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Thread: Calculating LNB Output Frequency (Project Part II)

  1. #1
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    Calculating LNB Output Frequency (Project Part II)

    I'm trying to figure out how to know what the LNBF output frequency is for a given transponder, and in particular what's realistically the maximum frequency I should expect.

    Using the maximum FTA downlink frequencies I could find:

    Max CBand Transponder: 4192 MHz
    Max KuBand Transponder: 12180 MHz

    ... Here are the calculations I'm doing. Anyone know if these are correct?

    (C LNB L.O.) - (Transponder) = 5150 - 4192 = 950 MHz
    (Transponder) - (Ku LNB L.O.) = 12180 - 10750 = 1430 MHz

    ... so, that means that the highest LNB output for C band comes from the lowest transponder frequency, and the highest LNB output for Ku band comes from the highest transponder frequency. Right?

    Min C Band FTA Transponder: 3706MHz

    5150 - 3706 = 1444MHz



    The reason I'm asking is because I've been reading conflicting things, and more importantly that the lnb output range is 950 - 2050, and I'd like to know what the realistic output range is. It might not make much of a difference, but if I'm going to play with the LNB output, it's nice to know my max frequency is 1.5GHz and not 2.0GHz.

    Any reason I should be worrying about 2.0GHz? Looks like 1.5GHz will be enough.

    Whew. Thanks for any help from you folks that have spent too much time with this hobby. :P

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  3. #2
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    This is from memory, but . . .

    As I recall, the traditional LNB output range is 950 - 1450mhz
    All "traditional" (read: old) documentation will quote those figures (If I remembered correctly).

    Bandstacked LNBs give 950 - 2150 by outputting both vertical and horizontal signals simultaneously, but at different frequency ranges.
    I think 1650 - 2150 is the other polarity.

    Hopefully, with this info, whatever documents you are looking at will be more clear, even if I made a minor error on my numbers.
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  4. #3
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    I agree with Anole, the higher frequency range is to account for bandstacked LNBs. This is also why high quality cable is needed when dealing with bandstacked systems.
    Current systems: 3X GEOSATpro microHD PVRs; Digitrans DTE-7150 DVB/Digicipher II; SiliconDust HDHomeRun ATSC/QAM networked tuners; fixed 1 meter Channel Master dish with Eagle Aspen P870 FSS stacked Ku LNB; 2X 3ABN 36" dishes with Sadoun KBSL1 stacked Ku LNBFs on Powermax SG-9120B H-H motors; fixed Sadoun SD180G 1.8 meter dish with Eagle Aspen B1SAT stacked C LNBF; Winegard Square Shooter OTA DTV antenna

  5. #4
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    The two freq relationships are:
    C-band
    LO-Freq - Downlink Freq = IF Freq
    KU-Band
    LO-Freq + Downlink Freq = IF Freq

    The LO freq for C-band is almost always 5150, but the LO freq for Ku can be 9750, 10600, 10750, or 11250, depending on which band and/or lnb type you're using.

    As mentioned above, the usual CONUS bands are 3700-4200 for C-band and 11700-12200 for Ku, and the IF that your receiver will see is in the 950-1450 band. This means that for the usual bands, 3700 corresponds to 1450, and 4200 corresponds to 950, and for KU, 11700 corresponds to 950 and 12200 corresponds to 1450. Ie for C-band the IF freqs are kind of reversed, ie the higher downlink freq gives the lower IF. But in both cases, the lowest freq you'd normally see will be 950, and with a "STANDARD" lnb, the highest freq is normally 1450.

    HOWEVER, there are some signals that you'll run into, usually signals aimed at South America or Europe, that are below 3700 on C-band. Since C-band is reversed on the IF, 3600 will show up at 1550, 3500 will show up at 1650, etc, etc. so to receive below 3700, you need a receiver that will go above 1450 on the IF. I can't remember how low those C-band freqs actually go, and I can't remember if they are linear or circular (ie if circular, you'd also need a different LNB or a teflon insert), however those are the only signals you'd need to worry about on C-band.

    ALSO however, there are situations when you might need to go above 1450 on Ku, but these instances are usually LNB dependent. For example, if you're trying to tune the 11700-12200 band with a STANDARD lnb, the IF will be in the 950-1450 band, however if you are using a "UNIVERSAL" lnbf, which has 2 LO freqs, usually one at 9750 and one at 10600, then, you'd be using the 10600 LO freq to tune this band, and the IF's you'd see at the receiver would be 1050-1550. Also, if you're looking to receive the low Ku band, which is down below 11700, you'd be using the 9750 LO freq, and to get to just below the 11700 downlink freq, you'd need your receiver to tune up to 11700-9750=1950.
    Therefore, if you're looking for frequencies BELOW 11700 with a regular FTA receiver, you'd need that receiver to go up to 1950 on the IF.

    I say "with a regular FTA receiver, because every FTA receiver I'm familiar with will NOT tune below 950, so with a STANDARD lnb, it is not possible to tune below 11700. You need a universal LNBF to tune below 11700, and that means you need a receiver that will go up to 1950. HOWEVER, if you are designing your own receiver, or using a communications receiver for some reason, and can tune below 950, then you may not need to use a universal lnbf if the freqs you're looking for aren't too far below 11700, but I don't think a standard LNB will go quite as far BELOW 950 as it goes ABOVE 1450, so I think the performance would drop when you went too far below 950.

    Anyway, in general, the lowest you'd need to go is 950, and if you're interested in either the low C-band band, or the low Ku band, then you'll need to go perhaps as high as 1950 on the IF.

  6. #5
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    B.J. brings up another point - Ku band in South America.
    I've seen some LNBs with LO frequency of 10500 or 10300 (?), so if that matters, do more research.
    If you don't care, then of course, forget it. -

    And I left out the whole Universal LNB and its two LO because... I didn't care.
    ... and you mentioned 10750, so I thought you were concentrating on the North American satellites.
    But it was good to get it out into the light of day. -
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  7. #6
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    equant,
    I dont know if this will help, or is what you are looking for, but I did a lil' Excel sheet for cross calculating the IF vs C/Ku downlink freqs and vice versa for "popular" LO's a while back. Post 3 has the xls' for 10600 and 11250. Changing the LO "value" in the cells for another range of calculation that is not 5150,10600,10750, or 11250 is pretty straightfoward, even if you are not Excel savvy.
    I did the freq "charts" to illistrate the overlap of the C vs Ku desired freq to IF relationship. The lists of downlink freqs extend out of band for completeness. Here's the link to the thread - http://www.satelliteguys.us/fta-shac...converter.html

    Sounds like your embarking on another peoject.............????

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