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)