Data mismatch between Host and fpga

[CrazyEce91]

Hello,

   I have made a 4QPSK modulator on the host pc and I send the data to the fpga target an then i just read them back. When I do not use any pulse shaping filter the received data are exactly the same with the data I sent. When I use a pulse shaping filter the there is a mismatch  between the sent and received data, resulting in a fuzzy constellation. Why is this? Can I solve it using a matched filter on the receiver?

cheers  

[brandonj]

Hello

What hardware are you using and can you post code?

[CrazyEce91]

I am using NI 2942R (it has Kintex 7 FPGA). 

[brandonj]

Are you removing the filter from the VI or are you changing the parameters to change between working and non-working? Also, it looks like you might have started with the Modulation Toolkit examples. Which ones did you start with?

[CrazyEce91]

It's the PSK Tx example. the example lets you to choose what kind of pulse shaping filter are you gonna use. One of the options is to use none,just send the bits without any pulse shaping.If i don't use any filter I receive exactly what I sent, otherwise there is a mismatch between transmitted and received data. 

[brandonj]

I see. Can you reply with constellations you are seeing for both with the pulse-shaping and without?

[CrazyEce91]

Well guys I managed to solve the problem.The problems was that I was not downsampling the signal. This lead to transition points to show in the constellation, when I used downsampling I was able to sample only the constellation points and plot the succesfully. The problem was not occurring when I was not using any pulse shaping filter because that way my input was consisting only by the qpsk symbols points, so the constellation was correct regardless the sampling time. When I was using pulse shaping filter then my fpga filter was consisting from many values apart from the QPSK Symbols so i had to downsample in order to get the qpsk symbols and reject the transition points. I hope you understand what I am saying 🙂