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Re: [Discuss-gnuradio] What's wrong with this constellation?

From: Martin Dvh
Subject: Re: [Discuss-gnuradio] What's wrong with this constellation?
Date: Fri, 08 Aug 2008 17:28:38 +0200
User-agent: Icedove (X11/20071018)

Peter Monta wrote:
> Hi Mason,
>> ... The amplitude of the receiver output seems to spray out pretty
>> widely from the constellation point intended.
> It could be an AGC issue---is the AGC time constant long compared with a
> symbol?
> Is the distribution of constellation points uniform (each is picked with
> equal
> probability)?
> One thing you could try, if the channel permits, is to temporarily hold
> the AGC
> fixed to see if that gives a better constellation (perhaps at the wrong
> size;
You should indeed try this.
Do you sent (short) bursts of QAM or is it a continuous stream?
If they are bursts then all loops (carrier recovery, AGC, clock-recovery) can 
behave strangely.

If it is a continuous stream you could also try a feedback-AGC in stead of a 
forward AGC which should give much less gain noise (after it has

It is also possible that the amplitude noise comes from the analoge components 
in your TX-path or your RX-path.
If you have noise or hum on the supply voltage of any gain or mixer stage then 
this will result in amplitude noise.

You could figure this out by sending out a single carrier (no modulation, use ) 
and receive it with an SSB-receiver script or just usrp_fft.py.
generate carrier:
usrp_siggen.py -R B --const -i 128 -f rffreq -a amplitude -g rfgain
look at spectrum:
usrp_fft.py -R A -d  64 -f rffreq -g rfgain
look at oscope:
usrp_fft.py -R A -d  64 -f rffreq -g rfgain -S

(Use the same analog signal path as you use for QAM)
Now you can look at the spectrum, listen to it and look at it with an oscope.

If the signal is clean, your QAM-TX script or QAM-RX script is to blame.
Make sure you also look at LF noise or hum, like 50 Hz or even lower.
If you see noise, then there is something wrong with your setup.


> or if you're using decision feedback, try to get it to the right size
> :-) ).
> Cheers,
> Peter Monta
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