New LNBF, why is a new scan needed?

Discussion in 'Free To Air (FTA) Discussion' started by johann12, Jan 5, 2013.

  1. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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    Recently I swap LNBF out on my ku dish and my c-band dish.

    Every time when I swap a LNBF out, my S10 can not get a signal from the new LNBF unless I do a rescan of all satellites.

    Is there any reason for it and how could I avoid having to rescan all satellites.
     
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  2. ke4est

    ke4est Long Live FTA
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    Is the skew set right for each lnbf some c- band ones need to be set to 12 o clock and some at 3

    Sent from my DROID 2 using SatelliteGuys
     
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  3. SatelliteAV

    SatelliteAV SatelliteGuys Family
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    On KU are you swapping a standard LO 10750 with a Standard LO 10750 or an Universal 9750/10600?

    C-band are you swapping from or to a bandstacked model?

    Also, if the LNBF LO frequencies are drifting the receiver might not tune the transponder unless it is rescanned. Are the transponder frequencies changing for the same channels when rescanned?
     
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  4. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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    C-band LNBF
    old LNBF ....BSC 621 2D = universal C/KU
    new LNBF BSC 422 dual



    KU LNBF
    old 10750 single
    new 10750 the pll that came with the new geosat pro dish.
    new 10750 dual output


    I double checked skew when I swaped the LNBF's.

    I guess this is not normal since you ask those questions.
     
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  5. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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    I never checked if the transponder frequency changed, I rescanned everything already so I do not know at this moment.
     
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  6. northgeorgia

    northgeorgia SatelliteGuys Family

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    I think it can be normal. I recently swapped out a universal Ku band LNB with a different brand, and it seems its LO frequency drifts about 2 or 3 khz lower than it should be on most transponders. But I don't know much about it -- is it an issue with the electronics inside each manufacturer's lnb, or do other variables affect this? It's something I'm curious about too :)
     
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  7. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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    On the 621 2d the arrow was at 9 o'clock position, that is right.
    The 422 the 0 degree is at the 12 o'clock position.

    The ku LNBF are set all the same at the 12 o'clock position.

    All settings where made to the most southern satellite which is off by 1.2 degrees( if I remember right) from the true south.
     
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  8. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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    Well, I was wondering.
    Let's say I scan all satellites in and delete the unwanted channels.
    I then save my channel list on my PC for safe-keeps.

    If it should happen that I swap a LNBF out and i want to restore the channel list, the channel list from my PC may not help me, since I may have to scan the satellites again.
     
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  9. FaT Air

    FaT Air HOA Free Zone
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    From what I remember, last foray into parts swaps was late last spring

    I've noticed this when swapping dish/lnbf combos from one satellite to another, or replacing lnbf's.
    Think it's all to do with LO drift, And the accuracy of the LO, and the difference between that of your old vs. the new.
    The lower the SR the smaller the 'slice' of the transponder it's on. Meaning, usually, more affected. Usually come up with a slightly different freq after a fresh scan, especially on lower SR transponders. At least that's what I have noticed. Higher SR channels seem to still work after a lnb swap.
    I do remember also re-scanning all the satellites.
     
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  10. catamount

    catamount Active SatelliteGuys Member

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  11. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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  12. SatelliteAV

    SatelliteAV SatelliteGuys Family
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    LNB LO Frequency - 101

    What is an LNB and how does it work?
    When a satellite signal is reflected into the feedhorn, the signal travels down the feedhorn or a waveguide and is picked up by an antenna probe and fed to the electronics section of the LNB (Low Noise Block). If the LNB has an integrated feedhorn it is called a LNBF (Low Noise Block Feedhorn).

    The LNB consists of two sections:
    1. LNA (Low Noise Amplifier) which boosts the strength of the very weak satellite signal.
    2. Frequency downconversion circuit that lowers the frequency so it will travel through the coax line to the receiver with minimized losses.
    The LNB's down convertor circuit subtracts a specific frequency called an LO (local Oscillation) from the downlink transponder satellite frequency to provide the IF (Intermediate Frequency) signal that is outputted from the LNB. A DRO (Dielectric Resonator Oscillator) type LNB(f) uses a tuning slug to adjust the LO (local Oscillation) frequency and consumer PLL (Phased Locked Loop) types use an internal reference crystal.

