Re: [Discuss-gnuradio] The threshold of energy detection

[Marcus Müller]

Hi Yan,


The threshold block converts your signal to 1 if you've been above the "high" value, unless it has fallen below "low" again. Everywhere else, it's 0. See the "documentation" tab in the block properties, or the GNU Radio doxygen manual [1].

But: Parseval's theorem states that energy in frequency and in time domain are directly proportional, so your

stream to vector -> FFT->complex to mag²-> vector to stream>moving average

can simply be replaced with a

complex to mag²->moving average

if detecting the average energy in your signal is all you want , which seems probable, seeing that length(average)≈length(FFT).

I'm not quite sure what your original intention was when storing (FFT->mag²->average) samples in test_sensor/out.dat; the signal after moving averaging is neither really frequency nor time domain. Out of curiosity: what do you want with that data?

There's very valid applications for frequency domain filters (a moving average is actually but a low pass FIR filter, in principle), but the convolution that filtering usually means is often replaced with a multiplication in time domain, which is much less CPU-intense and mathematically equivalent!

Best regards,
Marcus

[1] https://gnuradio.org/doc/doxygen/classgr_1_1blocks_1_1threshold__ff.html#details
¹ For new applications, I recommend Qt rather than WX -- the latter isn't going to be around forever.

On 12.02.2016 12:02, Yan Huang wrote:

Hi all,

 

I’m using USRP B210 to do spectrum sensing, and the flow graph as attached file. It can get a result that there is a peak at 5.8 GHz which is the receiving signal frequency.

 

But if I change the Moving Average block to a threshold, there is no output like previous one.

 

I want to ask how can I get a result have a peak at 5.8 GHz  in frequency domain using threshold. I can only get 1 or 0 in time domain not in frequency domain.

 

Is there anyone can give me some advice? Thanks in advance.

 

 

Yours Sincerely

 

Yan

 

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