[time-nuts] Digital Mixing with a BeagleBone Black and D Flip Flop

Andrew Rodland andrew at cleverdomain.org
Thu Oct 9 01:34:28 EDT 2014


Simon,

This is a fantastic idea and I have every intention of trying to
replicate it at home with tools on hand. Thanks for sharing, and I
hope you can show off some results.

On Wed, Oct 8, 2014 at 1:09 PM, Simon Marsh <subscriptions at burble.com> wrote:
> I've been a lurker on time-nuts for a while, most of the discussion being
> way over my head, but I thought there may be interest in some proof of
> concept code I've written for simple digital hetrodyne mixing using just a
> BeagleBone Black and an external dual D Flip Flop.
>
> The idea is based on the following article which describes creating a
> digital DMTD with an FPGA for clocks @ 125mhz:
>
> http://www.ee.ucl.ac.uk/lcs/previous/LCS2011/LCS1136.pdf
>
> My setup follows the same principle, but scaled down to 10mhz to make it as
> simple as possible (and not require an FPGA).
>
> The hardware side is just a 74AC74 dual flip flop to sample the input clocks
> being tested. Instead of having a helper PLL for the mixer frequency, I
> simply have a 3rd, de-tuned oscillator. The output from the two flip-flops
> together with the mixer clock are fed to the BBB.
>
> On the BBB, the approach is to do as little as possible in real time using a
> PRU core, and then post-process on the ARM core afterwards.
>
> The BBB PRU has a 16-bit, asynchronous, parallel, capture mode, where 16
> GPIO pins can be latched based on an external clock (described in section
> 4.4.1.2.3.2 of the TRM for those interested). In this case, the external
> clock is the mixer oscillator. All the PRU needs to do is wait for the
> sample to take place, read the GPIOs and store the results in main memory.
> The PRU is plenty fast enough to capture samples @10mhz and, in theory at
> least, each PRU could sample up to 16 clocks simultaneously (depending on
> whether the relevant GPIO pins were free).
>
> Once the sampling is complete, the ARM core can process the results in its
> own time, and this includes any more complicated algorithms for de-glitching
> etc
>
> The theoretical minimum time resolution depends on the beat frequency and is
> described in the article, for example with a beat frequency of 50 hz the
> minimum resolution is 50 / (10000000 - 50)*10000000 = ~5E-13. In practice
> the available accuracy is determined by the stability of the mixer clock and
> noise of the setup. The impact of this noise is described in the article as
> glitching and there are some suggested ways for processing this out. I'm
> trying this on an open bench, with basic oscillators, using pluggable
> breadboard and lots of hanging wires, I'm not at risk of getting near the
> theoretical limit quite yet :)
>
> Note that the BBB itself has no impact on the accuracy or noise of the raw
> data. Once the input is latched at the flip-flop, the only bit of critical
> timing required is to ensure that samples can be captured fast enough and
> that the flip-flop state is captured when it is stable (i.e. not
> transitioning).
>
> I make no excuses that this is very simplistic, and there are many, many
> ways that it can (should!) be improved. For me the next steps will probably
> be:
>
> 1) Get off the breadboard and focus a bit more on getting the signals to the
> flip-flop with a 'reasonable' amount of noise.
> 2) Improve the PRU code so that it stores transitions and not just the raw
> samples, this would offload a significant bit of work from the ARM core,
> save a load of memory and allow continuous streaming of data (instead of the
> current one shot approach).
> 3) Experimentation with different algorithms for processing the data on the
> ARM.
>
> I don't think anyone has posted a similar set up, so any feedback on whether
> the approach is viable or I'm wasting my time are welcome.
>
> I've posted the code to Google drive for anyone to take a look. It shouldn't
> be too difficult to reproduce if someone wants to, but again please remember
> it's just 'prove it can be done' code.
>
> https://drive.google.com/open?id=0BzvFGRfj4aFkblAwcWxGNHdCSDg&authuser=0
>
> Cheers
>
>
> Simon
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