DIGITAL CONNECTION

In most digital modes, setting your transmit audio level is usually the only adjustment needed, but it is really important to set it correctly. Too low or, even worse, too high, and your signal isn’t as good as it could be. Too low, and the signal-to-noise ratio is reduced. Too high, and the signal becomes distorted, often with excessive sidebands (“splatter”). With our recent forays into high-speed packet, several readers asked how to best set the FM deviation on a 9600-baud packet link, so let’s have a closer look at that.

At 1200 baud, the two AFSK tones used in packet radio are plainly audible at the receiver, but at 9600 baud, the signal sounds more like white noise than audio tones. This means that you are able to set the deviation pretty close using your ears at lower speeds, but once we get into the higher speed region, we need to use instruments.

The classic method of measuring deviation is using a deviation meter. My old Cushman CE-3 radio service monitor has such a meter as one of its many functions, and for years it was used to set transmit audio levels throughout the New