Low noise block downconverter ( LNB )
Have you ever wondered what is an LNB and what is an LNB LO frequency ? Here is some information about LNBs that I hope will help explain matters.
The abbreviation LNB stands for Low Noise Block. It is the device on the front of a satellite dish that receives the very low level microwave signal from the satellite, amplifies it, changes the signals to a lower frequency band and sends them down the cable to the indoor receiver.
The expression low noise refers the the quality of the first stage input amplifier transistor. The quality is measured in units called Noise Temperature, Noise Figure or Noise Factor. Both Noise Figure and Noise Factor may be converted into Noise Temperature. The lower the Noise Temperature the better. So an LNB with Noise Temperature = 100K is twice as good as one with 200K. C band LNBs tend have the lowest noise temperature performance while Ka LNBs have the highest (worst).
The expression Block refers to the conversion of a block of microwave frequencies as received from the satellite being down-converted to a lower (block) range of frequencies in the cable to the receiver. Satellites broadcast mainly in the range 4 to 12 to 21 GHz.
|Input freq. band C, Ku, Ka GHz from sat. waveguide||Loc Osc. (LO) freq.||Output L band into cable.||Notes|
|3.4-4.2||5.15||950-1750||invert output spectrum|
|10.95 - 12.15||10||950-2150||Invacom SPV -50 SM|
|11.7-12.75||10.75||950-2000||Invacom SPV -60 SM|
|12.25-12.75||11.3||950-1450||Invacom SPV -70 SM|
|11.7-12.2||13.85||2150-1650||DISH FSS inverted|
|12.2 - 12.7||14.35||2150-1650||DISH PRO inverted|
All the above illustrate a simple LNB, with one LNA and one LO frequency.
More complex LNBs exist, particularly for satellite TV reception where people wish to receive signals from multiple bands, alternative polarisations, and possibly simultaneously.
Ku band LNBs typically have two alternative local oscillator frequencies, for example 9.75 GHz and 10.6 GHz with the higher frequency option selected using a 22 kHz tone injected into the cable. Such an LNB may be used to receive 10.7 - 11.7 GHz using the lower 9.75 GHz LO frequency or the higher band 11.7 - 12.75 GHz using the higher 10.6 GHz LO frequency.
A few Ka band LNBs may have as many as 4 local oscillator frequencies.
The LNB shown above has one wire going into the waveguide to pick up vertical polarisation. If the input waveguide is circular is can support two polarisations and it can be arranged for there to be two input probes at right angles, thus allowing two alternative polarisations to be selected (vertical or horizontal, or left hand or right hand circular polarisation, LHCP or RHCP), either one or the other. Dual polarisation LNBs may commonly be switched remotely using two alternative DC supply voltages. e.g. 13 volts makes it receive vertical polarisation and 19 volts make it receive horizontal polarisation.
Useful links to suppliers who do professional LNBs for sale suitable for VSAT applications:
I have just picked these suppliers as ones I know of well. You are welcome to suggest others to me firstname.lastname@example.org that you may have found, that give good service.
If both input probes have their own LNB amplifiers etc you have effectively two LNBs in the same module, which will have two output cables, one for each polarisation. Many variants on this theme exist, with options also for multiple bands. Such a "Quad LNB" might thus have 4 outputs, for each polarisation and each of two bands. Such an arrangement is attractive for a block of flats, or head end antenna, which need to feed multiple indoor satellite TV receivers with the viewers all wanting all permutations of the two polarisations and two frequency bands.
The DISH-PRO system (as an example) uses stacked blocks of frequencies in the LNB output, one block from each polarisation. This means no switching of polarisation and that all signals are available simultaneously all of the time at the set top box end. The DC is supply normally fixed at 19 to 20 volts. High quality RG6 cable essential for working to 2150 MHz
In the case of a circular polarisation system the input waveguide contains a polariser which converts dual circular polarisation to two linear polarisations, extracted by two probes at right angles to two low noise amplifiers. See the image above.
Example frequency conversions are shown below
Input 12200-12700 R - 11250 LO = 950 -1450 IF output
Input 12200-12700 L - 14350 LO = 2150 - 1650 IF output
or, for a linear polarised satellite system with a limited 500 MHz bandwidth. (AMC-21 linear FSS ?)
Input 11700-12200 H - 10750 LO = 950 -1450 IF output
Input 11700-12200 V - 13850 LO = 2150 - 1650 IF output
The gap in the output spectrum makes it easier to combine filter and avoid interference and noise
All LNBs used for satellite TV reception use dielectric resonator
stabilised local oscillators. The DRO is just a pellet of material
which resonates at the required frequency. Compared with
quartz crystal a DRO is relatively unstable with temperature and frequency
accuracies may be +/- 250 kHz to as much as +/- 2 MHz at Ku band. This
variation includes both the initial value plus variations of temperature
over the full extremes of the operating range. Fortunately most TV carriers
are quite wide bandwidth (like 27 MHz) so even with 2 MHz error the indoor
receiver will successfully tune the carrier and capture it within the
automatic frequency control capture range.
