BladeRF astronomical interferometer

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BladeRF astronomical interferometer

Postby dlonard » Sat Dec 26, 2015 10:47 am

Hello Everyone,

I thought I would post some of my work on using three BladeRFs as SDR interferometers for radio astronomy. If you follow the links provided below, you can see some of the data from work that I've done at 60 MHz and 140 MHz. I haven't tried out the various approaches that are emerging to synchronize the data streams from multiple BladeRF's yet, but I am very keen to use 1 PPS synchronization. To align data right now, I am using a FFT/IFFT process to synch data in post processing. The links provide some code that I've written to do this and for using a GPU to function as a FX correlator.

There are a few kinks in my set up that occasionally leads to dropped samples, but most of the time, I am able to record data at 15 MSPS simultaneously from three radios.

If anyone else is doing any work using BladeRFs for interferometry, I would be glad to hear about your experiences.

David



https://www.youtube.com/watch?v=ISEfbBJxiK0
https://www.youtube.com/watch?v=ZlmvZa6bcTs
https://sites.google.com/site/amateurra ... ferometry/
https://groups.google.com/forum/#!forum ... rferometry
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Re: BladeRF astronomical interferometer

Postby jynik » Sun Dec 27, 2015 11:34 am

Hi David,

Awesome work, thanks for sharing!

I know you're waiting for the synchronized RX functionality, which I'll be looking to merge soon. There's a branch and some associated documentation here that I'll look to finish reviewing and testing. One important detail that will change is that the Mini exp pin used will have to change -- we used pin 1 as the input for the 1PPS / 10 MHz input - just a heads up.

Cheers,
Jon
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Re: BladeRF astronomical interferometer

Postby dlonard » Sun Dec 27, 2015 2:54 pm

Jon,

Thanks, I look forward to the 10 MHz/1 PPS synch functions going mainline. I'm still a ways off from being able to correlate signals in real-time, so I am content to process everything after recording right now. Without 1PPS synching, individual BladeRFs are about ~5000 to 10000 IQ sample pairs off when started off from Gnuradio at 15 MSPS on my machine.

David
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Re: BladeRF astronomical interferometer

Postby jynik » Sun Dec 27, 2015 6:44 pm

Hi David,

Sorry for the confusion. As I re-read this, I articulated things poorly. Just wanted to clarify things, as not to confuse other people as well.

The 1 PPS / 10 MHz input is something entirely different - this functionality, now in master, allows the on-board VCTCXO to be tamed by supplying 1.8V 1 PPS or 10 MHz input source to pin 1 on the mini expansion header. The purpose of this is to minimize frequency error and to allow people to re-calibrate their device. We'll be working on a blog post about how to do this with a COTS GPSDO.

The synchronized (or "triggered") capture functionality up in that development branch allows the mimo master to gate sample reception/transmission on all of the slave devices; this is the functionality I think you might find particularly useful. This is done using an I/O pin on the same mini expansion header. What I feebly tried to articulate was that this conflicts with the aforementioned pin assignment, since these two things were developed in parallel; we will change this pin when we merge this trigger-based.

What is currently not in the active "TO DO" list is having the bladeRF set a supplied timestamp value on the next PPS pulse. However, I will talk to the guys about what it would take to get this into the queue.

Hopefully that sheds a bit more light on what's to come. I'll try to touch base with you once it's all ready, and will be looking forward to future developments in your projects! :)

Cheers!
Jon
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Re: BladeRF astronomical interferometer

Postby dlonard » Sun Dec 27, 2015 9:11 pm

Thanks for the clarification, yes I am more interested in the ability to synchronize devices, preferably on a 1 PPS pulse from a GPSDO. I assume that a master radio could send a pulse to slave radios as an alternative, then no extra hardware would be needed. I think it would be straightforward to characterize the delay and use that for synchronization for those of us that do not need higher levels of frequency accuracy.
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Re: BladeRF astronomical interferometer

Postby jynik » Wed Dec 30, 2015 1:09 pm

Hi David,

You are correct about the "alternative" case -- with the pending changes we have up in the dev-synctrx branch, you would simply need to wire J71 (pin TBD) on the master to each of the slaves (on the same J71 pin); no extra hardware would be needed.

I'll be in touch once we have that merged!

- Jon
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Re: BladeRF astronomical interferometer

Postby dlonard » Wed Jan 27, 2016 9:45 am

I've done some more work to get three BladeRFs to cross correlate data in 'real time' now for those who are interested. A grc flowgraph and a Python script communicate over a TCP socket now, so data doesn't need to be stacked up on big solid state drives and post-processed. See the link below.

Once the sample stream synchronization is incorporated into the BladeRF master code, I will ready to do sustained astronomical interferometry observing.

David



https://groups.google.com/forum/#!topic ... haMXMHdO4o
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Re: BladeRF astronomical interferometer

Postby jynik » Mon May 23, 2016 7:01 pm

Hi David,

I apologize for it taking so long, but the sample stream synchronization has been merged to master. I still need to work on porting the original writeup (by Jan) to a stand-alone document, but for the time being I've added a very simple example here.

I hope you're still up for working on your interferometer, as I know I'll be excited to see your results!

Best regards,
Jon
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Re: BladeRF astronomical interferometer

Postby jynik » Mon May 23, 2016 7:02 pm

P.S. I'll have FPGA v0.6.0 up for download tomorrow afternoon!
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Re: BladeRF astronomical interferometer

Postby dlonard » Tue May 24, 2016 6:04 am

Jon,

I'm excited to try it out and thanks for the example. I've got three radios that need to be synchronized and I think I should be able to figure it out. Ultimately, I would like to be able to execute the triggering from within a GnuRadio flowgraph. I will keep everyone posted on my implementation.

Thanks,

David
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