EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: dentaku on June 03, 2017, 02:31:11 am
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I have a question about the impedance of BNC connectors and cable.
My Keysight DSOX1102G scope has a Gen Out port on it but I don't have any cables with BNC connectors. If I wanted to make up (or have someone do it for me) a short cable with BNCs on both ends, what should I look for?
I see lots of BNCs and cable are for 75ohm equipment with things like CCTV cameras.
BUT...
Shouldn't I be looking for 50ohm connectors and cables (RG58?) for my purposes? The Wavegen on this scope can be set to High-Z or 50ohm and the connectors all have the thick dielectric material.
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You should look for 50ohm connectors and 50ohm coax.
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You can certainly buy pre-terminated 50 ohm BNC cables from many places.
If you want to terminate some yourself, here's how (you need the right tools):
https://www.youtube.com/watch?v=ktQVwfo-s9w (https://www.youtube.com/watch?v=ktQVwfo-s9w)
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You can certainly buy pre-terminated 50 ohm BNC cables from many places.
If you want to terminate some yourself, here's how (you need the right tools):
https://www.youtube.com/watch?v=ktQVwfo-s9w (https://www.youtube.com/watch?v=ktQVwfo-s9w)
Ha! I just finished watching your video before checking back if anyone had answered.
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Btw, half the cheap crap cables sold on eBay as 50 ohm are really 75. I have the network analyzer scans to prove it. Look for cables made from rg58 or rg142 or similar.
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The wavegen output impedance does not change when you select 50 ohms or High-Z. It is always 50 ohms. What is changed is the output level to match load impedance of the target device. Use 50 ohm cables and connectors.
I recommend you watch this video: https://www.youtube.com/watch?v=tClE8s6RZdg (https://www.youtube.com/watch?v=tClE8s6RZdg)
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Btw, half the cheap crap cables sold on eBay as 50 ohm are really 75. I have the network analyzer scans to prove it. Look for cables made from rg58 or rg142 or similar.
The miniature coax in soft 174 or firmer 316 are also a good candidates for test cables where you don't mind the higher loss (since the cables are so short).
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The wavegen output impedance does not change when you select 50 ohms or High-Z. It is always 50 ohms. What is changed is the output level to match load impedance of the target device. Use 50 ohm cables and connectors.
Agree.
50 ohms at output wave impedance, this is no load impedance. If you will set output for your generator to 50 ohms, that is mean, that your target device (scope) should have 50 ohms load impedance on it's input.
Since, your scope do not have option for setup it's input at 50 ohms load impedance, than you need BNC feedthrough terminator (that have just 50 ohm resistor inside between central conductor and shield).
So, you don't need setup output of your generator to 50 ohms.
If you want connect to your scope any equipment for measurement signal level with 75 ohms load impedance (any TV equipment), then you just need 75 ohms load impedance feedthrough terminator. With this load at the end of BNC cable you should no worry about right signal levels at your scope input.
As for wave impedance, abstract from dielectric properties of the BNC connector, only one difference between 50 and 75 ohms - that is central pin thiner in 50 ohms BNC.
Just for connecting your Gen output to your scope input you can easy use 75 ohms cable without any problem. I suppose you do not need cable with 200m length :)
Any companies that doing broadcast TV installations use 75 ohms cables/connectors, in satellite equipment 50 ohms more often used, old coax Ethernet network also used 50 ohms.
For your 50 cm cable length and 20 MHz max frequency at the output just don't worry about it. ;)
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You can certainly buy pre-terminated 50 ohm BNC cables from many places.
If you want to terminate some yourself, here's how (you need the right tools):
There's an opportunity for misunderstandings here: When w2aew refers to "termination", he just means installing the plugs, I think. But in the context of impedance-matched connections, "termination" often refers to establishing a defined impedance at the endpoint of a connection. And in fact, if you connect your wavegen output via a 50 Ohm BNC cable to a high-impedance input (e.g. on your scope), you should also install a 50 Ohm termination resistor.
Back to the original question: A clear vote for buying pre-made cables with the BNC connectors already installed. Installing the connectors yourself requires a bit of practice.
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And in fact, if you connect your wavegen output via a 50 Ohm BNC cable to a high-impedance input (e.g. on your scope), you should also install a 50 Ohm termination resistor.
Probably yes..
Just discovered DSOX1102G datasheet (my scope A, not G model), the wave gen output have 50 ohms impedance.
And 50 ohm feedthrough terminator (http://www.ebay.com/itm/1pc-Feed-Through-Termination-PL-50-50-0-5-DC-1GHz-2W-PINTEK-/140955427128?hash=item20d1992138:g:tlgAAOxyZw5Rak0z) will required for connecting output to the input.
But again, I do not think that need much worry is coax cable itself and BNCs on it 75 or 50 omhs wave impedance.
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You can certainly buy pre-terminated 50 ohm BNC cables from many places.
If you want to terminate some yourself, here's how (you need the right tools):
There's an opportunity for misunderstandings here: When w2aew refers to "termination", he just means installing the plugs, I think. But in the context of impedance-matched connections, "termination" often refers to establishing a defined impedance at the endpoint of a connection. And in fact, if you connect your wavegen output via a 50 Ohm BNC cable to a high-impedance input (e.g. on your scope), you should also install a 50 Ohm termination resistor.
Back to the original question: A clear vote for buying pre-made cables with the BNC connectors already installed. Installing the connectors yourself requires a bit of practice.
Thank you for that! Experienced folks will intuitively understand the difference, but I certainly can see that my poor choice of words, or lack of explanation, can lead to confusion between terminating a bare cable (installing a connector), and terminating a transmission line (impedance matching).
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Would there be any reason, considering I'm using very short cables, to avoid certain types of 50ohm coax that are smaller and even more flexible than RG58 or RG142B?
I'm talking about premade cables too. I don't have any crimping tools.