Satellite - Choosing the proper LNB

Satellite - Choosing the proper LNB

Some manufacturers or distributors promote the LNB's announcing very low values. Is this realistic? This article explains why you should be careful with this subject and points to consider when choosing a good converter. It also discusses what the phase noise more significant at the reception that the noise factor. 

To understand the different parameters that define a LNB, it is best to analyze your performance.

The schematic of figure 1 further allows the distribution of the signal, from the moment it is collected in small antennas (H and V) to its output by taking F in Satellite Intermediate Band (BIS).

We realize that each of the antennas is followed by a triangle; this represents an amplifier. It is the first of the chain and also the most important because the signal level is very weak. This amplifier has to introduce minimum noise, like all electronic circuits that follow.

The fact is that the noise never dims the contrary, when it is amplified, this will be more. It is this principle that requires us not to use existing facilities in complementary amplifiers between the LNB and the digital receiver.

If we follow the distribution of the signal, we will see a rectangle, Band Pass Filter or bandpass filter (BPF) which allows to get rid of unwanted frequencies. This filter is followed by a mixer (MIX), which acts as BIS frequency = received frequency minus local oscillator frequency (OL), plus other filter Low Pass Filter or low pass filter (LPF) and the two stages final amplification.

This succession of circuits dedicated to well-defined functions are all sources of signal degradation; all of them must meet two requirements: minimum noise and phase noise distortion, too low.

Remember that every driver or semi-conductor, when it is crossed by a stream, is the source of an "atomic" and thermal agitation. For current there are free electrons in motion. To this stirring power dissipated essentially by rubbing, which will correspond an increase in temperature of the driver or the semiconductor corresponds. 

From this phenomenon, a temperature called "noise temperature," which is given in Kelvin (K) is defined. This is related to the power dissipated by the intermediary of the Boltzmann constant, the actual temperature of the conductor or semiconductor and the frequency band in which the component works. Immediately we understand that this temperature noise (noise factor and rightful) can not be zero unless the component is at 0 K, that is, to -273 C. As this temperature or noise factor noise depends directly on the ambient temperature, the higher it is, the worse the noise figure; this explains the degradation of the characteristics of a LNB, when it is too hot.


Defining the noise factor 

The real LNB noise factor F is defined from a reference temperature T0 by a simple relation F = KTO LNB. If T represents the temperature LNB LNB noise, the noise figure is explained by the relation F = 1 + T LNB LNB / or 290, T = 290 LNB (FLNB-1). For the "noise figure" Noise Figure NF or, in English, simply turn the noise in decibels with NF = 10log10 F or F LNB LNB NF = 10/10 ratio.

Take a concrete example: If a LNB has a noise figure of 0.6 dB NF, the noise factor F will worth 1.14 LNB and LNB noise temperature T, 42.96 K. Table 2, We find the correspondence between noise figure (in decibels, dB) and noise temperature (K). Thus, we find that the lower the value of noise, also lower the noise temperature.

Similarly, in the table are two columns: one gives the noise level in dBuV and the other, the same level in uV, calculated at a load of 75 Ohms. This correspondence is the definition of the noise temperature, which corresponds to a power dissipation in watts.

It is therefore logical to convert this temperature noise level corresponding signal. It is the minimum value of the noise level as observed at the output of the LNB. Remember that what is meant by "signal / noise" or S / N actually corresponds to the ratio of the amplitude of the useful signal to the amplitude of the noise present in the signal: the greater the relationship (also explained in decibels), the better the "readability" of the signal in relation to such noise.


Noise factor and digital signal 

In the case of analog signals, the noise and the signal / noise ratio are two key elements. In the case of digital signals, if these parameters are to retain good values, these are not the only important: the phase noise is the most important parameter significant and often.

Why is this so? For the transmission of digital signals using the quadrature amplitude modulation or QAM, which allows to obtain, from two signals I and Q christened, a constellation of points corresponding to the transmitted symbols.

In the case of satellite (DVB-S) they are used only four points or states, which correspond to the four bases of a square. This particular modulation is called 4-QAM or Quadrature Phase Shift Keying (QPSK). In the case of cable (DBC-C) and digital terrestrial television, 64 stages to provide a stronger signal is needed: we are talking of 64-QAM. If these different stages or points do not occupy their positions (the four bases of the square to the QPSK) data decoding is disturbed: pixelations and cuts appear in the image.

This dispersion of the stages results in a gap that affects the transmission: it is what is technically known as phase noise. These four stages should remain stable for demodulation is assured, regardless of the frequency of signal transmission, especially the laying of the BIS, that is accurate.


Phase noise 

Returning to the LNB, we understand that all circuits amplification, filtering and frequency conversion can be the source of the phase noise.

To avoid these problems all circuits must be perfectly studied and be subject to measures, so that they can see these effects. This is the reason that a serious manufacturer must take into account the outcome of the measures and not be content only to give the value of the noise factor to digital reception.

To appreciate the dispersion phase noise of these points is measured by a 360 ° cycle. This measurement is made in relation to the local oscillator frequencies in a given frequency band in relation to this one (1 kHz, 10 kHz, 100 kHz and 1 MHz). The measurement result is explained in decibels per Hertz cycle or dBc / H. 

This type of measurement can only be performed in a laboratory. We give the minimum values ​​of the spectral phase noise of -50 dB @ 1 kHz, 10 kHz @ -75 dBc and -95 dBc @ 100 kHz.

As an example, Figure 3 reproduces the characteristics of a brand LNB Swedish Microwave (SMW). It noticed that the given values ​​are better than the minimum required. We can also note that the noise factor is given only 0.8 dB.

Everything we have said so far applies not only to the LNB, but also to all players in the transmission components, especially the switches, which may also have phase noise, like a polarizing magnetic.

Carefully studying Table 3, we look at the line "Output VSWR". Here, VSWR refers to the ratio ROS or standing waves. Also this parameter is important in a facility: Explains the power to facilitate the transit of the signal between the source and the receiver and make the most of energy: the most important is the ROS, the lower the energy transmitted. But if you do not arrive at your destination, then it means that creates disturbances.

To set a measurement, the value should not exceed 2 ROS (corresponding to 89 percent of the energy transmitted); This value appears in the features table 3. In addition, this value should not be exceeded in the entire width of the band BIS. Otherwise accidents will occur, as the disappearance of certain programs (at the frequencies corresponding to high values ​​of ROS).

In a word, an LNB, like any other part, has to have a linear response as possible. Its characteristics are to be stable throughout the BIS band.

For a manufacturer is tempting to give the best values, but what frequencies? Long ago, the LNB's integrated a chip that listed their control characteristics throughout the BIS band and manufacturers often granted the worst and not the best value.

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