LNB Specification Guide
The LNB (Low-Noise Block Downconverter) receives the signal collected by the satellite antenna, amplifies that signal, and then downconverts those signals to a lower frequency, typically L-Band, which is better suited to transmission over a coaxial cable to your receiving device. Typically, these are mounted outdoors at or near the feed of the antenna.
A good LNB will do this without adding too many spurious signals of its own.
Below is a list of the common specifications associated with LNBs and what they mean.
When choosing an LNB for your application there are a few main items to be looking at :
- Input Frequency Range
- IF or Output Frequency
- Local Oscillator Stability
- Internal or External Reference
- PLL vs. DRO
- Gain Flatness
- Group Delay Variation
- Local Oscillator Intermodulation Products
- Local Oscillator Phase Noise
- Spurious Components at LNB Output
- LNB Output Return Loss
The input frequency or receive frequency of the LNB will need to cover the entire frequency range that the satellite broadcasts on for your specific application. If you are unsure of this, you can either ask your satellite provider, or look at your link budget.Back to top
This is the downconverted frequency that comes out of the LNB. This range is the range of frequencies that your receiver should tune to if you are going to be able to see all the frequency ranges covered by the LNB. In most cases a reciever will take in L-Band (950- 1750 or 2150 MHz).Back to top
The L.O. stability is the variance in the frequency of the Local Oscialltor (L.O.) that can be expected with time, temperature, voltage, humidity, and/or vibration. Typically you want this to be something significantly lower than your carrier width.
Most carriers today are larger in size and the days of 9600 baud satellite carriers or even 64 kbps carriers are rare. A 1 MBPS data carrier even at a spectral efficiency of 5 b/hz would occupy 200 Khz of satellite space so a +/- 50 KHz stability in the LNB would be more than adequate.Back to top
Most LNBs rely on an internal 10 MHz reference based on either a Temperature Compensated Crystal Oscillator (TXCO) in the case of a PLL LNB or a Dielectric Resonance Osciallator (DRO), however using an external reference LNB has the advantage of using a 10 MHz signal sent up the cable by an internal receiverm and those signals are typically more stable since it is easier to maintain a stable constant temperature indoors.
External Reference PLL LNB accuracy can be extremely good if you use a oscillator source such as a one based on a GPS receiver. In the case of a Teleport, external reference LNBs are recommended and the 10 MHz suppoly should have a phase noise characteristics, however make sure in this case that the power supply is not located next to a motor or a fan of which either will give our vibration and an oscillating magnetic field.Back to top
PLL oscilators are more frequency accurate and stable. Becasue of this a receiver connected to a PLL LNB can lock to a particular signal faster. While a DRO LNB may drift in frequency +/- 3 MHz, a d PLL will usually reduce this drift to something much smaller, as small as +/- 100 Khz or much less.
Assuming your carrier has a 1 Mbit/s bit rate, locking on to that could be difficult or impossible with a traditional DRO LNB that has a L.O stability of +/-750 Khz. Even if the reciever is able to obtain a lock, it is likely to keep losing lock due to the frequency drift in the LNB.Back to top
The LNB Gain is the amount the incoming signal is amplified. At first glance, the higher the gain you could get would be the obvious thing to look for, however this is not the only critereon that you should be considering when it comes to LNBs. When you have a large antenna looks at a high powered group of satellites, the gain could be so high that it could overload the front end of the receiver. You could have too much gain.
Even if the receiver can handle a massive amount of signal, there can be problems within the LNB itself when a large amount of amplification is involved. This leads to the generation of artifacts, spurious signals and distortion products that are akin to the distorition that you would get from turning up the volume on your radio. This distortion will interfere with the reception of your signals.
So unless you have specific reasons for an ultra-high gain LNB, then look for a gain of around 50 - 60 dB.Back to top
In order for the demodulator or receiver to work effectively, the gain at all frequencies with in the band should be the same. Typically this is not a difficult requirement to meet except at the edges of the band as long as the LNB is built correctly.Back to top
Group Delay is the time shift experienced by signals of differing frequencies. If the high frequency signal components are delayed significantly more or less than the low frequency signal components, then the demodulator in your receiver is going to have challenges.
An analogy that has been used to describe this is a stop light. Imagine that the time it takes for the red light to turn off is is significantly longer than for the green light to turn on. There would be a period when both lights are on, and this would likely cause accidents.Back to top
The Local Oscillator in the LNB itself is another source of interfering signals. Utilizing a properly balanced mixer, unwanted intermodulation products should be at least 60 dB down. There is of course, a wanted intermodulation product as well, that is the frequency translated signal fed out of the coaxial output connector.Back to top
A LNB should frequency shift the incoming signals to the IF output frequency and amplify them without altering the vital phase modulation in the signal. Adding further phase modulation to the signal is the worst thing that a LNB could do. Above all other variables, phase noise performance is one of the most important determinants of performance of a digital LNB. Ideally, a low noise factor LNB should have low phase noise too, but a low noise factor by itself is no guarantee of good reception.
There comes a point where the additional reduction of white noise within the LNB is pointless since the noise picked up by the antenna will be many times more significant than the noise contribution of the LNB. A good analogy here is a hearing aid. If you are picking up too much noise and just amplifying it, then everything is just getting louder. Rather than crank up your hearing aid, you may be better served to cup your hands behind your ears and try to get a better signal by blocking out some of the background or ambient noise from the sounds. In other words, get a better point, or a more accurate dish.
Microwave noise figures are a notoriously difficult thing to measure since there is no such thing to measure them with. All noise factor measurements are inferred from running and experiment which attempts to distinguish between internally generated noise and noise naturally present in the testing environment and the measuring instruments. When people start worrying about a fractions of a dB, it is time to start asking how these measurements are made and how repeatable the test results are.Back to top
Spurious components at LNB output is a catch-all category covering all the possible sources of interfering signals. The bigger the negative number here, the better.Back to top
This loss is a measurement of by how much spurious signals are coming up the LNB coaxial cable and re-entering the LNB are reduced before re-emerging from the socket. These signals can originate from the receiver and/or from signal reflections caused by discontinuities in the coaxial cable run. These rogue signals can cause interference and be responsible for the mysterious inability to receive certain transponders from a satellite. Needless to say, this value should be as high as possible, and a high return loss will help to reduce the level of these unwanted signals.Back to top
Like anything the more you spend, the better you will get, but also like anything that is not the whole story. From the above information, here are a couple of takeaways :
- Read the specification sheet, know what it is telling you.
- Noise Figures are only part of the story, and according to some satellite providers, they are not even relevant.
- If you are suffering from a weak signal, a better solution is a bigger dish.
- PLL Oscillators are reccomended for todays more advanced higher order modulation networks.
- Determine your required L.O. Stability based on the carrier you are locking to. There is no reason to get a +/- 5 kHz L.O. LNB when you are receiving a carrier that is 5 MHz wide.