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ADALM-Pluto: How to change settings with quick settling

Question asked by RobB Employee on May 16, 2018
Latest reply on May 17, 2018 by travisfcollins

Hi,

One of our users of the ADALM-Pluto is applying it in labs for their communication course. Extending on the possibilities in the lab, they want to change the settings of the receiver quickly. Using GNURadio they encounter 1 sec stall / blank time after doing a change. 

I myself checked with SDR# and e.g. a frequency change seems to be immediate. 

Below the description (without pictures) and attached a pdf with the same description and the pictures:

 

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The software package we currently use is GNURadio. This package communicates with the PlutoSDR by using the LIBIIO library (over usb). Below you will find a straight-forward block diagram to transmit and receive a single sine wave with the same device:

 

[BlockDiagram]

 

As you can see, I also implemented a numeral input variable for the local frequency (ID: carrier). When this block diagram is compiled into the corresponding python code, the following GUI is created, which allows us to change the local frequency 'on the fly':

[FFT plot]

 

 

In the waterfall display below, we can see the FFT plot over time where the amplitude is represented by a color. I think this is where the miscommunication happened; Whenever the local frequency is changed, the device cannot transmit or receive any samples to or from the device for ~1 second (between ~5.1 and ~5.2 seconds in the graph below). We called this a 'reboot' in our previous e-mail because the device is unresponsive for a noticable amount of time, but we probably should have called it an 'unavailability' since the device is not actually rebooting, but merely changing the local frequency attribute.

[Waterfall]

 

Our purpose is to have many adalm plutos communicate with each other over a set of carrier frequencies. For this to happen, each unit needs to claim an available (unused) frequency and do a frequency sweep to find other devices transmitting on other frequencies. If we need one second to switch per frequency, the sweeping process will take too long. We need to get this 'transition' time down to a few milliseconds - preferably even less - otherwise the device won't be usable for our purpose.

 

I looked through the code and found the following:

 

The python function that we get from GNURadio to change the local frequency is this:

 

[python def set_carrier]

 

The 'set_params' method above calls the following C function from the gr-iio library: https://github.com/analogdevicesinc/gr-iio/blob/85cf3200691ddf57f84001f0489f0a0ce4c34532/lib/pluto_sink_impl.cc#L66

 

If I follow the chain of all functions that call each other, the C function linked from the gr-iio repository above links back to this command from the libiio repository: https://github.com/analogdevicesinc/libiio/blob/560027bfa271c7c994ca5c383c9030f26a5b1d88/examples/ad9371-iiostream.c#L95

 

Assuming I observed this chain of functions correctly, does that mean that since GNURadio is directly writing the lo_fr (local frequency) attribute to the ad9371 chip using the libiio library, that there is no more optimal or faster way of doing this? Does this mean that we are stuck with the delay of 1 second to change between frequencies? Is this a limitation by the method of communicating between the petalinux mcu with the ad9371 chip inside the adalm pluto or a limitation from communicating with the device externally over usb? Is there other software we can use (than GNURadio) that allows us to do a frequency sweep at a higher speed?

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Could you please help with how to getting the settling of settings for the Pluto fast?

Thanks!

Best regards,

Rob

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