HackRF
blacklist hackrf
After a system-restart, to get the updated modprobe working, the hackrf worked under ubuntu 15.04 with the upstream
packages (Firmware version: 2014.08.1) out-of-the-box.
7.2 hackrf_set_sample_rate fails
Question: I’m trying to run hackrf_transfer and hackrf_set_sample_rate fails. The libusb_control_transfer call in
hackrf_set_sample_rate_manual is returning with LIBUSB_ERROR_PIPE.
Answer: Follow the instructions to update your firmware.
7.3 What is the big spike in the center of my received spectrum?
Question: I see a large spike in the center of my FFT display regardless of the frequency my HackRF is tuned to. Is
there something wrong with my HackRF?
Answer: You are seeing a DC offset (or component or bias). The term “DC” comes from “Direct Current” in elec-
tronics. It is the unchanging aspect of a signal as opposed to the “alternating” part of the signal (AC) that changes over
time. Take, for example, the signal represented by the digital sequence:
-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
This periodic signal contains a strong sinusoidal component spanning from -2 to 9. If you were to plot the spectrum
of this signal, you would see one spike at the frequency of this sinusoid and a second spike at 0 Hz (DC). 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.
There was a bug in the HackRF firmware (through release 2013.06.1) that made the DC offset worse than it should
have been. In the worst cases, certain Jawbreakers experienced a DC offset that drifted to a great extreme over several
seconds of operation. This bug has been fixed. The fix reduces DC offset but does not do away with it entirely. It is
something you have to live with when using any quadrature sampling system like HackRF.
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.
7.2. hackrf_set_sample_rate fails 25
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