Blonder tongue AMM-806, Reference Manual

Page: 192 / 285

185
Broadband RF Network Powering
When determining the power supply location in a cable system, it
is important to know OHM’s Law, cable loop resistance, amplifier
current drain and the minimum operating voltage required for
amplifier operation.
The cable resistance is included in the manufacturers specification
sheet. Resistance is usually stated as center conductor, shield,
and loop. Loop resistance is the sum of the center conductor and
shield resistance.
The amplifier current drain and voltage requirements are also part
of the manufacturers specifications.
The ideal location of the power supply is such that the current
supplied flows equally both, away from, and towards the
headend. The total current supplied should be between 80-90%
of the power supply’s rated output. Current passing through
the amplifiers is usually limited to about 6 Amps. Since power
supplies are available with various current handling abilities, one
with slightly higher capability than required should be selected.
Determining power supply placement, refer to the figure on the
next page.
1. First determine the loop resistance for each span of cable.
Example: Loop resistance = 1.7 Ohms/1000
ft. x 800’ = 1.36 Ohms.
2. Locate the last amplifier, determine the
voltage drop across the span of cable.
3. Using the current draw of the amplifier, determine
the voltage drop across the span of cable.
Example: 0.375A x 1.36 Ohms = 0.51 volts.
4. Since the minimum operating voltage for the amplifier
is 43 volts, the voltage level at the beginning of the
800’ span would be 43.5 volts (43.0 + 0.51).
5. At this point, the total current flow in the coax will increase
due to the addition of another amplifier. Current flow
in this span of cable will be 0.75A (0.375 + 0.375).