How does one of the most important parts of your satellite system work?

Below is a simple explanation of what a modern day LNBF is, and how it works.

C band LNBF	Ku band LNBF

The abbreviation LNB stands for Low Noise Block. LNBF stands for Low noise Block with Feed horn.   This is the metal or plastic cased unit that is mounted at the focal point of your antenna. Its purpose is to receive the very low level microwave signal from the satellite, amplify it and change the signals to a lower frequency band and transmit  them down the coaxial cable  to the indoor satellite receiver.

The expression low noise refers to the amount of noise passed by the wide band amplifier stages after the signal has been received by the receive probe and passed the signal through the wide band amplifier stages. The original GaAsFET’s have given way to the use of high electron mobility transistor (HEMT) FET’s. The lower the Noise Temperature the better.

The expression Block refers to the down conversion of a block of incoming microwave frequencies as received from the satellite. This block of frequencies (12.250 – 12.750 GHz) is down-converted to a lower (block) (950- 1450 MHz) of frequencies before they can be demodulated by the satellite receiver.

Figure 1

Figure 1 shows the input waveguide on the left, which is connected to the receiving probes mounted in the feed horn throat

As shown in the diagram, there is a probe mounted through waveguide that receives the incoming microwave signals. These satellite signals pass through a band pass filter which only allows the intended band of microwave frequencies to pass through.

The signals are then amplified by the wide band Low Noise Amplifier and then are mixed by a local oscillator which converts the incoming block of frequencies to a much lower block of frequencies which are controlled by the actual local oscillator frequency.

 The mixer output products are the difference between the incoming frequencies and that of the output of the mixer stage. Normally frequencies spanning a 500MHz block after conversion by the mixer stage is the block of frequencies (950 – 1450) MHz or depending on the actual local oscillator frequency these can alter from (950 – 1450 or (1600 to 2051) or what ever the unit’s local oscillator frequency determines.

Figure 2

Figure 2 Open LNBF showing the printed circuit board

 The second band pass filter selects only the "wanted" block of frequencies and suppresses any out of band frequencies making sure that only the desired block of frequencies is then amplified up to 65 to 70 Dbm so that the L band signals can combat cable attenuation from the satellite dish to the indoor satellite receiver. Most modern day satellite receiver will operate quite happily on RF signals ranging from -25Dbm to -62 Dbm.

 The modern day LNBF receives its power from the satellite receiver and sends the RF signals down the cable back to the satellite receiver. Polarization switching is accomplished by Varying the voltage to the LNBF from 13v to 20v which will adjust the receive probe mounted in the actual wave guide from Vertical to Horizontal or vica versa. Normally 13v will select Vertical and 20v will select the Horizontal probe

 Normally the output frequency is derived by subtracting the local oscillator frequency from that of the incoming satellite signals Input frequency.

Example: kordia freeview frequency 12456- lo11300 = 1156MHz
Example: kordia freeview frequency 12456- lo 10750 = 1706MHz 

Examples of input frequency band, LNB local oscillator frequency, and output frequency band are shown below.

 Overview of C and Ku band frequencies.

             
To work out what the output L band frequency is from any C band input frequency the C band input frequency must be subtracted from the local Oscillator frequency.

Input frequency 3,700 MHz subtracted by 5150 MHz (Lo) = L band frequency 1450MHz
Simply substitute the input and local oscillator frequencies as required.

5150 – 3,700 = 1450 MHz

To work out what the output L band frequency is from any Ku band input frequency the Local oscillator frequency must be subtracted from the incoming satellite frequency.

Input frequency 12456 MHz divided 11300 MHz (Lo) = 1156 L band frequency MHz
Simply substitute the input and local oscillator frequencies as required.

12456 – 11300 = 1156 MHz

Purchase your LNB's from JX

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