# Software Defined Academy Programme almost ready **URL:** \ **Category:** GNU Radio\ **Created:** [May 24, 2015, 3:19pm UTC](https://www.ruby-forum.com/t/software-defined-academy-programme-almost-ready/242067 "2015-05-24T15:19:16Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![Detlef\_R](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/d/58f4c7/32.png) [@Detlef\_R](https://www.ruby-forum.com/u/Detlef_R)\ **Post date:** [May 24, 2015, 3:19pm UTC](https://www.ruby-forum.com/t/software-defined-academy-programme-almost-ready/242067/1 "2015-05-24T15:19:16Z")
Dear List, the programme of the Software Defined Radio Academy on June 27 2015 in Friedrichshafen (sub-conference to HAMRADIO) is almost complete. But it is already now so interesting that I don’t want to wait any longer and present the list of talks to you: > **[sdra-2015.de](http://www.sdra-2015.de/pages/programme.html)** > > This domain may be for sale! As soon as we have an exact time schedule for the talks (which is not yet fixed), I will send another note. I’ll be happy to welcome you to the SDRA if you plan to attend. Please note that you’ll get the entire HAMRADIO (biggest European HAM festival) plus the MakerWorld with one single ticket. > **[HAM Radio Friedrichshafen | 23. - 25. Juni 2023](https://www.hamradio-friedrichshafen.de/)** > > Die internationale Amateurfunk-Ausstellung ist die Plattform für Funker aus der ganzen Welt. vy73 / br markus dl8rds ---
**Author:** ![Markus\_Heller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/d2c977/32.png) [@Markus\_Heller](https://www.ruby-forum.com/u/Markus_Heller)\ **Post date:** [May 28, 2015, 8:19pm UTC](https://www.ruby-forum.com/t/software-defined-academy-programme-almost-ready/242067/2 "2015-05-28T20:19:35Z")
I have a question concerning connecting two DVB-T dongles on the same clock source for interferometric (or passive radar) measurements, as described at > **[$16 dual-channel coherent digital receiver](http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html)** > > I have been playing around with the cool RTL dongles (more on rtl-sdr dongles on  superkuh's web page  or rtl-sdr.com ) that you can buy on... I have assembled the same system with one dongle used as oscillator on a 28.8 MHz resonator and the second one as a slave to this clock. All works fine, solved the issue when the oscillator would not start, now I have a reliable source of measurements. Initial tests (these are R820T-based dongles) exhibits significant random phase drift which I attributed to heating of the chips (they get above 50 degC when running continuously), so after gluing two heat sink with heat-conducting epoxy, I more or less managed to get a stable phase measurement when recording a same oscillator (200 MHz) with the two dongles and displaying the phase as angle(conjugate(signal1)\*signal2). The question is as follows: at [http://jmfriedt.sequanux.org/ph\_tout.pdf](http://jmfriedt.sequanux.org/ph_tout.pdf) I have shown one graph, quite representative of all my experiments, displaying the evolution of the phase difference between both dongles connected to the same 200 MHz oscillator. I _always_ start with a quite stable phase difference (red curve – inset in a zoom of this particular measurement) after plugging in my USB hub fitted with the two dongles and starting gnuradio-companion for recording the dongle I/Q stream (notice the abscissa sampling rate of 10 Hz =\> the full graph is about 1-hour long). After about an hour, I stop recording the red curve, and all I do is switch off gnuradio-companion and start it back =\> green curve with a quickly falling phase. Switch off again, disconnect-reconnect USB hub, restart an acquisition =\> blue curve. Same procedure =\> magenta curve. Can anyone hint at an explanation as to why I always start with a reasonably stable phase difference (yet not constant – is the phase fluctuation indeed due to heating of the fractional PLL in each RF frontend, drifting below the feedback loop time constant ?), but more worrisome why I always get this huge drift after launching a new acquisition ? The fact that I always get the same slope hints at a sofware/hardware communication issue, but how it is possible since both dongles are clocked by the same source and receive the same commands from the software ? Thanks, JM – JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France ---
