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Author Topic: Cant make heads or tails out of my spectral display! (technical discussion)  (Read 5544 times)
VA2FSQ
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Posts: 511




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« on: October 12, 2013, 10:19:26 AM »

I'm using a KX3 feeding the IQ output into a Sound Blaster XFi card.
I've tried a few thrid party SDR apps, (SDR-radio, SDRSharp, HDSdr) and they all are about the same in display.  They show a signal which is symmetric around the center frequency and as you tune the radio the peaks seem to split and go to the left and right instead of acting as a panadapter should and the peaks follow the frequency.  When I see the demos of these software, it doesn't look anything like that.

Ok so that's the simple part.  I'm writing my own software to do this as well.  I';m using a third party tool kit that provides a .NET framework.
So this is how it looks:

DSAudioIn: Here you define the audio capture the output of which goes to
FFT:  The spectrum output goes to
Scope:  A graphical interface to display the signal.

Now when I did this, I get what is expected, i.e. I see a peak where a signal is, but as I tune the radio, the peak continues to the edge of the graph and then it travels back to the center!  It acts the same as the other products out there!  What the heck is going on?

Now possibly, here is a hint.  In my app, I can use the stereo input which then feeds directly to the FFT, or I can use the left channel only (I?) or the right channel only (Q?) and nothing changes!  It looks the same.

I've tried to read the literature but I still have not seem an actual diagram as to how one decodes an IQ signal to a signal vs frequency spectrum.
Can anyone give me a hand here?

Thanks
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VA2FSQ
W6RMK
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Posts: 651




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« Reply #1 on: October 12, 2013, 11:12:06 AM »

If you feed I only or Q only into an FFT as the real part, and zero the imaginary, you'll get a spectrum with a mirror image.  Depending on how your spectrum display works, you'll get the DC peak at the left, and at the right, with sample rate/2 in the middle.

Here's a question, how many complex points going into the FFT, and how many are being displayed? How are you converting the complex output of the FFT into the display (real^2+imag^2), then 10*log10 for dB?
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VA2FSQ
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Posts: 511




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« Reply #2 on: October 12, 2013, 03:47:21 PM »

Hi Thanks for the response.
It seems that I have worked out a few issues. I can now get the spectrum to display in NAP3, SDR Radio etc but even with their auto image rejection and minimizing the amplitude differences between channels, I'm still see a fair bit of images crop up at times.
Not too impressive so far.

Anyways, my software does not yet handle the complex signals which is of course an issue.
So given that I have I data in one channel and Q in the other, I presume I have to feed both into the FFT? The conversion to the display is handled by the software I am using.  It's basically giving me the same thing as the other apps with a mono input.

Thanks for your help.  I have a lot of learning to do!
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VA2FSQ
K5TED
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Posts: 728




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« Reply #3 on: October 12, 2013, 06:41:40 PM »

Did you inadvertently let the Unicorn Juice out of the KX3?
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VA2FSQ
Member

Posts: 511




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« Reply #4 on: October 12, 2013, 07:38:25 PM »

Every thing is solved now. Turns out it was a bad antenna connection leading to noise and screeing up the IQ channels. Fixed.
Also fixed thanks to to the hints is my software. This library I use is fantastic. Zero lines of code and I have the spectrum.
What I was missing is feeding 2 seperate channels the real and the imaginary into a complex buffer and then feeding it to the fft.
Thanks for the hints
Tom va2fsq
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VA2FSQ
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