Technician: Automatic Gain Control

This week’s Tech Class question is pulled from sub-element 4 (Amateur radio practices and station set up), section B (AGC) [T4B12]

What is the function of automatic gain control or AGC?

A. To keep received audio relatively constant
B. To protect an antenna from lightning
C. To eliminate RF on the station cabling
D. An asymmetric goniometer control used for antenna matching

Let’s examine these answers one by one logically, starting from the bottom up.

“An asysmmetric goniometer?”  Considering a goniometer (yep, its a real thing) is a device used by physical therapists to measure the range of motion of, say, your elbow or knee…  I think we can eliminate that one.

RF on the station cabling? That’s done by making sure your station is all properly grounded, connected properly, and if its still a problem, placing a few ferrites on your cables to eliminate the rest.

Protect from lightning?  Nothing will completely protect your antenna from lightning, but proper grounding might help a bit.

Also consider that none of these are related to “gain” in any way, shape, or form.  The only choice that could even be related has to be related to audio.  Your volume control is labelled “AF (Audio Frequency) Gain” for a reason.  AGC circuits have been in radio receivers since the early days, basically it tries to eliminate any hard spikes from the signals, originally to protect your speaker from being overdriven, but now just to keep you sane.  Ideally you won’t even notice it working.

The answer, then, is A) To keep received audio relatively constant.

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