Design Guidelines for Large Satellite IF Distribution Systems
As a starting point, assume a level of -35 dBm per transponder out of the LNB. This will of course vary with satellite footprint EIRP antenna size and LNB gain.
A simplified calulation to determine LNB output level is shown below.
In clear sky conditions: LNB output (dBm) = Footprint EIRP (dBW) + 30 - Path Loss (dB) + Antenna gain (dB) + LNB gain (dB).
Assume path loss = 206 dB (at 12.45 GHz); antenna gain = 34.0, 38.5 or 40.7 dB for antenna size of 0.46, 0.60 or 1.0 m (at 12.45 GHZ); typical footprint = 51 dBW; typical LNB gain = 56 dB.
Example:. LNB output level = 51 + 30 - 206 + 34 + 58 = -35 dBm
Design for a level at secondary distribution points in the range -45 dBm to -30 dBm per transponder. This will typically be in the range of -10 dB to +5 dB relative to LNB level.
Home run systems are those where each outlet has its own coaxial cable from the outlet back to the secondary distribution point. In a home run system, the secondary distribution points will typically be located at a point convenient to feed a particular group of outlets, this may be at the headend.
Systems serving several floors are often more conveniently designed with a cascade, tapped-trunk architecture. Here the secondary distribution points will typically be located at a central point on each floor.
Some systems will be most conveniently designed as a combination of home run and cascade networks, forming a `tree and branch" structure.
Design for a level at the customer outlet of -55 dBm to -35 dBm per transponder.
Perform calculations of signal level at the highest frequency in use. In the United States this is generally 1450 MHz. In South America, it is generally 1750 MHz.
2. Measurement of Signal Level
An installer of medium and large satellite IF distribution systems requires a spectrum analyzer. The spectrum analyzer will allow him to measure the signal level of each transponder. Without a spectrum analyzer, a system may be constructed, and if it does not immediately function correctly without adjustment, the installer is " in the dark" as to why it doesn't work properly. He will then probably resort to changing parts in a random manner; wasting both time and money. The following are just a few of the problems that can be instantly recognized with a spectrum analyzer.
a. Signal levels within the desired range.
b. Acceptable flatness across the spectrum ie. the difference in level between lowest (950 MHz) and highest transponder (1450 MHz).
c. No unusual notches or "suck-outs". This effect is usually called by poor coaxial cable or type F connectors.
d. No ingress or other interfering signal which may cause certain transponders to be degraded.
The following are guidelines to the use of a spectrum analyzer with digital QPSK signals as found in DBS TV applications. The total occupied bandwidth of a digital DBS signal is 24 MHz with the power distributed fairly evenly across the 3 dB power bandwidth of approximately 20 MHz. As a result, the appearance on a spectrum analyzer when viewing the 16 transponders at 950-1450 MHz IF is similar to viewing a square wave on an oscilloscope.
The occupied bandwidth is much wider than the resolution bandwidth (IF filter) of most spectrum analyzers which are typically 1 MHz or 3 MHz in their widest setting. As a result, the displayed signal level is much lower than the actual level.
The correction in dB to be applied depends on the ratio of the analyzer's resolution bandwidth to 20 MHz from the following formula:
dB correction = 10 log (20/RB)
RB is the resolution bandwidth in MHz
This figure calculates to 13 dB for an analyzer with a I MHz resolution bandwidth or 8 dB for an analyzer with 3 MHz resolution bandwidth.
Some analyzers have been specifically designed for this application and use IF filters wide enough to pass the digital OBS signal without truncation. In this case, no correction need be applied.
