PlutoSDR Rx support

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PlutoSDR Rx support

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Implemented in v3.7.0

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Interfaces with the libiio library: https://github.com/analogdevicesinc/libiio
Udev rules are there: https://raw.githubusercontent.com/analogdevicesinc/plutosdr-fw/master/scripts/53-adi-plutosdr-usb.rules

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Got it working on GNUradio so that's a start. There is a hack to fool it to think it has an AD9364 inside: https://wiki.analog.com/university/tools/pluto/users/customizing So there should be a switch to force AD9364 limits (frequency, sample rate, bandwidth) although not guaranteed. Extended frequency range works at least down to 100 MHz.

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Had a glance at the AD9363 reference manual. It is quite feature rich and has everything like the LMS7002D but the hardware NCO. That's nice! Hardware decimators/interpolators and FIR filters. Feature rich AGC and gain control. We'll see how this translates with the libiio interface (a bit weird). I think it means that I will base the GUI on the Lime's

Interesting doc on driver hence parameters to be used with libiio (I think...): https://wiki.analog.com/resources/tools-software/linux-drivers/iio-transceiver/ad9361

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Got a revelation today about the very painful and badly documented libiio interface for the AD9361/3/4. What is refered as "filename" in libiio actually refers to the virtual file names found in the /sys/bus/iio/... structure when a device is directly attached to the host hardware. This is what they use to set parameters in the gr-iio module. This information can help make the connection between the not so badly documented paramerers here: https://wiki.analog.com/resources/tools-software/linux-drivers/iio-transceiver/ad9361 and the libiio interface.

Link to the GNUradio gr-iio module: https://github.com/analogdevicesinc/gr-iio
Direct link to libiio API documentation: http://analogdevicesinc.github.io/libiio/

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Another source of inspiration or verification: https://github.com/jocover/SoapyPlutoSDR
Also there is a forum dedicated to PlutoSDR: https://www.plutosdr.com/viewforum.php?f=5

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Modified the libiio ad9361-iiostream example to work with Pluto in a transponder configuration. It works!
Got a similar example working with C++ wrapper classes. Slowly getting to it!

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Despite all efforts this still does not work support is dropped
It's a fail.

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This post has comments and links about the very unclear sample rate setting:
https://ez.analog.com/thread/98077

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Implementation in SDRangel is impossible due to libiio and gr-libiio code obfuscation and lack of documentation.

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Examples like this from AD documentation are just plain wrong:

            for (p_dat = iio_buffer_first(rxbuf, rx0_i); p_dat < p_end; p_dat += p_inc, t_dat += p_inc) {
		const int16_t i = ((int16_t*)p_dat)[0]; // Real (I)
		const int16_t q = ((int16_t*)p_dat)[1]; // Imag (Q)

		/* Process here */

	}

I and Q samples are processed one after the other in a loop iteration for each and not as an (I,Q) group.

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This last discovery seems to have unlocked the progress on development. So PlutoSDR support at least for Rx should be possible.

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A reasonably working version for Rx is now on dev branch.