembly for a 50 ohm cable. Not 75.

[miguelvp]

I haven't had the need to get new RG-58 cables (50 Ohm) since I do use network cables for that purpose.

Is there any difference between using RG-58 intended for networking or RG-59 if you need a 75 Ohm cable but intended for video, equipment instead of using what the scope manufacturers supply?
For example using RG-58 instead of RG-142, a bit more attenuation and single instead of double braided, but does it matter that much?

I've only used monoprice for HDMI cables so far, but I do like the quality of their products so I bet they are a good source for competitive priced 50 and 75 ohms cables:
http://www.monoprice.com/Category?c_id=102&cp_id=10221&cs_id=1022101
http://www.monoprice.com/Category?c_id=102&cp_id=10203&cs_id=1020307

[Richard Crowley]

Baseband analog video tends to use 75-ohm as does digital video.
EDIT: And consumer RF (i.e. cable and satellite and antenna TV) uses 75-ohm.
Professional/scientific RF and the old 10Base2 coax ethernet used 50-ohm cable.

Actually, for very short jumper cables at baseband signals, there wasn't amy real problem wtih using 50-ohm coax for 75-ohm connections.
And back in the analog baseband video days, it was very common to see 50-ohm BNC connectors used everywhere, even for 75-ohm cable.
IME, it has only been in the era of HD digital signals (3G-SDI, etc.) that we are more conscientious about using 75-ohm BNC connectors.

[IconicPCB]

There is the questionof mechanical compatibility of 50  and 75 ohm hardware.

Precision connectors used to get made less precise if mated with incorrect impedance counterparts.

[Richard Crowley]

IME, 50-ohm and 75-ohm BNC connectors are mechanically interchangable.
I had been using (50-ohm) BNC connectors for several years before I even knew they were impedance-specific.
I had never even seen a 75-ohm variety BNC connector.

The only difference appears to be the dielectric plastic between the inner pin and the outer shell.
The 50-ohm variety appear to have the inner cavity nearly completely occupied by the white plastic dielectric.
While the 75-ohm variety appear to have almost no dielectric insulation in the "working cavity" of either the male or female BNC gender.

[IconicPCB]

They are almost compatible.

The centre conductor pin diameters are different and from memory the 50 Ohm pin is likely to damage the 75 ohm female part of the connector and deform it beyond 75 ohm spec.