14-05-2013, 12:19 PM
Internal Technical Report on DTH -GMRT Co-existence
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Overview of DTH Systems
Direct to Home are nothing but the Direct Broadcast Satellite Television and Radio Systems. Geostationary satellites play an important role for DTH systems. In general, DTH service is the one in which a large number of channels are digitally compressed, encrypted and beamed from very high power Geostationary satellites. The programs can be directly received at homes. Also, DTH transmission eliminates local cable operator completely, since an individual user is directly connected to the service providers.
An individual user has a small dish usually 45 to 60cm in diameter and Low Noise Block Converter (LNBC) pointed towards satellite. At home digital receiver i.e. Set top box is connected to TV which receives digitally multiplexed channels from LNBC and gives RF output for TV.
The satellite transmission is usually in Ku-Band. The digital channels are first multiplexed and then QPSK modulated before transmission. The small dish along with LNBC receives the signals and LNBC converts these Ku band signals to Intermediate Frequency based on the local IF which is typically 10.7GHz. Now, the set top box receives the down-converted satellite signals and performs the demodulation and de-multiplexing and finally D to A conversion before making signal competent to TV.
The DTH receivers available in the Market are affordable and the use of such systems is nowadays increasing dramatically in urban as well as ruler areas.
Various Spectrums in LNB Cable
For estimating possible RFI from DTH systems, one must know the exact spectrum available in the cable from LNB (Located at roof tep) and the set top box (Located in the house). Because the chances of radiation are from the cut to the cable, bad connector installation or bad cable itself. So to understand the spectrum, we have tapped the cable as shown in the following block diagram.
DTH-RFI estimates for GMRT
As seen from the various spectrums, lot of broadband as well as narrow band noise does exist in the LNB cable, which has no use to DTH reception but may cause RFI to GMRT, if the part of the conducted power gets radiated out. Two types of noises are observed, one broadband and another narrow band. One has to treat the two types in different manner. Our approach is to estimate the possible RFI to GMRT from DTH, by computing the minimum separation distance of DTH system from GMRT antenna, so that lTU-R requirements are met.
Controlling LNB
To understand the exact spectrum at LNB o/p, it is important to control LNB without the use of set top box. This can be achieved by simulating the signals given by the set top box to LNB by some other circuitry with proper impedance matching. The set top box gives 22kHz tone to LNB to set LO as well as proper DC voltage for power supply and polarization selection.
So to start with, we have used the Intersil 8038 Function Generator IC to generate 22KHz tone and then added proper DC voltage to it. Following Fig. shows the circuit diagram.
Conclusions
Under the (perhaps pessimistic?) assumptions of the radiation behaviour of the coax cable and connectors' used in the DTH system as in pages 15 and 16,
1.Broadband noise will have negligible effect on GMRT Observations, as the minimum separation distance is 90 meters with the assumption that there is no DTH system in 100 meter circle from any of the GMRT antennas. Care must be taken for arm antennas.
2.Narrow band noise can cause RFI, in spectral line observations below 400MHz, iflocated at about 2 km from a GMRT antenna.