The following procedure may be found useful for measuring satellite signal levels at IF with a spectrum analyzer:
1. Start by taking care not to apply DC to the analyzer unless designed to handle it.
2. Set the frequency span from approximately 900 to 1500 MHz. This will allow any slope in the system to be identified.
3. Set the input attenuator to an appropriate level. Expect approximately -35 dBm per transponder out of the LNB.
4. Set the analyzer to the widest resolution bandwidth.
5. Maximum video filtering is useful in achieving a reduced noise display.
6. Read the level at the top of the "square wave".
7. Add the correction described above to this level.
3. System Size Limitations
The same factors that limit the reach of a CATV system apply to satellite IF distribution systems. Specifically:
a. Intermoduation Distortion (IMD products). These are mixing products created by amplifier non-linearities. We recommend that the final component in a cascade have no worse than -40 dBc IMD products at its output. See the section on line extender amplifiers (page 9) for gudelines on input levels to limit production of lMD products.
b. Noise. As signal levels are kept down to keep IMD products in check, noise can start to degrade the signal. This problem is less severe than in CATV due to the high Ll"lB gain and relatively low C/N signals.
c. Slope build up. It is difficult to maintain a flat response with many units in cascade.
In summary to limit the effects of the above, we recommend that distribution systems be designed for no more than four active units in any one cascade line from the headend to the customer outlet. Active units include headend amplifiers, line extender amplifiers and active multiswitches (not passive multiswitch models 62221FD and 62321FD); eg. one headend amplifier, two line extender amplifiers and one active multiswitch in any one line from headend to customer outlet is the maximum cascade we suggest.
4. Home Run Distribution Systems
Home run systems are those where each outlet has its own coaxial cable from the outlet back to the secondary distribution point. Refer to the example in Figure 1. The secondary distribution points will typically be located at a point convenient to feed a particular group of outlets. In small systems, this may be at the headend itself.
In a voltage switched system, mutiswitches or a single output switch are connected to each pair of cables at the secondary distribution points.
The signal level at the secondary distribution points may be calculated relative to the LNB level for the system in Fig. 1 as follows:
At the highest frequency in use (1450 MHz in US): Total loss between LNB and secondary distribution points = 2801 IFD loss + 2201 IFD loss + Cable loss = L dB 52161FD gain at 1450 MHz = 16 dB, therefore, Level at secondary distribution point = LNB level + 16 - L dBm.
Fig. 2 shows a voltage-switched, home-run system with 64 outlets. Each multiswitch could be located in a convenient position to feed a group of four outlets. The multiswitch input signals could be taken from the headend in Fig. 1.
Home run systems are the easiest to set up and give fewest problems, particularly in the deployment phase of the project. This is unlike cascade systems where a problem in one component is likely to hurt all outlets "down the line".
5. Cascade Tapped-Trunk Distribution Systems
Refer to the example in Figure 3. These systems are particularly convenient for serving several floors with a secondary distribution point on each floor. An example of such a system is shown in Fig4. In voltage switched Systems, multi- switches are located at the secondary distribution points, forming a "tree and branch" structure. When only two voltage switched outputs are required on each floor1 multiswitch taps models 62221FD and 62321FD as described on Page 17 and 18 provide an economical and attractive solution.
When calculating the level at the output of each tap, consider:
The tap loss,
The thru loss of previous taps in the cascade,
Prior cable losses.
The gains of a headend amplifier and any line extender amplifiers.
An example of signal level calculations for a cascade, tapped trunk system is shown on Page 18.
When it is required to install a multiswitch or single output switch on each floor, dual polarization directional tap, Model 2112 IFD as described on Page 8 is recommended. When the -12 dB tap value is too low, an attenuator can be placed on the tap output. Attenuator Models 2803 IFD, 2806 IFD and 2810 IFD are recommended.
In cascade systems it is of extreme importance to use only components having high return loss characteristics in the trunk path. This is to minimize ripple in the passband. Dual directional tap model 2112 IFD and the two output multiswitch taps Models 6222 IFD and 6232 IFD have excellent return loss characteristics.
When it is required to run several trunk lines by placing splitters on the output of the headend amp, Model 52241FD may be preferred over Model 52161FD.
