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Thread: Coherent GEM CO2 laser RF cable lengths.

  1. #31
    Join Date
    Feb 2019
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    2495mm is from center pin to center pin distance right ?

  2. #32
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    Oct 2021
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    Quote Originally Posted by mixedgas View Post
    VSWR sensor is overly sensitive or they are doing something weird with the matching networks.
    Glad to see you have it working.

    Steve
    I did see Coherent documentation that VSWRFault can indicate on the first firing attempt, like for the first second, if the tube has not been used in the last few hours. The gas actually does have difficulty ionizing and the output will be low and unstable. The VSWRFault is indicating ionization probs as intended, but it might not actually be a failure in the tube just a brief, transient initial stall.

  3. #33
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    Oct 2021
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    Quote Originally Posted by macona View Post
    I used these.

    https://www.rfparts.com/r161075040.html

    You will need a crimper to use these connections. There are crimpless types of connectors too, just more expensive.
    Wow, always wondered about this. Hey, what cable type is used with these on a Coherent? I have an orange aftermarket cable marked "E170516-7 CATVP (11AWG)"

    I went looking and found someone with E170516-7 cable, but it say "6AWG". This is strange, I would think a different gauge would be a different part number entirely. This also begs the question, is this 11AWG orange aftermarket stuff necessarily the best choice for the task? The 6 AWG would be thicker, but resistive losses in the copper may be insignificant to begin with. I don't know if the 6AWG would even fit inside the RF crimp ends you list.

    The stock Coherent cable is notably thinner outer diameter than this aftermarket 11 AWG.

  4. #34
    Join Date
    Apr 2012
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    Beaverton, OR
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    I have an orange cable from coherent. LMR400 is what coherent uses, going heavier would do no good. 400 can handle about 2kw at 100MHz

    These power supplies are incredibly sensitive to mismatch. Literally a couple inches can cause a head from firing reliably to not. I had made one that was a couple inches too short and it would not fire reliably and added an elbow and a nipple to route the cable which brought the cable length to where it should be and it runs perfect every time.

    The E numbers are not a good number to use for finding wire, its mostly a identifier. You need the other numbers on the cable, like I mentioned above, LMR-400 is used for the smaller coherent heads, my Rofin uses LMR-600

  5. #35
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    Oct 2021
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    OK, I think you're right! I have an OEM Coherent cable that is thinner with a braided silver metal shield as the outermost layer. I don't see any marking for the coax itself, like you'd normally find printed every foot or whatever to show what type of coax was used to make it, because it's a braided metal jacket and that's not practical to mark.

    I'd gotten it into my head that the orange cable was aftermarket, but no, there was no reason to think that. It's OEM from a different period. It is labeled LMR400 further down, neat!

  6. #36
    Join Date
    Feb 2022
    Location
    Aberdare, Wales, UK
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    3

    Default Can you provide more detail on the procedure

    Quote Originally Posted by macona View Post
    Ok, so using the NanoVNA I put a connector on one end of the cable and started cutting pieces of cable off till I got to what appeared to be a perfect open at 100MHZ on the smith chart. Cut a touch more off to compensate for the center pin and crimped it all up. And it works. Finally. This is using LMR-400-UF which has a stranded copper core and it more flexible than the standard LMR-400. Total length from end of connector to end of connector is 2495mm
    Hi,
    I know this thread is almost a year old, but I am looking to use a NanoVNA to build a cable for a Coherent laser.

    What aspects of the Smith chart were you looking at exactly, the green Marker 1?
    Meaning if the marker is above the horizontal it's too short, or below the horizontal it's too long?



    If this is the case, you then just kept cutting back until the green marker was on the horizontal at 0 degrees?

  7. #37
    Join Date
    Apr 2012
    Location
    Beaverton, OR
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    Yes, the green. Make sure you compensate for any adapters you put on there.

    I dont remember if too long was above or below the horizon. You could add something to the end and see which way it moves it.

  8. #38
    Join Date
    Feb 2022
    Location
    Aberdare, Wales, UK
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    Thanks for confirming, it's appreciated.

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