    Correct LO Frequency:
    For this example we will reference a standard KU-band LNB with an LO frequency of 10750 receiving a satellite signal on downlink frequency 11700mhz.

    1. The LNB receives the downlink transponder satellite signal at 11700mhz and subtracts the LO frequency of 10750 and outputs an IF signal of 950mhz.
    • DL - LO = IF (11700 - 10750 = 950)

    2. The IF signal of 950mhz travels down the coax to the satellite receiver.


    3. The receiver's tuner then blind scans and logs an IF frequency of 950mhz and displays Signal Level and Signal Quality readings. The receiver also calculates the downlink frequency to display for the user. To do this calculation, the user inputs the LO frequency of the connected LNB then the receiver adds the LO to the IF and displays the downlink transponder frequency.
    • LO + IF = DL transponder frequency (10750 + 950 = 11700)

    4. If a receiver is pre-programmed with a transponder frequency of 11700 and the user sets the LO frequency to 10750, the receiver's tuner will look for a satellite signal at 950mhz IF. Most receivers have an AFT (Automatic Fine Tuning) circuit that searches for the signal on frequencies near the specified IF frequency. Most receivers will adjust the tuner to find a signal that is 3 - 5mhz away from the center of the specified IF. On GEOSATpro receivers we program the AFT range for up to 10mhz. With a typical receiver, if the IF frequency is not within 2 -5mhz of the specified frequency, the receiver will not find it.



    Incorrect LO Frequency:
    Why might the LNB LO frequency be incorrect? This may happen on a DRO type LNB due to a misaligned tuning slug at the factory, vibrations loosening the tuning slug or the seated PCB, heat / cooling expanding or shrinking the mechanics of the PCB/tuning slug or the aging of the LNB. PLL units have very little drift as they are referenced to the crystal and will only slightly vary from the factory setting. PLL LO frequency outside specification would be due to manufacturing error or mechanical failure.

    Now imagine if the Standard LNB LO frequency had drifted to from 10750 to instead be 10740. The LNB receives a downlink satellite signal at 11700, down converts by subtracting 10740mhz and outputs an IF of 960mhz.
    • DL - LO = IF (11700 - 10740 = 960)

    The IF signal travels down the coax to the receiver. The tuner now will blind scan and find the signal at 960mhz. The user has inputted that the connected LNBF LO is 10750, so the receiver add 10750 to 960 and displays 11710mhz as the transponder frequency. The receiver will log the transponder in the blind scan mode and be able to display programming, but the displayed transponder frequency will be wrong.
    • LO + IF = DL (10750 + 960 = 11710)

    A receiver's tuner with a preprogrammed transponder frequency of 11700 and user inputted LO frequency of 10750 will look for a satellite signal at 950mhz IF. Since the LNB is sending the transponder at 960 IF, the receiver will probably not find a satellite signal as it is beyond the tuning range of the AFT circuit.
    • Receiver Tuning: 11700 - 10750 = 950mhz IF
    • LNB Output: 11700 - 10740 = 960mhz IF
    • IF Difference: 960mhz IF - 950mhz IF = 10mhz IF offset
    • Receiver IF AFT Range: 5mhz - 10mhz = 5mhz beyond tuner AFT range

    Bottom line: When a satellite receiver is looking for a channel on a specific frequency, but the LNB is sending the transponder on a different frequency, no channel will be found.
     
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    Last edited: Jan 6, 2013
  13. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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    Very very very well explained. I understood it the first time I was reading it. Thank you for taking the time to post this.

    So in my case it could be a low quality LNBF or a miss-tuned LNBF or a drifted LNBF because of temperature etc. etc. etc.
    The PLL should be the closest to the transponder frequency since it is using a crystal for oscillation as a reference point.

    In other words when I scanned with the old LNBF and the new LNBF, the frequency may not be the same. If the frequency difference is way greater than the receivers stored frequency,then the receiver can't handle this since it is out of specs and the channels do not tune in.

    Thank you.
     
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  14. SatelliteAV

    SatelliteAV SatelliteGuys Family
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    Exactly! Now with the PLL LNBF, the Blind Scan transponder frequencies will be as accurate as the receiver's firmware provides.
     
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    Last edited: Jan 5, 2013
  15. ke4est

    ke4est Long Live FTA
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  16. johann12

    johann12 Thread Starter Active SatelliteGuys Member

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