NOW, DO THE WORK!
Now that we have seen samples of the cable loss data tables that are published by various industry cable and passive device suppliers, it is time to get the specific cable loss data and passives loss data together and do the work. First thing to do is to create a simple line drawing of the cable routes from building to building and/or within buildings. The key features should be indicated. Use a triangle to represent an amplifier providing forward gain from left to right on your chart. Use round circles to represent splitters and directional couplers in the trunk or feeder lines. Use your choice of small squares or Octagons to represent the taps. Make these large enough to write a two-digit number inside the square or hexagon or adjacent if desired. The number will represent the tap loss value. See the graphics below for examples.
Using a combination of the symbols above, create a trunk tree diagram with the cable distances between desired cable splitter or tap locations. As each leg of the proposed distribution is drawn, the choice of splitter versus directional coupler will become evident as a result of the cable length or passive losses on each leg. Draw in the location of the required taps and whether they will need to be 2-port, 4-port, or 8-port taps at each location. When this phase of the drawing is completed, the loss of each length of cable can be calculated. Tap values will be assigned when RF signal levels are calculated.
Now that we have a sample layout, the RF output level of the amplifier can be added to the diagram and the signal levels in and out of each length of cable and passive device can be determined. In the diagram above, the single TV set shown has a 60-foot length of RG-6 drop cable attached to an 8-port tap. The desired signal level at the TV set should be not less than 0 dBmV at any frequency. An ideal signal level should be around +3 dBmV to +5 dBmV. Due to the differential slope attenuation of the coaxial cable and the passive devices, the signals arriving at the TV set may have a different strength at the highest and lowest RF channels. This difference value (slope) should not exceed 12 dB maximum. The smaller the slope difference, the better for the TV tuner to select the desired channel with minimum interference from the other TV channels.
SIGNAL LEVELS - FILL IN THE BLANKS
A computer spreadsheet is an easy way to determine RF signal losses for high and low forward RF distribution frequencies. If a two-way active design is desired, the highest and lowest return path frequency losses should also be calculated. A sample spreadsheet is shown below. Remember to enter the data in a format that will account for the loss per 100-foot values given by cable manufacturers. Entering the values with the decimal point moved two places to the left will allow direct footage entries.
The table above shows cable footage and passive device descriptions in the first column on the left. Cable losses and passive losses are shown in the second and third columns. In a spreadsheet, the formula in each of these cells would be entered to multiply the cable footage in the first column by the loss per foot near the top of each frequency column. The formula in cell B4 would take the form of +$A4*B$2 and display the value 0.48 dB loss as shown in the 5 MHz column to the right of the 300 feet of cable distance in the first column. The dollar signs in the formula will insure that each formula copied below that cell will point to the cable distances in column A and the correct loss of the cable per foot at that frequency. That same formula can then be copied into all the cells in that column and in columns C, D and F. When a cable footage is entered in column A, the attenuation at 5 MHz will appear in column B. When a passive device is entered in row cell in column A, enter the insertion loss for that device at the designated frequencies, replacing the formula in the appropriate cells on that row in B, C, D and F columns. With all the losses in place, the RF signal levels can be found and displayed in columns E and G.
The RF amplifier output level at 50 MHz is entered in cell E2. The 750 MHz output level is entered in cell G2. Each cell in column E and G, beginning with row 3, will have a formula which takes the signal level from the cell above and subtracts the loss of the adjacent cell to the left from the value above. The formula in cell E3 would read +E2-D3 and display the value of 31.1 dBmV. Copy that formula in all cells below that one in column E. A similar set of formulas would be in column G to calculate the signal levels at the output of each device or after a length of cable at 750 MHz.
Looking at the chart, it becomes obvious by looking at the RF signal levels in columns E and G, that the cable and passive losses are reversing the slope output of the amplifier as the signal propagates down the signal path. This chart represents the path from the RF amplifier to the end of cable A as shown in the sample diagram on the previous page of this article. It appears that one more tap and a short length of cable may fit into this scenario before a second amplifier is required. Be sure that the highest frequency has at least 3 to 5 dBmV more signal than the rated noise figure of the amplifier that is going to be used. This is discussed in the technical article "Amplifier Operational Sweet Spot." A cable equalizer installed in the input of that next amplifier will insure a uniform carrier-to-noise ratio (C/N) for the RF signals. Choosing the correct equalizer is also covered in a technical article titled
"Fixed Equalizer Selection Theory and Chart."
With all this said, it is time to build your own system, using real cable and passive loss data from your system as it exists or is proposed to meet the cable distances involved. It may be necessary to increase the output levels and slope of the amplifiers used in the system to meet spacing requirements. Larger cable sizes may have to be used. Lower bandwidths ease this problem, but signal levels reaching the television sets are not negotiable. Good Luck! Call or email me if you need additional help. Jerry K. Thorne, Sales Applications Engineer, Quality RF Services, Inc. (Note: Since Jerry has moved on to other endeavors, you may contact jimgoins@qrf.com with any questions.)
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