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September 25,
1997
Camera
power and video use same coax cable
Frank Cox, Linear Technology
Corp, Milpitas, CA
Because
remotely located video-surveillance cameras do
not always have a ready source of power, it is
convenient to run both the power and the video
signal through one coax cable. One way to
accomplish this task is to use an inductor to
present a high impedance to the video and a low
impedance to dc. The problem with this method is
that the frequency spectrum of a monochrome video
signal extends down to at least 30 Hz. The
composite-color video spectrum goes even lower,
with components at 15 Hz. These figures entail a
large inductor. For example, a 0.4H inductor has
an impedance of only 75 ohms at 30 Hz, which is
approximately the minimum necessary impedance.
Large
inductors have a large series resistance that
wastes power. More important, large inductors
have significant parasitic capacitance and stand
a good chance of going into self-resonance below
the 4-MHz video bandwidth, thus disrupting the
signal. The circuit in Figure 1 takes a
different approach to the problem by using all
active components.
The
circuitry at the monitor end of the coax cable
supplies all the power to the system. IC1,
a power op amp, forms a gyrator, or synthetic
inductor. The gyrator isolates the low-impedance
power supply from the cable by maintaining a
reasonably high impedance over the video
bandwidth while contributing only 0.1 ohms of
series resistance. This op amp needs video
bandwidth and enough output drive to supply 120
mA to the camera. The selected part has a
guaranteed output current of 250 mA and a 3-dB
bandwidth of 60 MHz, making it a good fit.
Because the video needs capacitive coupling, no
need exists for split supplies; the circuit thus
uses one 24V supply. This supply also gives some
head room for the voltage drop in long cable
runs.
The
camera end uses a 12V fixed-voltage regulator to
supply 12V to a black-and-white CCD video camera.
IC4 supplies the drive for Q1,
a fast, high-current transistor. Q1,
in turn, modulates the video on the 20V-dc line.
The collector of Q1 is the input to
the 12V regulator. This point is at ac ground,
because it is well-bypassed by capacitors, as IC3
requires. IC1 is configured to deliver
20V to the cable. Because the 12V regulator at
the camera end requires 1.5V dropout voltage, the
series resistance of the cable can produce the
6.5V voltage-drop balance. The 20V output of IC1
gives head room between the supply and the video.
IC2, a video-speed op amp, receives
video from the cable, supplies some frequency
equalization, and drives the cable to the
monitor. Equalization compensates for
high-frequency roll-off in the camera cable. The
values for R1 and C1 yield
acceptable monochrome video with 100 ft of RG58
B/U cable. (DI #2087)
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