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Time for me to come clean about dBm, dBµV & dB

Discussion in 'DIRECTV Installation/MDU Discussion' started by veryoldschool, Jul 16, 2012.

  1. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Looking at the meters on the market:

    Applied Instruments AITS2 AI Turbo S2 $849.99
    Looks like a great meter, though it doesn't have the IV test, but gives the user all the information needed, and more.

    AIM $499.99 is great "but" only works with DirecTV.
    I can see a big advantage with this since it will run the IV test at the dish, which means if it passes, you're done working on the roof.

    ACUTRAC III Plus $205.99
    This does measure the Ka-lo signals, but doesn't look to measure the CNR/SNR

    These all don't measure Ka-lo:

    Applied Instruments SuperBuddy 29 $719.99
    BIRDOG ULTRA $599.99
    BIRDOG USB PLUS 4.0 $399.99
    Trilithic SAT LITE $219.99
     
  2. HoTat2

    HoTat2 Hall Of Fame

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    Los...
    Isn't the RF front end up to the demodulator of any superhetrodyne radio receiver actually an RF down-converter for all received signals to a fixed lower IF one?

    Agreed, and the lower the NF for a given gain of the first stage the less received signal strength at the dish is needed to achieve the necessary minimum C/N ratio, correct?

    Well as with its analog FM predecessor, the greater the received S.S. to the point where the LNB amplifiers are sufficiently saturated the greater the receiver quieting. So an over 40 dB drop in the no signal noise output is well obtainable isn't it?
     
  3. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    I think you're on the right path, but what you're talking about is in the receiver as demodulation is done at the end.
    The FM demodulation is what gives it the SNR of about 50 dB [if I haven't forgotten] for analog TV and FM radio.
    The SWiM adds another conversion and has an AGC, but still only processing the same signal and SNR.
     
  4. HoTat2

    HoTat2 Hall Of Fame

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    Los...
    Sure, but that 50 dB SNR after demodulation is strongly dependent on the C/N at the RF input.

    Yes at threshold you would typically have something like a 50 dB SNR after demod. for analog FM with around a 1 dB increase for every single dB increase in receiver input C/N ratio above threshold.

    But drop below threshold and its like a falling off a cliff where that 50 dB will go south fast at a rate of maybe 20 dB for every one dB drop in C/N ratio.

    And I've always felt digital radio is much the same behavior with about a 1:1 rate decrease in the BER of the demodulated signal when the RF input is above a minimum C/N ratio as threshold. Whereas its a steep increase in BER ("cliff effect") per dB decrease in C/N below threshold.
     
  5. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    This is sort of why I wasn't seeing the similarity as the SAT receiver works with a SNR of 8 dB or less.
    I don't think we're talking apples to apples anymore, so not sure of the value for this thread.
     
  6. ZandarKoad

    ZandarKoad Mentor

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    The newest batch is -14. Not sure who they are from, but those seem to be the ones causing all the problems. I don't have a whole lot of issues with the -01s.

    Yes, 103 even usually fails. What I end up doing is asking the customer to report over the phone the signal strength of the weakest transponder on 103(ca) which is usually 9, while I peak. This works about half the time. The other half the time I end up swapping either the LNB or the whole dish to get the lowest transponders on 103(ca) above 85.

    It seems as though you are using the term "dither" interchangeably with "peak" which is a little confusing for me. For me, dithering is not peaking, but is guessing at the peak by choosing two points on either side of the peak that are identical then setting to the mid point. Peaking is seeing exactly what you need to see on your testing device (be it a meter or a TV with a receiver hooked up) and making adjustments accordingly.
     
  7. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    "To dither" is to detune to a give value, then peak it and go past to the same detuned value, on the other side of the beam, and the turn back exactly half to find the center.
     
  8. ZandarKoad

    ZandarKoad Mentor

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    "To dither" is to tell the IV CSR whatever he wants to hear so he'll grant you a waiver. :)
     
  9. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    I'd call that BSing to get the job done, but "whatever" ;)
     
  10. dielray

    dielray Legend

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    I actually meant dither as in the way VOS described.

