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Re: [Discuss-gnuradio] Ettus N210 GMSK 9600

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] Ettus N210 GMSK 9600
Date: Wed, 23 Mar 2016 22:22:06 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 03/23/2016 10:08 PM, Dan CaJacob wrote:
Derek is spot on.  0 dB into any USRP is screaming loud.  Most of the units are very sensitive, so it's perfectly fine to err on the side of too much attenuation to start and dial the gain as you need.  It depends on the USRP type, but signals much above -15dB can physically damage the hardware if you set the gain too high.
I'd actually said most of this to Tom privately, but forgot to copy the list.

It's instructive to consider what kind of "normal" signals one might expect to receive on a receiver intended to be connected to an antenna.

Consider your average +70dBm (10kW) FM station.    10km away from the TX, we expect about 90dB path loss, which means that the
  theoretical power seen by your antenna would be about -20dBm.  Perhaps a bit more, since the TX antenna won't be strictly isotropic.

At 50km away, that's 100dB of path loss, giving about -30dBm at your antenna terminals (ballpark, hand-waving).  Now, a signal of
  -30dBm is actually considered fairly strong, and your typical FM receiver will produce a very nice, nearly-noise-free output from
  signals entering your antenna at levels of -30dBm.

The RX inputs of most SDRs (Ettus radio are no exception) are designed for the kinds of signal levels that are typical in "connected to
  an antenna" environment.  They are less well suited to "connected to a laboratory signal generator", without significant attenuation.

The RF signal levels floating around in a lab are typically much "louder" than you might find "on air", so if one is used to lab conditions,
  it may not be obvious that a sensitive receiver designed for "from the air" reception would be unhappy in laboratory signal-level
  conditions.  So, keep the levels very modest indeed.

On Wed, Mar 23, 2016 at 9:35 PM Derek Kozel <address@hidden> wrote:
Hello Tom,

The input power to the USRP daughterboard should not be above -15dBm. I would recommend putting as much attenuation as you have available inline and raising the gain until just before a timesink shows the signal clipping, as Andy says having the maximum attenuation set will be raising your noise figure.


On Wed, Mar 23, 2016 at 4:28 PM, Andy Walls <address@hidden> wrote:

>                              From:
> Tom Golden
>                           Subject:
> Re: [Discuss-gnuradio] Ettus N210
> GMSK 9600
>                              Date:
> Wed, 23 Mar 2016 13:14:18 -0600
> ______________________________________________________________________
> Here's my flow-graph along with a snapshot of the constellation and
> FFT.
> Thanks!!
> -Tom

Hi Tom,

A couple of things:

a) The polyphase resampler isn't going to work well without a filter
definition in the taps field.

b) The constellation sink will not display anything meaningful without
sample timing synchronization; it is not useful at its current position
in the flowgraph.  The constellation sink also doesn't display anything
useful for an FSK modulation normally; (G)MSK being an exception, if
treating it like a PSK modulation vs. FSK.

c) You don't have any coarse or fine frequency synchronization.  That
will cause you major problems, if trying to treat GMSK as a PSK
modulation.  It will cause you minor problems, if treating GMSK as an
FSK modulation.

d) Timing recovery blocks usually want a signal that has peaks (which
you get by putting the signal through a matched filter), and those peaks
should nominally be scaled to +/- 1.0.  You don't have a matched filter
or an AGC before the Clock Recovery block.

e) The USRP's 0 dB gain setting is actually the USRP inserting the
maximum attenuation it can (e.g. 37 dB of attenuation).  That can kill
your signal to noise ratio.  You may want to consider adding "gain" as
long as the time domain signal doesn't look clipped (sometimes hard to
tell with FSK).

f) You may wish to look at what Nick Foster's gr-ais does to demodulate
a 9600 baud GMSK AIS signal.  It will probably give you a nice starting
point; just ignore the stuff about correlating to a preamble.

If you share your datafile somewhere, I might be inspired to whip a
flowgraph that works on it. :)  But that could rob you of the learning


> On Wed, Mar 23, 2016 at 1:01 PM, Marcus D. Leech <address@hidden>
> wrote:
>         On 03/23/2016 02:48 PM, Tom Golden wrote:
>                 Hi,
>                 I'm a novice gnu radio user.  I'm using gnuradio with
>                 an Ettus N210 cabled to a modem transmitting GMSK
>                 9600bps.  This is just for a test to verify the modem
>                 transmit bits.
>                 I'm having issues with resampling.  The N210 clock
>                 can't be set to a multiple of 9600, so I'm attempting
>                 to resample. I've tried various mechanisms but the
>                 output after resampling to 96000 is too noisy to
>                 successfully decode bits.  I've tried the GMSK demod
>                 block as well as the combination of Quadrature
>                 Demod->Clock Recovery MM->Binary Slicer - and neither
>                 works.
>                 I've also played with the Polyphase clock sync but I
>                 don't see any noticeable difference. Can anyone
>                 recommend a solution?
>                 Thanks!!
>                 -Tom
>         For a first step, it would be useful for you to share your
>         flow-graph with the list.

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Very Respectfully,

Dan CaJacob

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