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How Far Can I Run a BNC (Video) Cable?



Hardware: Image Acquisition (IMAQ)>>Analog Image Devices>>PCI-1408

Problem:
I want to acquire images from a camera that is some distance from my computer. How far can I run my BNC cable?

Solution:
There is not a definitive answer. A good approximate answer is 10 meters, if in a relatively low-noise environment. If the environment is noisy (power supplies, etc.), then this will add noise to the signal and cause a poor image, reducing the maximum length of the cable.

Make sure the BNC cable used has the same characteristic impedance (Zo) as the camera and framegrabber being used. Most cameras and National Instruments framegrabber have a characteristic impedance of 75 ohms. Characteristic impedance (Zo) does not vary with length. Reflections due to impedance mismatch can result in "ghost images." Avoid this by using the correct impedance and by avoiding on-cable connectors; don't create a 10-meter cable by connecting five 2-meter sections with BNC adapters.

Attenuation (opposite of amplification) occurs as the cable length increases, causing the video's peak-to-peak voltage to decrease. This can be accounted for using Measurement & Automation Explorer (MAX) with NI-IMAQ 2.2 and later. Select the channel you wish to acquire from on the framegrabber, and select Properties. On the Basics tab, adjusting the white reference level (Volts) adjusts the gain applied to the image. If the signal's peak-to-peak voltage has been reduced, then lowering the white reference level will restore the signal amplitude and display contrast.

If your system requires placing the camera over 10 meters away from the computer, you can use a video line amplifier (not sold by National Instruments) to boost the analog signal over the cable. As mentioned above, the amplifier must have a characteristic impedance of 75 ohms. There are several different models with different power characteristics available in the market today. The model you should choose depends on how far you need to place the camera from the computer.

Related Links:
KnowledgeBase 1649IHHL: What Do Gain, White, and Black Reference Mean in IMAQ Analog Image Acquisition Boards?

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Report Date: 10/13/1999
Last Updated: 09/11/2012
Document ID: 1QCBD9YJ

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