**Author:** ![Markus\_Heller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/d2c977/32.png) [@Markus\_Heller](https://www.ruby-forum.com/u/Markus_Heller)\ **Post date:** [May 28, 2015, 8:47pm UTC](https://www.ruby-forum.com/t/software-defined-academy-programme-almost-ready/242067/3 "2015-05-28T20:47:04Z")
This is largely because this $10.00-apiece hardware was never designed for this class of application. When you’re doings things that require phase-coherence, you have to design your radios to support it. There are at least two PLLs involved here–one on the R820T chip, and another, as far as I can tell, in the RTL2832U chip, which does the conversion to baseband from the low-IF of the R820T. I was never able to get my RTL receivers to be phase-coherent in any useful way, there was always a slow phase drift, that was unpredictable. On 2015-05-28 14:18, [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > get a stable phase measurement when recording a same oscillator (200 MHz) with > the > phase. Switch off again, disconnect-reconnect USB hub, restart an acquisition =\> > blue curve. > Thanks, JM > Links: ---
**Author:** ![Markus\_Heller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/d2c977/32.png) [@Markus\_Heller](https://www.ruby-forum.com/u/Markus_Heller)\ **Post date:** [May 28, 2015, 8:55pm UTC](https://www.ruby-forum.com/t/software-defined-academy-programme-almost-ready/242067/4 "2015-05-28T20:55:15Z")
Thanks for your reply. Indeed designing phase coherent receivers is my daily job activity (partly), but the fun of hacking DVB-T receivers is to find ways of using these for applications they were never intended for. I have indeed read your posts concerning the difficulties in reproducing interferometric measurements, a great source of inspiration. At the moment we are considering quantifying the phase drift (switching from a reference oscillator to the unknown signal), but I wonder nevertheless why some have achieved excellent coherence and some (including myself) are failing. Nevertheless I’d like to actually identify the source of the drift (PLL bandpass, temperature-related setpoint drift, software configuration ?). Thanks, JM > > Initial tests (these are R820T-based dongles) exhibits significant random > > phase drift > > for recording the dongle I/Q stream (notice the abscissa sampling rate of 10 > > Hz =\> the full > > since both dongles are clocked by the same source and receive the same > > commands from the > > software ? > > > > Thanks, JM > > ## Links: > > [1] [Kilpisjärvi Atmospheric Imaging Receiver Array: $16 dual-channel coherent digital receiver](http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html) > [2] [http://jmfriedt.sequanux.org/ph\_tout.pdf](http://jmfriedt.sequanux.org/ph_tout.pdf) – JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 Besancon, France ---
**Author:** ![Markus\_Heller](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/m/d2c977/32.png) [@Markus\_Heller](https://www.ruby-forum.com/u/Markus_Heller)\ **Post date:** [May 28, 2015, 11:57pm UTC](https://www.ruby-forum.com/t/software-defined-academy-programme-almost-ready/242067/5 "2015-05-28T23:57:36Z")
Hi, I guess I should respond, as I’m responsible for the blog posting that you linked. I tested the relative phase stability using amplified noise that was fed via a splitter onto the two dongles. I cross-correlated the noise and found that there was a deterministic frequency difference between the channels. I recall this was a fraction of a radian per second. I just calculated what this small deterministic frequency difference was and removed it. I found that I could use the same constant during the whole experiment and it also didn’t change after power cycling the dongles. This is how I produced the flat line IQ plot that is in the blog posting. Based on your plots, you are mostly also seeing a constant deterministic frequency difference (the linear slope on traces 3 and 4). Trace 3 has a phase jump for some reason and I don’t know what is happening with trace 1. In my tests, I only measured data for several hours at a time. I didn’t see any loss of lock or weird behavior during my experiments other than what I indicated above. I have heard from several people that they have managed to reproduce the dual rtl\_sdr dongle configuration and to even use it for passive radar. I have even seen people taking the proper engineering approach and feeding a clock with a fanout buffer. I haven’t seen any cross-correlation plots from these devices though. I have had issues with heating on the smaller dongles. In these cases the down converter stops working. A heat sink fixed this issue. Keep trying, I think you are almost there. Try recabling the clock differently or try using another pair of dongles – there is a lot of variability between the dongles. Try looking at the clock signal with a scope to see what the levels are with a working individual dongle. I assume you are doing this for fun (that’s why I did it at least). There are much easier ways to get multiple coherent channels into your computer with much better fidelity. juha