| 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! Email me
if you need additional help. Jerry K. Thorne, ( jthorne at
n2prise dot org). You will have to enter the email address manually and put
in the proper punctuation. [ @ sign and the period ] |