When system losses are such that amplification is required in a cascade system, dual polarization high level line extender, Model 5217IFD as described on Page 9 is recommended. Amplifier Model 521 7IFD may be cascaded with an amplifier placed in the line after losses equivalent to the amplifier gain at the highest system frequency (1450 MHz in the US), ie. after each 18 dB of cable loss and tap thru loss, place an amplifier. Such a system is shown in Fig. 5. Note the use of the 521 6IFD as the headend amplifier. This is particularly suitable for use with the 521 7IFD as the amplifier sections are identical.
Care must be taken to keep intermodulation distortion products down to an acceptable level. IMD products of -40 dBc out of the final amplifier are generally considered the limit of acceptability. This will limit the number of amplifiers in a cascade and/or the signal level into each amplifier. Refer to the guidelines on Page 9 for keeping IMD products down to an acceptable level. Channel Master products are specified at an input level for -40 dBc IMD products, reducing the input level will significantly reduce the IMD products.
In cascade systems particularly, the slope or tilt of the signals can become a problem. This is the difference in level between the highest frequency transponder and lowest frequency transponder. The coaxial cable loss verses frequency characteristic is the main reason why transponders near the high end of the spectrum (1450 MHz) tend to become attenuated relative to transponders at the low end (950 MHz). Coaxial cable loss in dB at frequencies F2 and Fl follows the rule:
F2 loss (dB) Fl loss x square root(F2/F1)
Typical cable loss data is shown on Page 13. Try to keep the signal flatness within 7 dB at the customer outlet. Channel Master headend amplifiers, line amplifiers and high level line extenders include cable slope compensation to minimize this problem.
6. Carrying VHF/UHF Signals on the Distribution System At Channel Master, we recommend that in most cases, a third cable network should be used to carry VHF/UHF signals for off-air local broadcasts EXCEPT for the final "drop" to the customer's outlet. In voltage switched systems, this usually means between the muitiswitch output and the customer outlet. Most Channel Master multiswitches combine (diplex) the satellite signals and VHF/UHF signals on to one cable at each multiswitch output. At the customer's outlet, a 4001 IFD or diplexed outlet plate may be used to separate the satellite signals from the VHF/UHF off-air signals.
In particular, we do not recommend having satellite IF signals and VHF/UHF signals on the same cable where amplification is required in the system. In this case, the likelihood of problems due to intermodulation, signal level variations, slope, etc., is just too great.
Additional diplexing is another area connected with the carrying of VHF/UHF and satellite signals on one cable which can create problems. Here the extra group delay from each diplex filter builds up and can degrade the digital QPSK signal. This problem is usually evident on the lowest (near 950 MHz) transponder. in summary, one pair of diplexers is the maximum we can recommend using .
In a totally passive distribution network, VHF/UHF signals may be added (diplexed) on to the same cable as the satellite signals at the headend. See the example headend on page 18. The following Channel Master products are particularly useful in such networks. These products are passive (therefore no distortion products can be created) and ultra-wideband (40-2050 MHz): Ultra wideband multiswitch taps, Models 62221FD and 62321FD. See example network on page 17 The single output switch, Model 6101 IFD is used with the ultra wideband taps (eg Model 2112 IFD) and splitters (Models 2271 IFD, 2471 IFD and 2871 IFD). The VHF/UHF and satellite signals are then separated at the customer outlet. See the example network in Fig. 6. When designing a VHF/UHF distribution network, the system designer should consider using the passive components from Channel Master's MATV (806 MHz) range. These will generally provide a lower cost solution than the ultra wideband units. These products are: Splitters (2, 3, 4 and 8 way), Models 7992, 7993, 7994, 7998. Directional Taps, one output (9, 12, 16, 20, 24, 30 dB), Models 7109A thru 7114A.
Recommended distribution amplifiers for VHF/UHF signals are:
Model 7335C for for off-air VHF/UHF.
Models 7034C, 7054C for CATV VHF.
For a complete description of these products, request the Channel Master® MATV/SMATV catalog and the Antennas and Accessories catalog.
A typical 3 wire distribution network carrying VHF CATV signals from the ground up is shown in Fig. 7.
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