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gnuradiognuradio-companion

Why am I getting only zeros out of the VCO block in GNU Radio?

In GNU radio I am trying to use the frequency of one signal to generate another signal of a different frequency. Here is the flow diagram that I am using:

flow diagram

I generate a 50 kHz signal with a signal source block and feed this into a Log Power FFT block. I used the Argmax block to find the FFT bin with the most power and multiply that with a constant. I want to use this result as the input to the complex vco block to generate another signal with a different frequency. All vectors have a length of 4096.

However, looking at the output of the complex QT Gui Time Sink block, the output of the vco is always zero. This is strange to me because using a float QT Gui Time Sink to look at the output of the multiply block (which is also going to the input of the vco block), the result is 50,000 as expected. Why am I only getting zero out of the vco?

Also, my sample rate is set to 1M. I am assuming because of the vector length of 4096 that the sample rate out of the Argmax block will be 1M/4096 = 244. Is this correct?

I am running gnu radio companion on windows 10.

Solution

  • The proposed solution is not a solution. Please don't abuse the signal probe, which is really just that, a probe for slow, debugging or purely visual purposes. Every time I use it myself, I see how architecturally bad it is, and I personally think the project should be removing it from the block library altogether.

    Now, instead of just say "probe is bad, do something else", let's analyse where your flow graph falls short:

    • your frequency estimation depends on the argmax of a block that was meant for pure visualization purposes. No, the output rate is not (sampling rate/FFT length), the output rate is roughly "frame rate" (but not actually exactly. That block is terrible and mixes "sample times" with "wall clock times"). Don't do that. If you need something like that, use the FFT block, followed my "complex to magnitude squared". You don't even want the logarithm - you're just looking for a maximum
    • Instead of looking for a maximum absolute value in an FFT, which is inherently a quantizing frequency estimator, use something that actually gives you an oscillation. There's multiple ways you can do that with a PLL!
    • your VCO solution probably does what it's programmed to do. You just use an inadequate sensitivity!
    • The sampling rate you assume in your time sinks is totally off, which is probably why you have the impression of a constant output – it just changes so slowly that you'll not notice.

    So, I propose to instead, either / or:

    • Use the PLL Freq det. Feed the output of that into the VCO. Don't scale with a constant, but simply apply the proper sensitivity. Sensitivity is the factor between "input amplitude" and "phase advance per sample on the output in radians".
    • Use the PLL Carrier recovery. Use a resampler, or some other mathematical method, to generate the new frequency. You haven't told us how that other frequency relates to the input frequency, so I can't give you concrete advice.

    Also notice that this very much suggest this being a case of "I'm trying to recreate an analog approach in digital"; that might be a good approach, but in many cases it's not.

    If I might be as brazen: Describe why you need to generate that other frequency, for which purpose, in a post on https://dsp.stackexchange.com or to the GNU Radio mailing list [email protected] (sign up here). This really only barely is a programming problem, but really a signal processing problem. And there's a lot of people out there eager to help you find an appropriate solution that actually tackles your problem!