Merge multiple complex streams

GNU Radio
1

Playing around with GRC for a few months now, sorry if this question is
from a dummy.

I’ve a bunch of rtl dongles laying around, and wonder if it’s possible
to
combine more then one to a single baseband signal.

The goal is to create a super cheap full HF band web reciever like or
for
’ websdr’

I managed to create multiple channels, where the 2nd dongle is tuned to
an
offset, equal to the samplerate of the first. (for example 2 x 2 Msps =
4
Msps baseband)

Is such a thing possible with GRC?

Imre Biacsics

2

If you make an FFT block for each rtl-sdr source you can then interleave
them all together and take your pick using the ‘M in N’ block. At the
edges
of each rtl-sdr frequency it will probably get quite messy as the clocks
are not synchronised.

An example of the use of the FFT and ‘M in N’ blocks is in my scanoo_rx
app
which will give you a head start. The ‘M in N’ block is used to pick
out
the channel from the output of the FFT block:

https://raw.githubusercontent.com/m0mik/scanoo/master/apps/scanoo.com_rx.grc

I’ve always found that in order for it to work at first you must double
click something in the flowgraph and then click cancel… some random
erroneous bug in GRC.

Mike

3

How about the PFB synthesizer detailed by TR? I’d think all the dongles
would have to be phase locked and references tones (or comb) used to
calibrate the channels. If your signals are not crossing channels then
maybe you could get away without that.

Imre Biacsics wrote

I’ve a bunch of rtl dongles laying around, and wonder if it’s possible to
combine more then one to a single baseband signal.

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4

On 12.05.2014 17:49, Imre Biacsics wrote:

offset, equal to the samplerate of the first. (for example 2 x 2 Msps = 4
Msps baseband)

Is such a thing possible with GRC?

A couple of things, on top of what Mike said:

  • HF bands go lower than the frequency of the dongles. Keep that in
    mind.
  • As Mike said, clocks aren’t sync’d. So, if you’re using data from
    multiple dongles, that’ll most likely be corrupt.
  • To make things worse, they drift differently too. So, little chance in
    fixing things.
  • If all you want is an (inaccurate) “spectrum analyzer” for more than 2
    MHz, then this is actually not a bad idea. If you have N+1 dongles, you
    can use N of them to monitor N*2MHz of spectrum, and the other one to
    receive at frequencies you’re interested in.

M

5

On 05/13/2014 03:02 PM, Imre wrote:

[Discuss-gnuradio] Passive radar using my hacked dual coherent channel r
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All you channels will not only need to be phase-coherent, but also have
zero phase offset. That’s the only way you can reasonably simulate
having much wider bandwidth by “gluing together” a bunch of smaller
bandwidths.

My own experiments with the RTLSDR gear is that achieving phase
coherence across significant bandwidth appears to not be possible, even
with a common, high-quality master clock.

6

(Something went wrong with my mails, hopefully doing it right now, sorry
Mike, replied directly to you)

Using Mike’s example and some fiddling around:

http://www.livep2000.nl/code/grc/multichannel_input.grc

Tuning on a FM channel shows the mentioned mess,
interconnect the hardware clock? Maybe like this:

http://lists.gnu.org/archive/html/discuss-gnuradio/2013-09/msg00387.html

I’am give the polyphase synthesizer a look

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7

What about the passive radar - didn’t the author daisy chain the clocks
of
several RTLSDR?

Regards,
Vanush

8

On 14.05.2014 09:03, Vanush V. wrote:

What about the passive radar - didn’t the author daisy chain the clocks
of several RTLSDR?

That was two dongles sharing a clock. Not exactly daisy-chained, and it
didn’t look easily extendible :slight_smile:

M

9

We must not forget that each device has its own USB connection.
One reason that getting all in sync on a ’ per sample’ base must be
impossible.

Still a multi user system like the websdr can achieved by overlapping
the
channels and let the software decide what channel the user’s choice
fits.

For a nice graphical representation (Spectrum analyzer, waterfall) this
principle could work well.

Imre

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