Re: [Discuss-gnuradio] trouble with QAM mod/ WX GUI Constellation Sink

[Martin Braun]

On 17.07.2015 14:56, Bill Eaton wrote:
>> What I meant was that you plot the result of your timing + phase recovery
> 
>> into an XY plot, that would take care of those additional points
> you're seeing.
> 
> I have to confess that I don't even know what that means or how to go
> about accomplishing it. I have to take baby steps because I'm a baby.
> ;-) If you could point me to an example or some other things to try,
> that would be helpful.

OK, this is starting to get into basics of digital comms and this
mailing list is probably not a good place for that.

In brief, what you're plotting is the output of the QAM Mod -- which is
*not* a QAM constellation, but rather the complex baseband signal that
can be transmitted. This does include the QAM constellation.

> I looked through the examples that ship with GNURadio and didn't see
> anything obvious. Also, I looked through the GRC block tree to see if I
> could try something. There's a Synchronizers section. Something from there?

Try the examples in gr-digital.

> You thought maybe it was oversampled. It occurred to me that maybe the
> X-Y chart rate should be samp_rate/8, but I tried that and the chart
> still look largely the same.

That's not quite the right terminology, but if you plotted only a subset
of the samples of your signal, you would see the constellation.

It seems you're fine using GNU Radio itself, so I recommend you get an
intro book to digital comms and map what the say to the GNU Radio examples.

Cheers,
Martin


> 
>  
> 
> mlist_qam_constellation_xy.grc.png
> 
>  
> 
>  
> 
>  
> 
> -----Original Message-----
> From: address@hidden
> [mailto:address@hidden On
> Behalf Of Martin Braun
> Sent: Friday, July 17, 2015 1:20 PM
> To: address@hidden
> Subject: Re: [Discuss-gnuradio] trouble with QAM mod/ WX GUI
> Constellation Sink
> 
>  
> 
> On 17.07.2015 12:43, Bill Eaton wrote:
> 
>> From Martin Braun:
> 
>>
> 
>>>My preference is to build the demod/sync chain with it's own blocks,
> 
>>>and then just output the complex symbols
> 
>>
> 
>>> into either an X/Y plot (WX) or the QT Constellation Sink
> 
>>
> 
>> Even an X/Y plot ends up looking funny. I’ve attached a GRC file for
> 
>> X-Y, and when you turn on persistence, it looks better. But it’s very
> 
>> strange. Maybe my bits_per_symbol is wrong. Or maybe the sample rate
> 
>> for the X-Y chart should be different than the random sample rate?
> 
>> Here’s the 4 QAM X-Y.
> 
>  
> 
> Your plot looks fine, it's just oversampled (but perfectly
> time-synchronized, I'm guessing this is post-pulse shaping + noise?).
> 
>  
> 
> What I meant was that you plot the result of your timing + phase
> recovery into an XY plot, that would take care of those additional
> points you're seeing.
> 
>  
> 
> M
> 
>  
> 
>  
> 
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