# Noisy constellation Output

**URL:** <https://www.ruby-forum.com/t/noisy-constellation-output/231656>\
**Category:** GNU Radio\
**Created:** [August 21, 2014, 5:12pm UTC](https://www.ruby-forum.com/t/noisy-constellation-output/231656 "2014-08-21T17:12:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [August 21, 2014, 5:12pm UTC](https://www.ruby-forum.com/t/noisy-constellation-output/231656/1 "2014-08-21T17:12:49Z")

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Hi,

I am new to GNU Radio but have noticed something… When I create a PSK  
modulated signal and output that to a constellation sink, why is the  
signal so noisy? Why is the signal not in four perfect constellation  
points?

Signal Source -\> PSK Mod -\> Constellation Sink

Thanks,  
Luke

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**Author:** ![PEEL\_Luke](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/9fc29f/32.png) [@PEEL\_Luke](https://www.ruby-forum.com/u/PEEL_Luke)\
**Post date:** [August 21, 2014, 5:21pm UTC](https://www.ruby-forum.com/t/noisy-constellation-output/231656/2 "2014-08-21T17:21:58Z")

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On Thu, Aug 21, 2014 at 11:11 AM, PEEL Luke  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> Signal Source → PSK Mod → Constellation Sink
> 
> Thanks,
> 
> Luke

That’s ISI (inter-symbol interference) due to the transmit pulse shaping  
filter. The last of our new tutorials (still in beta) covers these  
issues:

[http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided\_Tutorial\_PSK\_Demodulation](http://gnuradio.org/redmine/projects/gnuradio/wiki/Guided_Tutorial_PSK_Demodulation)

Tom