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4.11.5 - Noise Blanker

You turn on the noise blanker with NBL. You adjust the noise blanker sensitivity with NBT. You adjust the noise blanker width with NBW.

Noise blanking is useful when impulse noise interferes with signal reception. Examples of this type of noise include harmonics of powerline noise at intervals of 50 or 60 Hz; arcing and sparking from streetlights, powerpoles, generators and car ignitions; and other impulse noise sources such as medical equipment, welding units, and industrial installations.

This function works by analyzing the sampled RF signal; it identifies short duration spikes in amplitude, and then removes them from the RF prior to signal demodulation.

Note: NBW represents the number of samples that define the width of a spike. Consequently, when you work at a lower sample rate, such as that used for a 50 KHz receive window, a single sample represents a different amount of time than it does when running 250 KHz wide. For this reason, with the same noise source, you'll find that the most effective NBW setting differs between the various sample rates. So if you change your sample rate, remember to revisit this noise blanker setting.

While noise characteristics vary widely and so no single blanker design can remove all of them, I think you'll be pleasantly surprised by just how effective the SdrDx noise blanker can be.

At 250 KHz bandwidth, try starting with settings of NBT 50 and NBW 20. For the same effectiveness at a 50 KHz bandwidth, adjust to NBW 80. If these settings do not reduce the noise, increase NBW or NBT and see if you can find a combination of settings that will help. Remember: not all noise can be removed, only impulse noise.

Caution: With the noise blanker in the NBL state, the more impulse information removed from the RF signal, the less precise the resulting waveform is in representing the signals in the band. There are two consequences to this: First, that the visible RF signal and waterfall will become distorted by the signal blanking, and second, that the demodulated signal will also become less precise but free of the noise itself. A second press on the button will result in NBL, and in this state, the RF display processing will not be affected by the noise blanker. This removes the potential for distortion, however in higher noise environments, the visible noise may present more of a problem than the distortion. Use the mode that is best for your purposes. Noise blanking is applied equally to the demodulated signal in either mode.

With strong noise blanker settings in NBL mode, you can expect to see signals in one portion of the display cause visual interference with the display of all other signals. This is normal, a consequence of information having been removed from the RF signal by the noise blanker. Try NBL mode and see if the presence of noise in the display is less annoying than blanker distortion is. Using SNF can ameliorate visual noise as well while still not presenting cross-signal distortion in NBL mode.

You can access the intensity setting of the noise blanker DSP process with Right-click NBL



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