Constellation interferense

[DDavid]

Hi guys,

I looking at BB on constellation.
For example if the link are compressed then map constellation I will see that distant (from 0,0 by diagonal) will be close .
What I am looking that all kind of interf or distortion how they influence at constellation map.

What i see is all samples in the constellation map are going in circles.

I hope that I explain my problem!

David

 

[vfone]

When you get circles in the constellation there is a carrier interferer (or more).

 

[DDavid]

Hi,

Please explain;
when you are saying |carrier interf" what you mean?????

David

 

[vfone]

Unmodulated carrier wave signal (CW).

 

[DDavid]

Sorry but I still don't get it!?

 

[GVVIN]

Explain your situation. What modulation from where you are getting the input, where you are measuring etc.

What Vfone told is, the signal you are seeing is not modulated.

Unmodulated carrier wave signal (CW).

 

[DDavid]

I have modem that by front end connected to A/D.
In programmable application (that wrote by us) I can see SNRin (A/D) and constellation.
What I saw that constellation of each symbol (in each quarter I have 4 circles
I am talking about regarding single circle) all samples are making circles.
Why???

 

[biff44]

I think everyone is wondering exactly what you mean by 4 circles. Are they really circles, like as drawn with a compass? Or are you talking about 4 large dots, that are filled in with smaller dots inside?

The latter is typical of all systems with noise. The noise can be thought of as part amplitiude, and part phase noise. Amplitude noise will make your AD samples vary along a radius from the origin. Phase noise will make your AD samples vary in an arc at a constant radius from the center. If it is thermal noise (as in noise figure) you get round spots that are half amplitude and half phase noise.

Is that what you see?

Rich

www.maguffinmicrowave.com

 

[DDavid]

Hi

i am sorry for confusion that i made
In the next fig you can see example (not my design)
What I mean that I have 4 large dots in first quarter
when i am make zoom for single dot
I have a lot of sample, few of samples make circles

 

[biff44]

Your bottom picture is typical of a QPSK system that has a strong signal, and has roughly equal ammounts of AM and PM noise.

The top picture, who knows what that is. The bottom picture can turn into the top picture for a number of reasons. For instance, if the input signal power is dropped, the diameter of the 4 "dots" will start to increase because your signal to noise ratio is getting worse. At some point the dots will start to overlap a little. Then at some point, the thing in your system that tells the AD converter WHEN to sample the signal (a carrier tracking loop) will lose lock, and everything will smear around in a circular fashion.

Think of noise as just a random vector added to any of the individual dots making up one of the four spots. If the signal strength at the receiver input drops, the noise strength at the receiver input stays the same, the receiver will automatically increase the gain to try to see the smaller signal. So, even though your constellation still shows the same approximate radius, it is after the receiver has cranked in a much higher gain.
Rich

www.maguffinmicrowave.com

 

[DDavid]

top pic is zoom of bottom pic