If you want the LNB for the reception of narrow carriers, say 50 kHz wide, you have a problem since the indoor receiver may not find the carrier at all or may even find the wrong one. In which case you need a rather clever receiver that will sweep slowly over a range like +/- 2 MHz searching for the carrier and trying to recognise it before locking on to it. Alternatively it is possible to buy Phase Lock Loop LNBs which have far better frequency accuracy. Such PLL LNBs have in internal crystal oscillator or rely on an external 10 MHz reference signal sent up the cable by the indoor receiver. PLL LNBs are more expensive. The benefit of using an external reference PLL LNB is that the indoor reference oscillator is easier to maintain at a stable constant temperature. Ka band LNBs operate at such high frequency that they can need phase look loop frequency control unless the wanted carriers are very large bandwidth. An internal PLL uses a crystal oscillator in the LNB. An external reference PLL uses a 10 MHz reference supply from the customer's indoor modem or receiver.
All modern DRO LNBs are sold as 'digi-ready'. What this means is that some attention has been paid in the design to keeping the phase noise down so as to facilitate the reception of digital TV carriers. The phase noise of DRO LNBs is still far worse than for PLL LNBs. What good phase noise performance is really needed for is for the reception of low bit rate digital carriers and for digital carriers using high spectral efficiency modulation methods like 8-PSK, 8-QAM or 16-QAM modulation, which reduce the bandwidth required but need more power from the satellite, a bigger receive dish and better quality PLL type oscillators in both the transmit and receive chains.
The DC voltage power supply is fed up the cable to the LNB. Often
by altering this voltage it is possible to change the polarisation or, less
commonly, the frequency band. Voltages are normally 13 volts or 19
Perfect weatherproofing of the outdoor connector is essential, otherwise corrosion is rapid. Note that both the inner and outer conductors must make really good electrical contact. High resistance can cause the LNB to switch permanently into the low voltage state. Very peculiar effects can occur if there poor connections amongst multiple cables to say an LNB and to a transmit BUC module as the go and return DC supplies may become mixed up and the wrong voltage applied across the various items. The electrical connections at the antennas between the LNB and the BUC chassis are often indeterminate and depend of screws in waveguide flanges etc. Earth loop currents may also be a problem - it is possible to find 50 Hz or 60 Hz mains currents on the outer conductors - so be careful. Such stray currents and induced RF fields from nearby transmitters and cell phones may interfere with the wanted signals inside the cables. The quality and smoothing of the DC supplies used for the LNBs is important.
Some LNBs, such as those from Invacom, incorporate a receive band pass, transmit band reject filter at the front end. This provides both good image reject response for the receive function but also protects the LNB from spurious energy from the transmitter, which may destroy the LNB.
Check with a current meter that it is drawing DC current from the power supply. The approx number of milliamps will be given by the manufacturer. Badly made or corroded F type connections are the most probable cause of faults. Remember that the centre pin of the F connector plug should stick out about 2mm, proud of the surrounding threaded ring.
Use a satellite finder power meter. If you point the LNB up at clear sky (outer space) then the noise temperature contribution from the surroundings will be negligible, so the meter reading will correspond to the noise temperature of the LNB, say 100K (K means Kelvin*. 0K is absolute zero temperature, or -273 deg Centigrade, Kelvin units are the same magnitude as degrees Centigrade, so a room temperature of +17 deg C is the same as 273+17=290K). Boiling water at +100 deg C is the same as +373 K.
If you point the LNB away from straight up and down towards your hand or the ground, which is at a temperature of approx 290K then the noise power reading on the meter should go up, corresponding to approx 390K (100K +290K). If you have the LNB mounted in an antenna you should get the same effect, a big increase in noise output when the beam from the dish is pointed at the ground or trees etc. Pointing at the sun should give a much bigger increase, but be careful not to overheat the LNB horn if the dish surface is optically reflective (i.e. not a matt paint finish) and similar to a mirror and reflects light and heat.
Note that LNBs may fail on one polarisation or on one frequency band and that the failure mode may only occur at certain temperatures.
If you choose to try a replacement LNB in a VSAT system check the transmit reject filter and supply voltage - you don't want to be one of those people who keeps blowing up LNBs trying to find a good one !
If you have a very large dish, say 7m diameter and point it at a satellite whose signals are intended for reception by small 70cm diameter antennas then the 20 dB increase in total power of the signals into the LNB may be sufficient to overload some of the transistor amplifier stages inside. This is not always obvious. Measuring the composite output power of the LNB using a power meter is suggested and comparing this with the -1 dB compression point in the manufacturer's specification. An alternative is to do an antenna pattern test on both a high power and a low power satellite. Any non linearity problem with the high power satellite is then clearly visible. Special low gain or high power output level LNBs are available for use with large dishes.
* [edit 1 Feb 2016] I've updated the definition of "Kelvin". It has been pointed out that you should not use the expression "degrees Kelvin" but rather use the word "Kelvin" or the abbreviation "K" alone.
** [edit 1 Nov 2018] I've replaced the original text about the input signal and local oscillator signal being "scrambled up" in the mixer with better wording suggested to me in an email.
If your think there are errors in the above advice please suggest new text paragraphs and send to me by email. email@example.com You are also welcome to send me pictures of your LNB/antenna. Many thanks.
► Page created 21 March 2006, edited 1 Nov 2018
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