Thursday, February 2, 2012

Technical Report

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.
DD Direct: DD Direct is launched by Doordarshan. It operates from NSS-6 Satellite and gives 33 free to air channels and 13 radio channels. The transmission covers most of the India. The cost ofthe Dish, LNBC and Set top box is around Rs. 2500/-. With this setup only free to air channels are visible.
Dish TV: Dish TV is launched by Essel Group. The Dish TV has different set top box with Smart card facility to decode paid channels. The cost of the unit is around Rs. 4000/-. So user can watch paid as well as free channels and radio programs. The user has to pay monthly rental for paid channels. The entire 'Zee Network' channels are available on Dish TV. Dish TV Transmission is also from NSS-6 Satellite.
Specifications of NSS-6 Satellite: Location: 95 degree East No. of Ku-Band Transponders (36MHz wide): 60 Saturated EIRP: 44-55 dBW. Ku Band Uplink: 13.75 to 14.5 GHz Ku Band Down links: 10.95 to 11.2 GHz
11.45 to 11.70 GHz
12.50 to 12.75 GHz Modulation Type: QPSK Symbol Rate: 27.5 Mb/s. Downlink for DD Direct: 12815, 12534,12898 GHz. Look Angles for GMRT: Azimuth= 130.51 degree from North.
Altitude= 57.26 degree

Antenna & LNB Specifications
Antenna Specifications: Reflector
Size Type Frequency Antenna Gain 3dB Beam width 10dB Beam width Aperture efficiency Surface Accuracy Material Elevation Angel Range Cross Polar Discrimination on major aXIS. VSWRmax Noise Temperature Offset Angle FID ratio
LNB Specifications: up
Range Local Oscillator Noise Figure Noise Temperature Gain 65cm Offset Feed
10.7 to 12.75GHz > 36.75dBi @12.75GHz
3.2 degree
5.2 degree >70% < 0.01" Steel! Aluminum 15 to 50 degree >27dB
1.3 < 35 Kelvin 25 degree
1O.7GHz to 12.75GHz 9.75GHz or 10.6GHz 0.5dB 35 Kelvin @ 290 degree 55dB

Set Top Box
As per the diagram, the set top box accepts the entire down converted band and separates out the individual transponder frequency. Then signals are first converted to fixed IF and then QPSK demodulated. The bandwidth of QPSK signals is 27.5 Mhz as the bit rate is 27.5 Mb/s. It is observed that 11 digital channels are multiplexed in 27.5 Mhz bandwidth. The power supply for LNB, polarization selection signals as well as LO setting signals are send by the set top box itself by using the same cable between the LNB and set top box.
After the QPSK demodulation, the digital bit stream obtained contains several multiplexed channels as well as error control bits. The bit stream is processed to correct and detect errors, de-interleaved, and decrypted. A digital demultiplexer then extracts the bits for wanted channel, and send them to MPEG decoder, and finally generates analog Audio and Video signals with DIA converters to drive TV set.
The paid channels are encrypted, and a smart card having the correct key for decryption is required to view the paid channels. The key is provided by the paying monthly rent by the user.

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.
The Set Top Box we used has brand name 'Koship' and the LNBs are 'Colorado', 'Gardner', 'Broadcom' make.

Important Observations

I. LNB Polarization is changed by changing the DC voltage from the Set top box.
2. 14 V is used for Horizontal and 18 V for vertical polarization.

3.To set LO of the LNB, 22 Khz 1 Vppk sine wave is sent from set top box.

4.LNB IF output is broadband, typically 100MHz to 2.5GHz.

5.At LNB output, apart from the genuine satellite signals, the broadband as well as Narrow band noise exists.

6.For Broad band noise, peak power observed is -65dBmllOOKHz for Horizontal polarization.

7.Lot of spurious having broadband nature is seen in the GMRT band, which may be because ofthe inter-modulation products produced in the LNB cable.

8.For Narrow band noise, peak observed is -70dBm.

9.The Narrow band lines are because of the microprocessor and clock circuitry in the set top box, as even if the LNB is disconnected, the Narrow band lines exists.

10.Narrow band lines are more between 100MHz to 400MHz. is possible that this power can get radiated from the poor cable causing RFI to GMRT.
12.The set top box available in the market, usually consists of a embedded card which includes the majority of the set top box circuitry, SMPS and display card.
13.The set top box has 'loop out' terminal, which duplicates the LNB spectrum, so that one set top box can drive another one.
14.The experiment was conducted on 25 Feb 05, in which the DTH system was mounted on Guest House and kept ON -OFF -ON for 3 minutes each and central square antennas were pointed towards Guest House. We have not found any correlation between DTH ON and OFF and change of Power at GMRT Correlator output.

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.

Analysis of Narrow band noise

1.Let the wire along with the connectors, acts as a radiator with OdBi gain.
2.Let the radiated power from the cable is 60dB dOWll from the conducted power. (Assumption is based on the European Emission Standard EN50081-1 which perhaps would be the worst-case)
3.The peak narrow band power observed is -70dBmlHz.
4.As per the ITU-R standards for Radio Telescopes, the harmful level ofRFI for GMRT in spectral line mode is -239dBW/m/\2-Hz.
Calculations: Power in the cable, Pc = -70dBmlHz
Pc = -lOOdBW/Hz
So the radiated power is 60 dB down Pc;
Pr = -160dBWlHz.
Now the Flux at the distance R from the source is given by;

Thus if the separation distance of DTH system and GMRT antenna is ~2Kms, then the narrow band noise under worst case would cause RFI which will just touch the GMRT sensitivity.

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.

Though this circuit gives exactly the same signal and DC voltage as measured in the LNB cable, but when connected to LNB, circuit was unable to control LNB. This may be loading caused by impedance mismatch, and o/p stage of this circuit must be designed for 750hm. Additional effort to understand and solve the problem could not be put because of time constraints.


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.