Traditionally the quality of a LNB has been determined by the noise figure (NF). Currently, the values indicated in most of the technical specs are driven more by marketing criteria than real performance.
The problem is that the installer can’t probe the accuracy of the specs that he is given and becomes a “victim” in this war to achieve the lowest NF that re-sellers and importers of these products have started.
The truth is that the quality of the satellite signals obtained at the output of a LNB, are determined by the C/N (carrier to noise) more than what they are by the NF. If we consider the satellite HD multiplexes that use the DVB-S2 standard, the phase noise is an even more important parameter than the NF.
The quality of a DVB-S2/HD reception system is, obviously, reflected in the quality of the signal received and therefore can be measured with the common parameters that measure the quality of these type of signals: C/N, MER (Modulation Error Ratio), LKM (Link Margin) and CBER (Convolutional Bit Error Rate).
NF, C/N and H45. The importance of the measurement.
As it was mentioned in the paragraph above, the installer can’t measure the NF of a LNB. But he can measure parameters like: C/N, MER, LKM and CBER. The value of an extremely low noise figure doesn’t matter if, in practice, the measure taken from the parameters of the sensor set are worse.
In the graphic below we can see the response of two different LNBs. The red trace corresponds with a Televes 7475 0.5dB NF and the green trace is from a LNB of a competitor publishing a NF of 0.2dB.
At first sight we could evaluate the quality based on the signal levels for each of the traces and think that since the green is given higher levels it should be a better LNB. But we should look at the gaps without multiplexes in the graph,
||because it is only there where we can appreciate the noise levels.
The reality that this graphic is pointing out is that yes the LNB of our competitors has a higher gain but the C/N, that is what it is really important, is 3dB lower.
In terms of system efficiency, the difference in the performance of the two LNBs will be higher than the difference that you will get when using an 80cm dish instead of a 1.10m dish. In other words, an 80cm dish using a Televes’ LNB 7475 will give a better overall performance than a 1.1m dish with one of our competitors LNBs.
Even without using graphics like the one above, simply by measuring the C/N by means of the H45 Televes meter it would be enough to demonstrate the non-sense of evaluating the quality of the LNB based on the NF (0.2dB) parameter only.
The phase noise
In those LNBs that are going to be used to receive DVB-S2/HD services the phase noise is an even more important parameter than the NF; something that is completely ignored by those ones that assume a direct relation between quality and NF.
The DVB-S2 standard uses most commonly an 8-PSK (Phase Shift Keying) modulation. This modulation uses 8 symbols to transmit the information, as opposed to QPSK, modulation most commonly used to transmit SD (Standard Definition) services, that uses only 4. This difference makes the possibility of having interference between symbols in an 8-PSK modulation more likely than in a QSPK. So the “purity” of the local oscillator of the LNB is a key parameter to avoid this source of interference that could generate breakage of the picture. The parameter that measures the “purity” of the signals given by the local oscillator (LO) is the phase noise. Televes‘ LNBs have a phase noise higher than 75dBc@10KHz that is what Astra
specifies for satellite digital services. The graph below shows how the phase noise is measured by our quality control department. This is one of the several parameters that we measure when approving a LNB.
As it is known, in general, but particularly in the installation of DTT-SAT systems, the adjustment of the LNB skew is critical. For this reason it is convenient to have a LNB with a high value of polarities discrimination, taking care anyway of adjusting carefully this parameter.
- The NF is a parameter that has been intentionally overrated by some without very little technical basis. In the meantime, other paramters that are important for the quality of the LNB are been ignored.
- The quality obtained from a SAT reception system doesn’t depend exclusively on the NF and, as it has been explained, with DVB-S2/HD signals the phase noise is even more important.
- The H45 is a key tool for a modern and trained installer (there is no other way of evaluating the quality of the system without a proper spectrum anlayzer).
- Televes considers training as a key factor over information only.
To understand the information is even more important than the information itself and hence the reason why we won’t participate in a battle to publish the lowest NF value and instead will train our customers.
- We have to keep in mind that the quality of the system has to be evaluated as a whole, measuring several parameters. The specs from a technical sheet or the measurement of one parameter do not determine the quality of a system.