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Local RFI Signal Content – Four Basic Types

local rfi signal content

Let’s consider the four basic types of local RFI signal content. We will see when and where a noise canceler can make a critical difference to your radio listening. 

Before we start, a few thoughts about terminology. We are really talking about cancelling or reducing interference, not noise. Interference is something that makes it hard to discern a signal. It could be noise (e.g. atmospheric or thermal noise) but it could also be another signal that we might call “noise”.

Now let’s consider some rather simple forms of noise: tones and white noise. I call these simple because they can be analyzed and reduced with simple techniques based on statistics.

Tones, often called “birdies” are just sine waves. If a tone is on a nearby frequency, it also gets demodulated and interferes with your desired signal. Computer and network gear produces tons of birdies around your house. Usually they are quite weak and more annoying than disruptive. You can use a noise canceler, for example with a local noise antenna next to the source, but this is hard when there are many different sources.

Besides, your receiver has a tool that does a good job removing tones: the notch filter. Typically, notch filters are located in IF or AF stages. They can recognize tones because of their statistical regularity – high correlation over time – and just remove them. Some software defined radios also include tracking notch filters at the front end.

Second, white noise is similar to atmospheric noise – very broadband and statistically uncorrelated. Some local RFI signal content contains white noise. But, similar to tones, your receiver has a tool for that: noise reduction. NR can track and remove uncorrelated noise, typically by 10 dB or more.

Local RFI Signal Content – Getting More Complex

Impulse noise is generally power line and motor ignition noise. Basically, it is a voltage spike that gets radiated. In engineering terms, impulse noise is created by rapid change of voltage over time, or δV/δt. Short time domain impulses create very broad and strong frequency domain noise. Unlike tones and white noise, you can’t treat it; you have to remove it.

Enter the noise blanker, which has been around since 1936. NB works by effectively turning your receiver Off for the duration of the impulse. A separate circuit or algorithm senses the (very) sudden rise in the noise floor and uses a switch or gate to remove the pulse before it can get into the radio. Some NB are free running (reactive) while others are synchronous (predictive). The synchronous NB usually provides blanking at the frequency of your power lines – 50 or 60 Hz.

Finally, we must increasingly deal with complex interference – for which there are not yet any real tools built into radios. This is where the noise canceler shines by reducing the interference before it gets to your radio. With complex interference, local RFI signal content contains tones, impulses and white noise all together.

With software defined radio, I dream that one day tools for complex interference reduction will arrive. But they ain’t here yet.

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