Quality RF Services, Inc. CATV / MATV Design Fundamentals

Page 5

SIGNAL LEVELS - FILL IN THE BLANKS
A computer spread sheet 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 spread sheet 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.
A B C D E F G P3-500 Cable dB Loss per foot  >> 5 MHz 40 MHz 50 MHz RF Level 750 MHz RF Level 0.0016 0.0044 0.0050 35 0.0204 40 2-way splitter -3.8 -3.9 -3.9 31.1 -4.9 35.1 300 -0.48 -1.32 -1.50 29.6 -6.12 29.0 23 dB, 4-port tap -0.5 -0.7 -0.7 28.9 -1.4 27.6 80 -0.13 -0.35 -0.4 28.5 -1.63 24.9 20 dB, 4-port tap -0.6 -0.7 -0.7 27.8 -1.5 23.4 80 -0.13 -0.35 -0.4 27.4 -1.63 21.8 17 dB, 8-port tap -1.6 -1.7 -1.7 25.7 -2.6 19.2 80 -0.13 -0.35 -0.4 25.3 -1.63 17.6
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 spread sheet, 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 fomula 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 propogates 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, Applications Engineer, Quality RF Services, Inc.  jthorne@qrf.com       < Click for Technical Articles Menu >

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