75 ohm cable with 50 ohm BNC connectors

[ePOWEReng]

I am wondering if there is a large impact with using a 50 ohm BNC with a 75 ohm cable. We are using this with about a 50kHz signal. Is the dB drop going to be anything to worry about???

Thanks in advance for your help.

 

[VEBill]

The concern shouldn't be about loss, it should be about mismatch and reflections.

But 50 kHz is so low that you're not likely to notice the reflections over any reasonable length of cable.

That's why nobody worries about the characteristic impedance of audio connectors. 50 kHz is nearly audio.

 

[VEBill]

There is also a practical matter to consider. Many BNC connectors are designed for a specific physical size of cable (outside, and the center conductor). In other words, it might be difficult to make the connector parts fit directly onto the wrong size cable.

One way to make RG-59 or RG-6 cable fit onto a BNC is to use the normal F-connectors (easy) and then F-to-BNC adapters (easy).

 

[benta]

I agree with VE1BLL.
You're probably going to have trouble with the center conductor if you are using crimp BNC connectors. It's thinner in 75 ohms and may not attach correctly.
If you are soldering the connectors it's not much of an issue.

 

[zeitghost]

The other thing to remember is that the centre pin of one is a different size to the other. (Think the 50R centre pin is larger diameter).

Plugging a 50R jack into a 75R socket expands the centre receptacle too much & it is unreliable thereafter with 75R jacks.

 

[Noway2]

I agree with Ve1bll as long as you are certain that 50KHz is the bandwidth of interest. If you are talking about a digital signal that is clocked at 50KHz, the needed bandwidth can be a lot higher. If this is a digital signal, the edge rate will determine the bandwidth, which will be significantly higher than 50K. Depending on how the circuit is implemented, this may or may not pose a problem.

 

[zappedagain]

While this article is a bit old, I just saw this interesting article on 50 ohm vs. 75 ohm that looks worthy of a post:

http://www.gigatronix.com/pdf/BNCz50Vsz75.pdf

Here are a couple of tidbits:

"The BNC connector was designed as a 50 ohm connector for military applications and requires a modification to the interface to create a 75 ohm connector. To achieve this the dielectric is either entirely removed or reduced to a very thin wall..."

"The use of 50 ohm connectors on 75 ohm cable with analog
signals has little or no distortion effect at frequencies below 600MHz"