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What is CTCSS?
Article by Alan Hill, N5BGC

What is this tone that we need in order to use the Tesuque Peak repeater?

With the quantity of radios in place on Tesuque Peak, there is a very great potential for interference to our repeater receiver. Commercial radio equipment has been using this scheme for years to either cut down on interference or allow several to use the same repeater and have an almost private repeater.

The tone is injected to the modulator after the filtering of the microphone circuits. These filter circuits shape the audio response from 300 to 3000 Hertz to be fed into the modulator. In the receiver, this tone is filtered out and not heard. It is used in the repeater control circuits to allow the transmitter to be keyed only when that tone is present. There are about 35 tones spaced from 60 to 208 Hertz. They are generated either by an analog circuit or sharply filtered digital signal.

The tones are not typically passed through from the receiver to the transmitter in a repeater. It is normal to regenerate them so the audio can be shaped before it is injected in the transmitter. There is no tone on the repeater transmitter. We had problems with the transmitter interfering with the receiver if a tone was transmitted. When we removed it, the troubles stopped.

Alan N5BGC



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