Software Defined Academy Programme almost ready

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:

http://www.sdra-2015.de/pages/programme.html

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.

http://www.hamradio-friedrichshafen.de/

vy73 / br
markus
dl8rds

I have a question concerning connecting two DVB-T dongles on the same
clock
source for interferometric (or passive radar) measurements, as described
at
http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html
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
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

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, [email protected] 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:

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] http://kaira.sgo.fi/2013/09/16-dual-channel-coherent-digital.html
[2] http://jmfriedt.sequanux.org/ph_tout.pdf


JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044
Besancon, France

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

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