    Recently the LNBs I have received have been R2 -01s, -14s, and then R3 -01s. The R3 WNCs are cosmetically different from the R2s in that the casing is more similar to the -14s and the lens cap is curved. Your maximum signal levels are likely to be different than what I see. Between the 3 on a clear sunny day dithered(yes) to the 103 I would see a signal strength on 103(ca) transponder 16 of 90-94 with the -14s, 76-95 with the R3 -01s, and never below 95 on the R2 -01s. Dithering to the 101, I would see 88-94 on the -14s, 76-94 with the R3 -01s, and 94-95 on the R2 -01s. In my area I fail the 103 even when I see transponder 16 drop below 89. If you do an actual dither(on the 101), what values do you typically see? What values would you see with whatever your previous LNBs were?

    Out of curiosity what ODUs do you guys have? We have been with -20 (not sure the brand, stamped HDS) for the last few months.
     
  11. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    What does the details show for the failure?
    Guessing it would be the SNR, so next I'd be interested in the SNRs for all the 103 [a&b] TPs.
    I focused on the 101 for the attenuated LNB IV failures and SNR was the failure, so I then checked all the TP SNRs [of the even or odd fail] and found the lowest, and focused on it to find the pass/fail point.
     
  12. dielray

    dielray Legend

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    It's almost always the SNR that fails on the AIM. I have only seen the power level fail when the lines were very long. I have never seen the LNB Offset fail. If I am remembering correctly, I have seen the power level fail on the 101, but the SNR pass. I believe I had the AIM's location incorrectly set as at the ODU.

    The pass/fail point varies by zipcode, both with the AIM and the receivers.

    Not every transponder is checked, except with the AIM's EIV+. It used to be 2 from each sat, though now they check more. Next time I have to call in to get a waiver, I'll have to write down the ones they ask for(or if an ISS rep is reading this, perhaps they'll post).
     
  13. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    How large is your service area?
    I can see how the pass/fail would be sightly different at different locations, which points to some [good] value of the IV test, as it's "tweaked" for locations.
    I never entered a zip code, so was using the Southern Cal zip from who let me use it.
    I was fairly skeptical of the AIM & the IV testing, but ended up impressed by it as to how tight the pass/fail was to an optimum alignment.
    Now if it's raining while you're trying to do an install, this can suck. :lol:

    The LO offset would only matter if you had drifting in very cold weather.
     
  14. dielray

    dielray Legend

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    Each state is divided into service regions. I'm in MN08 which covers the far suburbs to the north and west and can get a little rural. The main cities and immediate suburbs are MN09.

    It used to be a very large pain. They now suspend IV and IV retest when the weather is not expected to be good.
     
  15. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    It sounds like only spots would be effected and CONUS will be constant.
     
  16. ZandarKoad

    ZandarKoad Mentor

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    You mean the message you hear when you call in to the IV system tells you they've suspended IV and IV retest when it's raining in your area. They never actually do. Or at least, they don't in my DMA. Never once in the last two years have I see "the continue option on IRDs that fail IV" during inclement weather.
     
  17. HoTat2

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    Los...
    Sorry, had to pretty much run out on errands all day yesterday;

    OK VOS, I just thought while one is digital and the other analog, I see similarities in their characteristics.

    Where analog C/Ku band LNBs receiving FM video have (or maybe "had") a high noise output too for no signal input which would tend to suppress or "quiet" under signal conditions.

    In a similar pattern the noise output of an LNB for digital DBS service would have a high noise level for no input signal as your measurements show which would then decrease as the digitally modulated RF carrier was received.

    I also feel that unlike analog though the error correction system range may result in much slower initial increase in BER as C/N drops below threshold, before the cliff effect (massive increase in BER) occurs and complete loss of signal.
     
  18. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    Not all LNBs are the same.
    DirecTV has built these to a price, and are spec'd to work with their system.
     
  19. HoTat2

    HoTat2 Hall Of Fame

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    Los...
    Then what in your view happens to the LNB high noise power output under no signal conditions when it locks onto a satellite signal?

    And for clarity, when you and others refer to the LNB being "locked" or "unlocked." Is that in reference to an AFC/PLL control loop circuit for the LO or something?
     
  20. veryoldschool

    veryoldschool Lifetime Achiever Staff Member Super Moderator DBSTalk Club

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    The locked is the LO to the input signal. The meter reads how much it's being corrected, when locked.
    "My view", well without being able to probe inside the LNB, with more than a meter, I don't know.
    Commercial grade equipment isn't "state of the art" [which is where most of my experience is] and this level of noise power is much higher than anything I'm used to, but within the designed parameter's of these systems.
     

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