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Measure Cable TV Signal Level — Parallax Forums

Measure Cable TV Signal Level

MikeKMikeK Posts: 118
edited 2009-02-26 02:38 in General Discussion
A colleague of mine has got a rats nest of RG-58 cable & splitters in his house (installed before he moved in). He's got cable TV, and he's having signal level problems at some, but not all, of the outlets. He's not sure if the signal levels are too high or too low. Is there a reasonable (low) cost way of measuring the signals, even if it's not especially accurate? Much of the wiring is behind walls, so tracing the wires by hand is out of the question.

Thanks,
Mike

Comments

  • LeonLeon Posts: 7,620
    edited 2009-02-18 15:34
    I'd use a scope.

    Leon

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  • Carl HayesCarl Hayes Posts: 841
    edited 2009-02-18 16:22
    The signal levels won't be too high -- doesn't happen.· If they're too low, he'll get noise (snow) on the screen.

    More likely, I think, is that he's getting ghosting on the screen because RG-58 isn't the right kind of cable for most video systems.· RG-58 is 50 ohm cable, while the standard for video systems is 75 ohms (RG-59 or RG-6, usually), and most devices like monitors, VCRs, etc. are designed to present a 75 ohm termination.· An impedance·mismatch will cause signal reflection at every juncture. which can in turn cause ghosting, sync problems, and other annoyances.

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  • LilDiLilDi Posts: 229
    edited 2009-02-18 17:08
    Running a video carrier signals thru more than two or three splitters will cause ghosting, color shift, signal attenuation, and other problems as well, but Carl is on the main problem here. Also if you are using RG-58 cable and RG-59 splitters, well... thats a problem too.
  • MikeKMikeK Posts: 118
    edited 2009-02-19 01:44
    Thank you all. The information was helpful. I passed it on to my colleague. I also learned some things.

    Mike
  • Craig EidCraig Eid Posts: 106
    edited 2009-02-19 02:13
    The typical low cost troubleshooting tools use a low frequency modulated carrier signal.

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  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-02-19 04:16
    MikeK,

    Unless your cable is analog (unlikely these days), there really isn't a fair comparison against the 64-QAM, 128-QAM, or 256-QAM digital signal that is going to be a "cheap" solution.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Craig EidCraig Eid Posts: 106
    edited 2009-02-19 06:20
    Are you certain the your friend has RG-58 cable instead of RG-59 or RG-6? You can measure the OD of the cable to confirm the type of cable using the OD's listed below.

    Cable······ Imped· Max Oper··· O.D.·········································
    Type······· (Ohms)· Volts····· Inches················
    RG-6A/U··· ·· 75··· 2,700····· 0.332·····
    RG-58C/U···· 50··· 1,900·· ·· 0.195·····
    RG-59B/U···· 75··· 2,300····· 0.242··

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    Craig Eid

    www.TriadRD.com

    Post Edited (Craig Eid) : 2/19/2009 6:49:45 AM GMT
  • LeonLeon Posts: 7,620
    edited 2009-02-19 11:42
    The cable type is usually written on the outer jacket at frequent intervals. Cheap TV coax is an exception, but it's coloured brown so confusion is unlikely.

    Leon

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  • Oldbitcollector (Jeff)Oldbitcollector (Jeff) Posts: 8,091
    edited 2009-02-19 15:46
    Thanks guys for bringing back some really bad memories of RG-58 bus networks. [noparse]:)[/noparse]
    {commonly used back in the early '90's}


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  • awesomeduckawesomeduck Posts: 87
    edited 2009-02-19 18:48
    Here's a quick hack. Plug your cable modem (assuming you don't use DSL) into the outlet in each room. Hook up your laptop directly to the cable modem and web browse to http://192.168.100.1 There you will find your tx and rx signal levels, at least for the frequencies your cable modem is using...which should be a good cheap sampling. This works on most, but not all cable modems.
  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2009-02-19 20:17
    awesomeduck,

    That's a cool way to do it... even better if your cable modem also serves as an IP phone (<- just more information).

    Realize though that with "192.168.100.1" you are talking directly to your cable modem, and not some website. If you come across a cable modem that does not work, then look on the box for an alternative IP address to use.

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    Beau Schwabe

    IC Layout Engineer
    Parallax, Inc.
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-02-20 07:20
    awesomeduck said...
    Here's a quick hack. Plug your cable modem (assuming you don't use DSL) into the outlet in each room. Hook up your laptop directly to the cable modem and web browse to http://192.168.100.1 There you will find your tx and rx signal levels, at least for the frequencies your cable modem is using...which should be a good cheap sampling. This works on most, but not all cable modems.
    Doesn't work with my Linksys cable modem.· I get an HTML page that tells me about the modem, but nothing about signal levels.

    Cool, though.· I didn't know cable modems had a IP address at all on the LAN side, much less what that address might be.

    Anyone know whether it works with D-Link cable modems?· Errr -- not meaning to hijack the thread.

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  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2009-02-20 07:44
    Carl,

    Try this address on your LinkSys modem: 192.168.100.1/RgConnect.asp. It's accessible from the modem's home page via the "Connection" link.

    -Phil
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-02-20 17:56
    Phil Pilgrim (PhiPi) said...
    Carl,

    Try this address on your LinkSys modem: 192.168.100.1/RgConnect.asp. It's accessible from the modem's home page via the "Connection" link.

    -Phil
    Hey, Phil·-- that works!· Many thanks.· I had no idea.

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    · -- Carl, nn5i@arrl.net
  • steve_bsteve_b Posts: 1,563
    edited 2009-02-26 02:08
    You could always just move a tv around hehe!! hope you got a strong back!

    Keep an eye out for coax "taps". They work the same as a splitter...except that a splitter takes an input and makes 2 outputs at 3dB down.
    A 4way splitter does worse....1 input and 3 outputs...1 output is 3dB down and the other two outputs are 6dB down...so even worse.
    If you can find taps, you have an inline loss with a tap off the side that is usually only 1dB down, even though the in to out loss may be 20dB (used in setting up distribution systems).

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  • CannibalRoboticsCannibalRobotics Posts: 535
    edited 2009-02-26 02:38
    Disconnect everything not necessary - use continuity tester to make sure anything not used is disconnected. Pick out the shiniest newest looking splitters.
    On the cables you still need, start changing connectors until signal gets better. (When has the cable guy ever showed up to your house without changing the cables or connectors?) There is a calculable length-loss in the cable but "controllable" RF signal strength over cable is ultimately a function of the mechanical connection quality. Unless the cables have been nicked, cut or moisture compromised, they are probably predictable in terms of loss and there is nothing you can do about it. Rule of thumb says a "good" connector is a 1dB hit on signal strength in the math. Few connectors have a rated life span of more than five years.

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