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What the LNB really incorporates in itself.

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  • alfred_1
    • Dec 2010
    • 146

    What the LNB really incorporates in itself.

    Allow me to give an indebt explanation (which I myself learned from my younger days) into the workings of that most important piece of electronic marvel, namely the LNB.

    The LNB incorporates in itself: An Amplifier, An Oscillator and a Mixer (down converter). We will explain each unit individually later on.

    The LNB receives signals from the Ku or Ka Band (we will take these two as an example) These Signal will be in the range of 10,700 to 12.800 GHz and the LNB will bring them down to what we term as the L-Band range that is to say from 950Mhz to 2150Mhz.

    Now if these Frequencies weren’t brought down to the L-Band range by the LNB we
    Would have to use ‘hallow ****s’ or ‘wave guides’ between Dish and Receiver to be able to handle that very high incoming Frequency of 10,700 to 12.800 GHz.

    The cost of such material and the design of the Receiver itself would have to be very high, if it wern't for the LNB.

    Now let’s take the items that incorporate the LNB one by one.

    The Amplifier as its name implies, amplifies the weak signal received from the Satellite, and it is important that it introduces as little noise as possible.

    The Oscillator (Low and High Band) Generates one or two very stable signals,
    One for High Band (10.600 GHz) and another for Low Band (9.750 GHz).

    The Oscillator Frequency is mixed with the incoming Signal and then down converted to the intermediate frequency which is the L-Band. The result of the mixing is the Sum and Difference of the two Frequencies.
    Let me state an example so as to understand better the down mixing system.

    If you have a Frequency of 10719Mhz entering the LNB and an Oscillator Frequency of 9750Mhz (low Band) then the resultant Sum and difference Frequencies are 969Mhz
    (i.e. 10719 – 9750 MHz) and also 20469MZ (i.e10719+9750) which we don’t need and therefore is filtered out.
    So the down converted Frequency is 969 MHz (L-Band)

    Let’s give another example:

    If the incoming Frequency entering the LNB is12713 and the Oscillator Frequency is 10600Mhz (High Band) then the resultant Sum and difference Frequencies are 2113Mhz (12713 – 10600) and also 23313 (i.e. 12713+10600) which we don’t need and therefore is filtered out.
    So the down converted Frequency is 2113 MHz (L-Band)

    So now we have a Frequency of 969Mhz (in the L/Band) which is passed to the Receiver containing all the information we need to generate our TV Picture and naturally also a frequency of 2113Mhz when the Oscillator chooses the High Band.

    The Mixer is nothing more than a circuit to mix the Two Frequencies, that is, the Incoming Satellite Frequency and the one generted by the Oscillator and then pass the resultant output to the Receiver.

    Finally we must also mention that the LNB also receives a Frequency of 22 Khz from the Receiver so that it can choose between Vertical and Horizontal Polarities.

    I sincerly hope that this explanation will further the knowledge of all those who are interested in the true workings of a normal LNB.