Tagged: spatial filtering

Noise Cancellation using Linrad and the Phase Coherent Capabilities of an RSPduo

The RSPduo is the latest product from SDRplay, and it's main defining feature is it's dual RX channels. The two channels allow you to simultaneously listen to two stations that may be at opposite ends of the spectrum. Of course the same could be done cheaper with two separate RSP1A devices, however the advantage of the RSPduo is that the two channels are phase coherent. This opens up a lot of technical possibilities such as active noise cancelling/spatial filtering of an interfering signal or unwanted RF noise.

The basic idea behind phase coherent noise cancelling is that you have one antenna pointed towards your signal of interest, and the other pointing towards the interfering signal or coupled to the unwanted noise source. The unwanted signal/noise can then be subtracted from the signal of interest, resulting in a cleaner signal. This can only be done if the two channels are phase coherent like they are on the RSPduo.

Over on YouTube and his blog, ICAS Enterprises has been experimenting with noise cancelling on his RSPduo [Part 1] [Part 2]. The blog is entirely in Japanese, but all the relevant screenshots are in English, and we have provided Google Translated links. SDRplay have also provided a description of the process on their blog. The idea is to use Linrad to process the sound output of the two RSPduo channels from SDRuno. Linrad has capabilities that allow you to subtract two signals from one another.

In his experiments he receives a known 50 MHz CW beacon, and then injects an interfering test signal into the system. Then using Linrads 'signal adaption' feature he is able to completely remove the interference. 

RSPduoでのノイズキャンセルの実験 - Linrad使用
RSPduoでのノイズキャンセルの実験 - Linrad使用

Demonstrating Two Channel Coherent Spatial Filtering In a GNU Radio Simulation

Over on YouTube VE6EY has uploaded a video that demonstrates spatial filtering (aka beamforming) working in a GNU Radio simulation. This is a technique that can be used with a 2-channel coherent SDR with to nullify local interference. One SDR is connected to an antenna for receiving the distant signal, and the second is connected to a noise probe that is designed to receive only the local noise source.

The demonstration is not performed with real SDRs, but with prerecorded signals, although it still shows the effectiveness of the technique. In the video VE6EY shows switch mode and powerline noise being nulled out from some AM music, and explains through a demo why phase coherence is required.

V36EY gives further information and a link to download the demo over on his blog post.

SDR Spatial Filtering Demo
SDR Spatial Filtering Demo