DIRECTV LNB Operation

Discussion in 'DIRECTV General Discussion' started by RawbyTheSheeth, Mar 2, 2021.

  1. Mar 2, 2021 #1 of 10
    RawbyTheSheeth

    RawbyTheSheeth New Member

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    Hi All,

    I have a few questions which I've had trouble finding answers for. I don't know much about the specifics of a DIRECTV system's operation, so any links to a decent overview would be nice.

    Unnecessary Background Information:
    I have an SDR that can handle the IF output from the DIRECTV LNBs, and I'm looking to measure received power spectrum from DIRECTV satellites over time to analyze the effects of weather on signal strength. I'm not interested in demodulating the signal or streaming anything, so I don't plan on getting a DIRECTV receiver. I'm looking to purchase a Slimline Dish, LNB, and a power inserter from Amazon to look at the T10/T12 transmitters at position 103. I'm trying to decide if I should just build my own LNB or not at this point.

    The Question:
    Will an LNB and power inserter output the IF without a DIRECTV Receiver present? Is the receiver necessary to pass instructions back to the LNB? I got the impression from somewhere that receivers send commands back to the LNB to select the channels, (at least in the SWM configuration).

    I've been trying to find drawings of the RF downconversion chain in the LNB too to see if the amplifiers implement some kind of gain control or if they're fixed, but I can't find anything. I suppose I should just buy an LNB and check it out myself by sweeping the power in and looking for a linear change in the output.


    If there's a better place to ask this, please point me in that direction, thanks!
     
  2. Mar 2, 2021 #2 of 10
    slice1900

    slice1900 Well-Known Member

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    If you buy a "legacy" (4 output) version it will work if you feed it the appropriate voltage/tone.

    However, if you can't demodulate the signal you won't be able to tell much about the effect of weather. Directv broadbands at ~15 db over the noise floor, so you will have a limited range to determine the effect of weather. Once attenuation/scattering due to weather knocks 15 db off the signal, it will be indistinguishable from the background noise so it won't tell you much. Ku and especially Ka are both hit a lot harder than 15 db during heavy rain.
     
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  3. Mar 2, 2021 #3 of 10
    RawbyTheSheeth

    RawbyTheSheeth New Member

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    Feb 26, 2021
    Alabama
    Yeah, I knew there wouldn't be much room and that I'm not getting any coding gain. I'm also aware that the satellite's transmit power is ambiguous since I'm simply looking at a received signal, so I've limited my expectations more to monitoring shallower fades on a shorter time-scale from effects when it isn't raining, namely tropospheric scintillation which is typically less than 2 dB. I'm hoping that this will also allow me to avoid issues with satellite transmit power variations depending on whether or not the sat's solar panels are in the sun.

    It definitely makes sense to me that the older non-SWM LNB would be best here. When you say "feed it the appropriate voltage/tone" does that imply that the LNB needs something fed in besides just the DC from the power inserter? If so what would that be?
     
  4. Mar 2, 2021 #4 of 10
    harsh

    harsh Beware the Attack Basset

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    Surely there are voltage regulators used in conjunction with the batteries making the presence or absence (or anything in between) of sunlight an utterly moot point.

    You'll also have to figure out how to defeat the AGC in the LNB or the measurements won't have any practical correlation with the satellite's output level.
     
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  5. Mar 3, 2021 #5 of 10
    P Smith

    P Smith Mr. FixAnything

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    very brave unsupported statement ! any proof ?!
     
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  6. Mar 3, 2021 #6 of 10
    RawbyTheSheeth

    RawbyTheSheeth New Member

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    Haha, very true if there's an AGC, I may as well just brassboard my own LNB and go from there. I can't find any documentation saying if there is or isn't one though. I know they're very common in superheterodyning.

    I have fallen into a trap of dealing with proprietary information, and I fear I will know nothing for certain without just buying crap and testing it.
     
  7. Mar 3, 2021 #7 of 10
    P Smith

    P Smith Mr. FixAnything

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    I did post pictures of LNB guts here...
     
  8. Mar 3, 2021 #8 of 10
    slice1900

    slice1900 Well-Known Member

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    AFAIK they don't have AGC in the legacy LNB, the LNAs are fixed gain.
     
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  9. Mar 3, 2021 #9 of 10
    slice1900

    slice1900 Well-Known Member

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    Look up info on DiSEqC. You select between 99/101 and 103, as well as odd and even transponders by feeding either 13v or 18v and either a 22 KHz tone or no tone to the LNB.
     
  10. P Smith

    P Smith Mr. FixAnything

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    SWiM LNBF have own protocol, two way conversation
     

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