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The next picture below was generated with the same setup. However this time, a GPIO Labs FM Notch Filter was inserted between the antenna and the RTL-SDR. You can see that almost all FM signals have been eliminated. The effect of an FM Notch Filter A FM notch filter is especially useful when a low noise amplifier (LNA) is added prior to the SDR to amplify and detect weak signals. Most LNAs have higher gain values at lower frequencies. The picture below shows a typical gain versus frequency curve. This results in the strong FM band signals becoming even stronger after amplification relative to other signals (for instance cellular) that the user might be interested in. FM notch filters suppress amplified signals in the FM band while preserving everything else Inserting a FM notch filter after the LNA can suppress the amplified FM signals before they impact the receiver. FM Notch filters are a very important accessory in any SDR kit as they help protect the receiver from saturation and enable the user to make signal measurements that would otherwise be impossible. The post FM Notch Filters – why you need one with most SDRs appeared first on Software-Defined Radio Simplified.
Source: https://www.onesdr.com/2019/11/21/fm-no ... most-sdrs/
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