.. _troubleshooting:

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Troubleshooting
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.. _bigspike:

There is a big spike in the center of the received spectrum
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If you see a large spike in the center of your FFT display regardless of the frequenecy you are tuned to, you are seeing a DC offset (or component or bias). The term "DC" comes from "Direct Current" in electronics. It is the unchanging aspect of a signal as opposed to the "alternating" part of the signal (AC) that changes over time.

.. figure:: ../images/dc_spike_grc.png
   :align: center

   DC spike


Take, for example, the signal represented by the digital sequence:

.. code-block:: sh

	-2, -1, 1, 6, 8, 9, 8, 6, 1, -1, -2, -1, 1, 6, 8, 9, 8, 6, 1, -1, -2, -1, 1, 6, 8, 9, 8, 6, 1, -1

.. figure:: ../images/dc_spike_example_plot.png
   :align: center

   Example signal

This periodic signal contains a strong sinusoidal component spanning from -2 to 9. If we plot the spectrum of this signal, you can see one spike at the frequency of this sinusoid and a second spike at 0 Hz (DC).


.. figure:: ../images/dc_spike_example_spectrum.png
   :align: center

   Spectrum of example signal

If the signal spanned from values -2 to 2 (centered around zero), there would be no DC offset. Since it is centered around 3.5 (the number midway between -2 and 9), there is a DC component.

Samples produced by HackRF are measurements of radio waveforms, but the measurement method is prone to a DC bias introduced by HackRF. It's an artifact of the measurement system, not an indication of a received radio signal. DC offset is not unique to HackRF; it is common to all quadrature sampling systems.

A high DC offset is also one of a few symptoms that can be caused by a software version mismatch. A common problem is that people run an old version of gr-osmosdr with newer firmware.

Solution
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There are a few options:

    #. Ignore it. For many applications it isn't a problem. You'll learn to ignore it.

    #. Avoid it. The best way to handle DC offset for most applications is to use offset tuning; instead of tuning to your exact frequency of interest, tune to a nearby frequency so that the entire signal you are interested in is shifted away from 0 Hz but still within the received bandwidth. If your algorithm works best with your signal centered at 0 Hz (many do), you can shift the frequency in the digital domain, moving your signal of interest to 0 Hz and your DC offset away from 0 Hz. HackRF's high maximum sampling rate can be a big help as it allows you to use offset tuning even for relatively wideband signals.

    #. Correct it. There are various ways of removing the DC offset in software. However, these techniques may degrade parts of the signal that are close to 0 Hz. It may look better, but that doesn't necessarily mean that it is better from the standpoint of a demodulator algorithm, for example. Still, correcting the DC offset is often a good choice.