title: How can I calculate the Noise Floor in GNU Radio Companion?
body: <p>To my understanding, the noise floor for each USRP may be different. I want to know how I can calculate the noise floor without physically going into the fft and spotting it out manually. I want to know if there is a block in GNU Radio that will calculate this, or if there is a stream of blocks I can use to calculate it. Please provide a block diagram in your answer ( <code>block 1 ---&gt; block 2 ---&gt; ...etc.</code>). </p>
  
  <p>For my application, let's say I have a QT GUI frequency sink that is showing all noise at the moment. I want to calculate the noise floor so that I have a value that represents "no signal present" ie. noise. Once I have this value, I plan to set a threshold 5dB higher than the noise floor to indicate that a signal has been detected. I've been able to kind of eye ball the average noise value from the QT GUI Frequency Sink but that's not good enough for me. I want to be able to calculate it and not have to look into the plot every time to update the noise value every time I change USRPs.</p>
  
  <p>For instance:
  <a href="https://i.stack.imgur.com/8qevm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/8qevm.png" alt="QT GUI Frequency Sink Showing Noise"></a></p>
  
  <p>You can see the average noise value for this is around -55dB. I want to calculate this without having to eye ball it. This way, when a signal gets transmitted at (in this example) 0Hz, then the power of the signal will increase and I can see if a signal was detected.</p>