LNB
A Low Noise Block (or LNC- Low Noise Converter), used for broadcast satellite reception, is usually affixed either in or on the satellite dish. An LNB helps keep the overall sound and picture of satellite TV from becoming greatly degraded, without the need of introducing a much larger dish reflector.
Its purpose use the super heterodyne effect; and amplify and convert a wide block (band) of frequencies to compensate the signal loss associated with a coaxial cable at high frequencies.
The term ‘low noise’ relates to the quality of the 1st stage input amplifier transistor, measured in Noise Temperature units, Noise Figure units or Noise Factor units.
A lower Noise Temperature rating is always better, and both Noise Factor and Noise Figure are easily converted into Noise Temperature units.
The term ‘Block’ refers to the conversion of a higher block of microwave frequencies (in the range 4 GHz to 21 GHz) being down-converted for the receiver to a lower block range of frequencies.
The “low-noise” part indicates that amplification and mixing takes place prior to cable attenuation. Since satellites operate at high frequencies, it is critical that the noise is controlled prior to signal processing.
For wide-band satellite television carrier reception (generally 27 MHz wide band), the tolerance (accuracy) of the LNB local oscillator frequency needs to be in the range of ±500kHz,. This makes low cost DRO’s (dielectric oscillators) feasible.
For reception of narrow bandwidth carriers (i.e. 16-QAM)- a highly stable, low phase noise dedicated LNB (local) oscillator is required that contains an internal crystal oscillator (or 10 MHz reference from the indoor unit) and a PLL (Phase-Locked Loop) oscillator, and naturally tend to be noticeably more expensive.
LNB Feedhorns (LNBF’s)
DBS (Direct Broadcast Satellite) dishes use an LNBF, which matches the antenna’s feedhorn up with the LNB. Small diplexers are used to distribute the resulting IF signal (typically in the 950 MHz to 1450 MHz range) and “piggyback” these signals with the same TV cable that carries lower-frequency (terrestrial) television signals from a typical outdoor (terrestrial) antenna.
The TV set receiver has another diplexer that separates the signals.
Universal LNB’s
A universal LNB can receive both polarizations and the full range of frequencies in both the Ku and C satellite band. It receives both polarizations simultaneously (through 2 different connectors), while other LNB’s have either switchable or adjustable polarization.
Typical LNB specifications are:
Local Oscillator (LO): 9.75 GHz /10.6 GHz
Frequency: 10.7 GHz-12.75 GHz
Noise Figure (NF): 0.7 dB
Polarization: Linear
Standard DBS LNB example:
Local Oscillator (LO): 11.25 GHz
Frequency: 12.2 GHz-12.7 GHz
Noise Figure (NF): 0.7 dB
Polarization: Circular
Typical North American C-band LNB specs:
Local Oscillator (LO): 5.15 GHz
Frequency: 3.6-4.2 GHz
Noise Figure (NF): 15 to 100 Kelvins (uses Kelvin ratings as opposed to dB rating)
Polarization: Linear
Dual and Quad LNB’s are multiple LNB’s contained in one package, to allow for multiple receivers (on one dish). It consists of 2 universal LNBs and only one “F” connector and coaxial cable connection to the converter box.
The Monobloc LNB, designed to receive signals from satellites that are spaced very close together, has only one output and only one (satellite) transmission is viewable at a time, while dish systems which have two or more separate LNB’s connected to separate receivers, and both transmissions can be simultaneously viewed or recorded.
Quad Universal LNB (a.k.a. Quad-Output LNB)
The Quad Universal LNB can accommodate four separate receivers, each receiver has independent control of band and polarization via 13v. and 17v. and 22kHz on/off switching respectively. It is primarily deployed in the Sky Digiboxes.
Two LNB outputs would go to one “Sky Plus” Digibox, leaving the other 2 LNB outputs for either 2 standard Digiboxes or to one additional Sky Plus Digibox.
OCTO LNB’s
An OCTO LNB is the same as above, except that it has 8 independent outputs.
Quattro Universal LNB’s
This is a 4-output LNB, specifically designed for use as a distribution system (for apartment complexes).
LNB’s can generally supply up to16 outputs for separate Digiboxes. Its The 4 outputs are:
Low band horizontal polarization
High band horizontal polarization
Low band vertical polarization
High band vertical polarization
LNB’s that are used for satellite TV reception contain DRO’s (dielectric resonator stabilized local oscillators), that resonate at the required frequency.
A DRO is relatively unstable, as compared to a quartz crystal resonator or oscillator.
Because most TV carriers are quite bandwidth-wide (i.e. 27 MHz band) even a 2 MHz error can successfully be received.
LNB Supply Power
The DC supply (in the 13v. to 19v. range) is cable line-fed to the LNB. Efficiently weather-proofing the outdoor connector is critical, as oxidation and corrosion occur rapidly. This is related to signal degradation.
Both the outer and inner conductors must make solid electrical contact.
High resistance will cause the LNB to switch permanently into the low voltage state and lead to overall signal deterioration.
The electrical antenna contacts between the BUC chassis and LNB are often times difficult to navigate, and ‘earth loop’ currents can also be a problem.
One can become severely shocked in discovering 50 Hz or 60 Hz AC Mains currents on the outer conductors. Proceed then with extreme caution.
Testing an LNB
Check the ammeter drawing the DC current from the power supply.
Poor quality and/or corroded F type connections are the most typical cause of concern.
The center pin of the F connector plug should stick out 2mm away from the threaded surrounding ring.
A satellite finder power meter reading will directly correspond to the noise temperature of the LNB. LNBs that fail on a particular polarization or particular frequency band, may do so only at certain temperatures.
Overloading an LNB
If you have a very large dish and aim it at a satellite with signals intended for small dish antennas, the overall 20 dB increase in signal strength may be enough to overload some of the internal transistor amplifier stages.
It is wise to measure the composite output power of the LNB (using a power meter) and compare this with the -1 dB compression point supplied by the manufacturer.
You can also do an antenna pattern test on a high power and a low power satellite and non linearity problems will become clearly discernible.
To solve this problem, you need a special LNB with low gain or high power output level if you have a large dish.
The “field expedient” means of fully testing an LNB is to hook it up to the desired dish (aligning the dish and LNB) and connect it to a satellite receiver.
Note the time of day and standard thermometer temperature, and check to see if each channel is there.
If all channels are present, take note and wait until for temperature variance of 40 to 50 degrees Fahrenheit, and perform the test again.
Missing channels can also be due to problems of the cable, receiver, or the dish.
Swapping the dish, the cable, the receiver and having in mind that not all receivers work with all LNB’s will aid your trouble-shooting endeavors.
A cheap satellite finder meter will give you the average strength of all frequencies, and an expensive one can let you target specific frequencies, but not indicate to you if frequencies are missing.
