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Coax filter

Discussion in 'Electronic Design' started by cameo, Feb 6, 2013.

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  1. cameo

    cameo Guest

    Years ago, when Comcast added Internet service to the pre-existing cable
    TV service in my neck of woods, the technician, after running a short
    pigtail coax cable from the wall outlet to a two-way splitter to serve
    both the TV and the cable modem, he also installed a small hex barrel
    shaped filter into the cable leg that runs from the splitter to the TV.
    The filter is about half inch thick and 2 inch long and looks made of
    stainless steel.

    Not being an electronics expert, I never really understood the purpose
    of that filter and I did not notice any difference either im my
    broadband modem or TV performance after I removed it. Can anybody
    enlighten me what that filter is supposed to accomplish? If anything, I
    would have expected something like that in the cable leg that runs from
    the splitter from the modem, not in the TV leg, but what do I know?
    BTW, the following code is stamped on the filter: SNB6-5/136 kV 182, if
    that means anything after all these years.
  2. cameo

    cameo Guest

    I see ... What frequencies are used for the TV and the broadband?
  3. Robert Baer

    Robert Baer Guest

    Methinks it is to reduce interference to the TV that is generated by
    the cable modem.
    That cable modem generates a lot of hash, and interferes with cell
    phones up to at least 5 feet.
  4. cameo

    cameo Guest

    You misread my post, Jeff. It's SNB6-5/136. not SNBR-5/136. But it might
    be too old a code to find any documentation on it now. I think the
    Comcast technician installed it about 15 years ago. There is no color
    code on it, BTW. It's all mice stainless steel shiny. It is also not a
    cylindrical shape as your picture shows, but hexagonal. Otherwise it's
    similar dimensions.
  5. cameo

    cameo Guest

    I only have a ClearQAM HD TV (doing double duty as a PC monitor) and the
    Motorola cable modem operating from that splitter. The TV is connected
    through an STB that only provided SD picture but I can bypass it to get
    the local HD signals of the local stations. They all are powered from
    the same power circuit via power strip.

    I also have an old analog set in another room that gets the TV signal
    through a Comcast DTA since Comcast dropped all analog signals in my
    area and encrypts everything except the local HD broadcasts. I do get
    good reception on both TV sets and good data speeds as well from the
    modem, despite the real old RG-59 cabling. Even with the coax filter
    removed. I get no telephony or other Xfinity services besides the
    broadband and Digital Economy TV package.

  6. What shit brand of cable modem are you referring to?

    The modern Motorola units don't do that. and they fire off at over

    And the TV (band) is inside coax and won't get anything injected into
    it, unless you (or your installer) are an idiot at cutting a proper
    fitting as well.

    The digital (read Internet service side) segment is yet another set of
    cable channels and the digital broadband 'service' is contained within
    standard 6MHz channel slots.

    So, the whole thing is packaged within the standard cable TV channel
    slot on the digital side.

    I thought the filter was for the cable modem, not 'for the TV side' as
    it were. On our system, it is a specific channel notch filter, which is
    what the numbering is about. It is no different than the "channel traps"
    they used to use to deny a customer a certain channel. If they bought
    that channel's subscription, they would come remove the "trap" from your
    taps out on the pole. That is a channel TRAP filter, which passes all
    channels except for the trapped channel.

    This is a channel PASS filter that filters all channels except for the
    digital one, and that branch feeds our cable modem segment.

    In your case, since it is on the cable TV feeder side, I suspect that
    it too is a channel TRAP filter (a narrow notch filter), which traps the
    channel the digital side is on and passes the rest.

    I know even the TV stuff is digital now. I am using the term to refer
    to Internet service in this post.
  7. It is a specifically set device to 'attenuate' a specific channel or
    band of channels. IOW, a notch filter.

    I doubt the feed is so hot they need to cap the leg feeding the TV
    band. But it does appear to be only a 6dB pinch. But that is a guess...
    as well.
  8. cameo

    cameo Guest

    Mi modem is Motorola's SB6121 that I bought myself.
    That's what I expected, too.
    That's how it was here, too, except it was the cable box on the
    street as we don't have poles in my street.
    Well, Comcast here now enforces channel subscription remotely by
    sending signals to their STBs and DTAs. Only the local stations can
    still be received with digital TV sets till they encrypt those, too.
  9. cameo

    cameo Guest

    You skipped that part in my last post, I'm afraid. I described the
    services in detail.
    OK, a picture is worth a thousand words, I guess. So here are two pics
    of my filter in question:
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