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attenuator for cable tv.

Discussion in 'Discussions, Opinions & Editorials' started by KD8GFC, Apr 21, 2012.

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  1. KD8GFC

    KD8GFC Ham Member

    I know this is not ham radio related but there are some smart people lurking on the Zed. Does anyone know were I can buy a Forward Pass attenuator for cable tv ?? I need one that will weaken the downstream but let the upstream pass untouched. I have found standard attenuators but not the forward pass type any help would be great. Thanks
  2. KA9JLM

    KA9JLM Platinum Subscriber

    I like the Tru-spec for something that works good for a good price.

    These are Return loss 20dB typical.

    When you say Forward Pass, are you meaning directional or DC Pass ?

    You may be wanting a Directional coupler / Tap.

    Last edited: Apr 21, 2012
  3. KD8GFC

    KD8GFC Ham Member

    What I need is to lower my downstream modem signal without making my upstream signal go higher. I would need a attenuator that only affects the incoming signal and not affect the outgoing or return signal. Thanks for your reply.
  4. KA9JLM

    KA9JLM Platinum Subscriber

    The pads for doing that are built into the line amps.

    Sounds like the cable company does not have their amps equalized properly. Or maybe you are doing something special ?

    If you know the Up / Down frequency of your system then you could build something.
  5. KD8GFC

    KD8GFC Ham Member

    They dont have it balanced right and getting a tech dont seem to do me much good. My down freq is in the 500 mhz band and my up freq is 29.600 mhz and 27.900 mhz. Thats why I thought I could get a attenuator thats lowers signals above 50 mhz but let 50 mhz and below pass. As you can see my internet workes ok but my modem signal stats are borderline high on the down if they change much more i will have drop outs.
  6. KA9JLM

    KA9JLM Platinum Subscriber

    You may be able to use a piece of cheap coax and let the frequency Roll off work for that purpose.

    Or build a simple low pass filter, that will roll off the highs to meet your requirements.

    Do you have a way to measure the signal levels ?
    Last edited: Apr 21, 2012
  7. KD8GFC

    KD8GFC Ham Member

    Sure do right from the modem itself my down is 9.1 dbmv snr is 40, up is 46 dbmv snr 32 the up is perfect but the down should be from -5 to +8 max dbmv all the snr's are good.
  8. KD8GFC

    KD8GFC Ham Member

    Here are the exact numbers from the Modem...

    Downstream Bonded Channels
    Channel Lock Status Modulation Channel ID Frequency Power SNR Correctables Uncorrectables
    1 Locked QAM256 8 597000000 Hz 9.1 dBmV 40.3 dBmV 0 0
    2 Locked QAM256 1 555000000 Hz 8.6 dBmV 39.8 dBmV 0 0
    3 Locked QAM256 2 561000000 Hz 9.0 dBmV 40.3 dBmV 0 0
    4 Locked QAM256 3 567000000 Hz 9.0 dBmV 40.3 dBmV 0 0
    5 Locked QAM256 4 573000000 Hz 8.8 dBmV 40.8 dBmV 0 0
    6 Locked QAM256 5 579000000 Hz 9.2 dBmV 40.8 dBmV 0 0
    7 Locked QAM256 6 585000000 Hz 9.0 dBmV 40.8 dBmV 0 0
    8 Locked QAM256 7 591000000 Hz 8.9 dBmV 40.9 dBmV 0 0

    Total Correctables Total Uncorrectables
    0 0

    Upstream Bonded Channels
    Channel Lock Status US Channel Type Channel ID Symbol Rate Frequency Power
    1 Locked ATDMA 8 5120 Ksym/sec 27900000 Hz 45.8 dBmV
  9. KO6WB

    KO6WB Premium Subscriber

    The lowpass filters to eliminate TVI from amateur transmitters does exactly that. The values may need to be played with and of course you can make it much smaller. Look in some of the older Handbooks that have different impedance.
    Good luck
  10. KA9JLM

    KA9JLM Platinum Subscriber

    It sounds like you are within target according to this;

    Downstream Received Power: the DOCSIS specification requires cable modems to function correctly with downstream power levels in the range -15 dBmV to +15 dBmV: power readings at or close to those extremes are likely to be unacceptable. However, when connected to a real-life CATV network, a cable modem might be functional over a narrower range than this. Accordingly, cable ISPs will specify an even narrower target range when commissioning their network: this target range will differ according to ISP.
  11. KD8GFC

    KD8GFC Ham Member

    Not sure thats up to date with the newer Docsis 3 specs. Comcast here says 8.0 is border line for docsis 3.
  12. KA9JLM

    KA9JLM Platinum Subscriber

    You could be correct.

    If that it what they say, and you are paying for it, then I would think that they should make it meet their own specifications.

    It looks like you had no errors, does it have a lot of errors at times, or what is the problem that you are having ?
  13. KD8GFC

    KD8GFC Ham Member

    I do get errors from time to time. The thing is Comcast wont do NOTHING till there is a major problem that I can see coming in the near future. I already know how to solve this issue if I could find the above mentioned anntenuator I know they make them just cant find were to purchase it. Thats why I asked here on the Zed if anybody knows were to get them. Thanks for all the replys.
  14. KA9JLM

    KA9JLM Platinum Subscriber

    I am sure you can buy them, custom made. There are many companies that can make whatever you want.

    But I would just make one for much cheaper. Or have your ISP provide one.

    There may be a software tweak in the modem where you can adj the level also.

    Good luck on your project.

  15. KA9JLM

    KA9JLM Platinum Subscriber

    Yes, they are easy to build for 75 ohm cable , with only low level RF to deal with.

    I think I would build it to strip off all of the cable TV channels also.
    Last edited: Apr 22, 2012
  16. KA9JLM

    KA9JLM Platinum Subscriber

    RF Tilt Attenuators

    Here is a place that has them.


    The LE7-750 RV Series are the one that you would want.

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