[Nich]

Hi all.


I have talked to a lot of people, and most of them say that the

BNC itself isn't 75ohm or anything else, but that it's the cable

that dictates it. Most electronic stores say it.


So what is right?


Nich

 

[matrix]

I know in my Mossier electronics catalog you can order BNC connectors for making up cables and the connectors themselves are different resistance like 50 ohm vs 75 ohm.

 

[jcmccorm]

They have to match the cable. There are 50ohm and 75ohm BNC connectors (that I know of).


Cary

 

[Nich]

Yes, but many people claim that putting a 50ohm BNC on a 75ohm

cable doesn't make a difference.


Nicholas

 

[Jay Wilson]

Hi Nich,

I think this is one of those things that you will always have 2 sides of the fence. With test equipment, there is a difference in a 75ohm cable/75ohm connector and a 75ohm cable/50ohm connector. Whether it's a visually justifiable difference is what will always be argued.


I'm of the opinion that it should always be the same. Since they're available, and can be measured, why not use the matched termination with your cabling?

 

[jcmccorm]

If it's at the end of the cable, source or far end, the effect of the connector impedance is probably negligible. If you've got them in the middle of the cable, because you wanted to add a "T" connector for example, you'll get reflections off the impedance mismatch. If it's video, you'll see ghosting because of it.


Cary

 

[Jay Wilson]

Quote:

Originally posted by jcmccorm If it's at the end of the cable, source or far end, the effect of the connector impedance is probably negligible. If you've got them in the middle of the cable, because you wanted to add a "T" connector for example, you'll get reflections off the impedance mismatch. If it's video, you'll see ghosting because of it. Cary

That's true. My comments were assuming it was just a straight through cable from source to projector.

 

[Nich]

"I think this is one of those things that you will always have 2 sides of the fence. With test equipment, there is a difference in a 75ohm cable/75ohm connector and a 75ohm cable/50ohm connector. Whether it's a visually justifiable difference is what will always be argued"


Please read this article:

http://www.extron.com/technology/arc...tifyingcables1


Nicholas

 

[jcmccorm]

Cool! Someone did all the work. That VGA connector looks nasty! Good thing I have Mike Parker's MP-1 (BNC's all the way...)


Cary

 

[Jay Wilson]

Yep. The article says just that. There is a slight difference on the equipment, but not enough to make a difference!

 

[spearce]

I say "75 ohm BNCs aren't that much higher in cost than 50 ohm BNCs", and "the cable is 75 ohm", so "why not just use 75 ohm everywhere and not worry about it.".


Interesting read on that article from Extron. Goes right back to the old debate on speaker wire: does that $20/ft cable really sound better than the $0.30/ft zip cord from Home Depot?


Given the price difference on being full 75 ohm and not (at least from my supplier here in the states), I just went 75 ohm on everything. But given that article, I can see why many would not see the need to spend the extra few cents per connector.

 

[Nich]

Spearce, yes of course, but I know that if I go to my local electronics

store(very well-known) and ask for 75ohm BNC's they'll just give me what

they have, whatever that might be.


Nicholas

 

[spitz]

While we're on the topic, has anyone used the snap n'seal compression BNCs for video cable? I've got the tools for them and I know in terms of "F" connectors, they are much more reliable. The killer is that they're really expensive.

 

[spearce]

spitz, you mean the crimp on BNC connectors? If so, its about all I've heard of anyone using for video coax.... I love the Canare tool and die and BNC connectors. They are expensive, but make a nice termination in 30 seconds or less.


The important part is to have the right die for the connector you are crimping.

 

[Vic C]

I have used the CABLE PRO compression fittings in fact I used them on my RGBHV cables for my projector. Very reliable and a great looking connector. Yes they are expensive but you do get what you pay for!

 

[BenY]

Let me put it differently,

What difference dose it make if the BNC female receptacle is soldered inside the unit,and the coax cable is stripped and connected in the open???

Is that connection adheres to the 75 ohm rule ???

I`ve seen it inside projectors,switchers,etc...

 

[spearce]

The connection adheres to the 75 ohm rule only if all parts between source and sink (projector) have a nominal 75 ohm resistance.


It doesn't matter how the cable is mated to the BNC female jack. Crimp, solder, push-on, etc. But soldered connections are most common in circuit board work and machine BNC jacks, while crimp are most on common cables.

 

[BenY]

spearce,

Cables and connectors do not have 75 ohm resistance.

Its 75 ohm impedance.

A crimp connection can maintain a true 75 ohm impedance,but solder connection cannot...

Yuval.

 

[spearce]

Doh! Thanks for the correction BenY. Foolish my trying to quickly post a message from work and getting myself mixed up.


Good point about the solder connection, I didn't think about the impedance being different due to the presence of the solder in the connection.


Clearly I am a softie (wimp, computer science) and not an EE.

 

[BenY]

spearce,

The problem is not due to the presence of solder,but due to the fact that the termination is not closed and center conductor left open,that part of the exposed cable presents a variation impedance in to the line,and causes whats called "Return loss" or simply put a "reflection".

Yuval.