abnormal behavior of constellation graph

[nauman2558]

i am doing dqpsk in one vi then outputting my modulated signal on daq card analog output channel then reading analog input channel of daq card from where modulated signal coming then demodulating my signal in another vi. the problem i am facing is that the constellation at receiver is not stable it gets into cirles then gets stable for some time, as far as its stable i get BER=0 and when its moving around we get BER= some value. can u plz tell me why its happening and is it due to my daq card or pc?

[LVbum42]

Nauman,

What DAQ card are you using? Can you give me a model Number?  Also, it would be great if you could a post screen shot of when your receiver is not stable, a screen shot when it finally stabilizes s, and an image of what you expect to see if possible.

Regards,

Isaac S.

[LVbum42]

Also, can you tell what kind of cable you are using?

[nauman2558]

sir i am using wire with BNC connectors at the end, i think its 50 ohm impedence wire. please have a look at the attatchment. my wire look like this...

[LVbum42]

Well it may be the cable, but I'd like to know what NI DAQ card you are using and a screen shot of your constellation map would definitely be a great help!

Regards,

Isaac S

[nauman2558]

sir i am atatching the png of my constellation graphs, it keeps on rotating and changing, when its stable for a while i get BER=0 other wise it has some value from .5 to .2. sir i am not using NI daq card. i am using keithley kusb 3116 usb data acquisition module. i have read in its literature that vi's linked to kiethley daq card, works same as daq mx.

[LVbum42]

Could you also upload your vi code?

Regards,

Isaac