Automatic Gain Control, Part 01:   How AGC Circuits Control Levels, And Why We Still Need Them In Mo


My company, 4Cable TV, was recently called in to consult on an automatic gain control (AGC) problem. The customer stated that the AGCs in his amplifiers that had been sent out for repair were no longer working. They would install the modules, then within two to three days, the levels would be off by two to three decibels.

I asked if they knew why the amplifiers had AGC modules. The customer said, correctly, “To keep the levels constant.” But, funny enough, they didn’t know what was changing. More and more, we run into folks in the industry who lack a working knowledge of important fundamentals. You see, what the customer didn’t know is that transmission loss in a coaxial cable changes with the temperature at a predictable rate of 0.1 percent per degree Fahrenheit. If it gets cold, the cable attenuation goes down, and if it gets hot, the attenuation goes up. This means that if your ambient temperature changes by 10 degrees, your cable loss will change by 1 percent.

It may not seem like much, but if you go to a place like Bishop, CA, where temperatures can swing 100°F in 24 hours, a lot can happen. Now let’s say that you have a three-amplifier cascade with 30 dB spacing, for a total of 90 dB of cable transmission loss. If you set your levels in the afternoon, when the temperature was 100°, then the temperature fell to a low of 0° overnight, your cable loss would have dropped by 9 dB. Without controls of some kind, the levels at the end of your line would have increased by 9 dB, and your system would be in trouble.

Today’s amplifiers are comparatively very temperature-stable. However, they do vary with temperatures by 0.5 - 1.0 dB, and usually the gain goes up when the temperature goes down, potentially adding another 1.5 - 3 dB to your already 9 dB too-high signal.

Fortunately, in a modern CATV system, we have AGC, or as it is also called, ALC, for “automatic level control.” Both terms are correct. The circuit controls the output level of the amplifier by automatically adjusting its gain.

In a modern AGC circuit (above), a sample of the output signal is split off using a directional coupler and fed to a band-pass filter, which selects one frequency or channel to used for measurement and control.

In the early days of cable, special frequencies, such as 73.5 MHz and 166.5 MHz, were inserted just for level control. As the industry matured, actual channels were used in lieu of pilots for more efficient use of the spectrum. |||||

Steven K. Ritchey is the founder and President of 4CableTV International

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