5. LOW NOISE CONVERTERS

The function of the Low Noise Converter (LNC), which is sometimes also called an LNB (Low Noise Block converter), is to:

• amplify the very low level incoming signals up to a level that can be demodulated by the satellite receiver.
• frequency translate the incoming signals to a lower frequency. It is more efficient to operate at a lower frequency because practical transmission lines have high cable losses at 4 GHz and especially at 12 GHz. The band that is conventionally used for transmission from the LNC to the satellite receiver is 950-1450 MHz.

Power for the LNCs active components is typically 12-15VDC applied between the inner and outer of the coax cable that connects the LNC to feed to the satellite receiver.

In addition to bringing the incoming signals to a suitable level for demodulation it is desirable that as little additional noise as possible be added as part of the amplification process.

Modern 12 GHz LNCs can achieve noise figures of approx 1 dB (or better). There has been dramatic improvement in LNC performance over the last 10-15 years. The WARC-1977 Broadcast Satellite Plan was based on an assumed LNC noise figure of 8 dB.

C band LNCs can have noise figures as low as 0.35 dB. Performance parameters of current generation C and Ku band LNCs are given in Table 2 at the end of this section.

Formulas that may be useful in LNC calculations are listed below.

Conversion of Noise Temperature to Noise Figure

Conversion of Noise Figure to Noise Temperature

Note: The term "Noise Factor" is sometimes encountered. It is the numeric version of the dB term Noise Figure ie Noise Figure = 10 log10 (Noise Factor)

Typical LNC gain figures are in the region of 50 - 60 dB. With such high gain stages the noise contributions from later stages will be negligible unless very long IF cables are used.

These gains can be achieved without overloading or intermodulation problems because (usually) satellite signals are of broadly similar levels. The wide level differences which sometimes occur with terrestrial systems are less likely in satellite systems.

LNC noise figure is affected by temperature. An indication of the sensitivity is given in graph below.

Figure 16: Indication of variation in Noise figure with ambient temperature [Ref 7].

In addition to Australia's high ambient temperatures, LNC designers should also consider the heat focussing effects of the dish (this mainly applies to prime focus dishes). To reduce this effect the dish may be coated with a suitable non reflective paint. It should also be kept in mind that during sun transits a very large amount of heat may be focussed on the LNC, and although signal will be lost during these times (due to the large increase in noise temperature) the LNC should not fail due to the heat stress during this period.

Table 2: LNC data (Source: AV/COMM 1995 Catalogue).

band  Noise        gain       Comments                Cat     Retail Cost    
      figure                                          No.     (A$)           
Ku    1.0 dB (     50-55 dB                           L1100   200            
      75K)                                                                   
Ku    0.7 dB (     50-55 dB                           L1200   260            
      50K)                                                                   
Ku    1.0 dB (     50-55 dB   low drift for SCPC      L1300   470            
      75K)                    reception                                      
Ku    1.0 dB       50-55 dB   dual polarity LNC       L1400   296            
      (typical)                                                              
C     25K (0.35    64 dB                              L1510   280            
      dB)                                                                    
C     20 K (0.30   64 dB      dual polarity LNC       L1551   320            
      dB)                                                                    

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