Author Topic: 50 ohms, BNC, coax, fuction generators, and termination of a signal.  (Read 6205 times)

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Offline george graves

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I'm a little lost on all of it.....

 - Why would test equipment (like scopes) have an input impedance of 50 ohms?   When a DMM might have an impedance of 10M ohms or more?
 - I'm having a hard time wrapping my mind around signal termination. I assume part of the puzzle I'm missing is it's not resistance, but high frequency impedance? 
 - Also - What's the difference between a 75ohm video cable, and 50 ohm cable used for test gear?  When is a wire, not just a wire?  How can two pieces of copper have 50 ohms?
 - and - how can I "T" a signal, so I can measure it on two devices at once? - Yet still terminate it properly so that I'm seeing what is really there.
 - How does function generators play a role in this.....and how do I set up a function generator to mimic a real world signal?

All of this see like a "trap for young players" - go slow - cause I R DUMB!

Thanks Dave!
« Last Edit: November 03, 2013, 09:16:39 am by george graves »
 

Offline NANDBlog

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Re: 50 ohms, BNC, coax, fuction generators, and termination of a signal.
« Reply #1 on: November 03, 2013, 09:53:18 am »
Scopes usually have 1 MegaOhm input impedance. And they are deigned to be used with probes, when probing circuits in general.
50 Ohm and 75 Ohm cables have different size pins. Dont mix them. Also, they are not 50 Ohm. Just measure them with a multimeter. Not to mention, the same cable comes in different lengths.
50 Ohm is the Characteristic impedance. It is sqrt ( L / C ), so the capacitive and inductive part  of the cable relates to each other that way.
You can buy T connectors. They have 2 male 1 female connector or something like this.
Function generator in fact could have 50 Ohm output resistance. You don't want to drain infinite current from it, do you? Someone correct me please!
 

Offline DL8RI

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Re: 50 ohms, BNC, coax, fuction generators, and termination of a signal.
« Reply #2 on: November 03, 2013, 10:44:26 am »
The 50/75 Ohm (there are other like 110 Ohm) are the Wave-Impedance, it has nothing to do with the DC-Resistance you can measure with your DMM. In high-frequency-applications the sources and sinks have to be matched for zero reflection. There were some conventions to what impedance a system is matched. Today 50 or 75 Ohms are the most common used systems. Since we talk about wave-propgation we don't have the same voltage over the system, but a dependency on location (the structures are big with respect to the wavelength). A 50Ohm-Cable behaves like a 50 Ohm-Resistance. That means: If the source "looks" into the cable it sees a 50 Ohm sink -> No reflection. If we have a missmatch on the other End of the cable, the reflection will happen there and travel back through the cable to the source. If its matched -> No reflection on the end.

A DMM is normally for low-frequency only, you want minimal current in Voltage-mode, therefore a nearly infinite input-impedance is wanted. The same for scopes. More advanced high-frequency scopes have often a switch to change from 10Meg to 50Ohm.

There is a large variety of good and bad and even worse (and thats BNC :) ) connectors. As NANDBlog said you have to be careful with 50, 75 connectors. They will maybe fit somehow, but you can damage them by interchanging them. And real RF-Connectors are very expensive.
« Last Edit: November 03, 2013, 11:06:54 am by DL8RI »
 

alm

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Re: 50 ohms, BNC, coax, fuction generators, and termination of a signal.
« Reply #3 on: November 03, 2013, 02:41:43 pm »
50 and 75 Ohm BNC connectors have the same center pin diameters, just pick any datasheet and compare. Or ask these guys who might know a thing or two about making BNC connectors. I guess people confuse BNC with N connectors (which do have a different center pin diameter) and keep repeating the myth. The only effect of plugging a 75 Ohm connector in a 50 Ohm jack (or visa versa) is a (usually minor) impedance mismatch.
 

Offline c4757p

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Re: 50 ohms, BNC, coax, fuction generators, and termination of a signal.
« Reply #4 on: November 03, 2013, 02:45:48 pm »
For a bit of an explanation as to what exactly a cable's impedance is, not just that it has to be matched to the load to avoid reflection, w2aew and I tackled that one in this thread, including making an actual measurement of it.
No longer active here - try the IRC channel if you just can't be without me :)
 

Offline NANDBlog

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Re: 50 ohms, BNC, coax, fuction generators, and termination of a signal.
« Reply #5 on: November 04, 2013, 06:55:40 pm »
50 and 75 Ohm BNC connectors have the same center pin diameters, just pick any datasheet and compare. Or ask these guys who might know a thing or two about making BNC connectors. I guess people confuse BNC with N connectors (which do have a different center pin diameter) and keep repeating the myth. The only effect of plugging a 75 Ohm connector in a 50 Ohm jack (or visa versa) is a (usually minor) impedance mismatch.
http://www.milestek.com/blog/index.php/2011/04/50-ohm-vs-75-ohm-bnc-connectors-explained/
No it is actually hard to mix BNC with something very different. The pins are different, you need a different tool, and I had cables and connectors which were "not playing nice together", i had to apply more force than usual to mate them. 5 seconds later I realized why.
 
 

alm

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Re: 50 ohms, BNC, coax, fuction generators, and termination of a signal.
« Reply #6 on: November 04, 2013, 09:07:26 pm »
I don't understand your point. You link to a page that states that "Both [50 and 75 Ohm] connectors can mate without damage", but then talk about mixing BNC with "something very different", whatever that is. Of course you're not going to be able to plug BNC in an N, SMC or TNC jack. All I'm stating is that a 50 Ohm BNC connector and 75 Ohm BNC jack (or visa versa) should mate without damage to either, and the page you link to states the same.
 


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