[question] problem with real OFDM spectrum

[kydong]

Hi all,

I have trouble with spectrum visualization of OFDM signal.

I implemented OFDM system on Stratix II ES180 board and convert the Q-signal to analog via on-board DAC 14bit. I got the result like the figure I attached.
I wonder why the firs edge and end edge of spectrum is far away to each other? the different is large: around 20dB (in the matlab simulation, I got the same level of them)

the second is, there is a mini circuit (transformer) before the DMA output, so, the DC component should be eliminated form signal, why I still get the DC component on the spectrum analyzer (see the figure I attached)

Thanks very much,

 

[cwjcwjcwj]

Do you mean you generated OFDM signal using FPGA and used on-board DAC to convert digital signal to analogue signal. And plot the spectrum using spectrum analyzer?

Did you add Q-component and I-component signals together?

 

[kydong]

Do you mean you generated OFDM signal using FPGA and used on-board DAC to convert digital signal to analogue signal. And plot the spectrum using spectrum analyzer?

Yes. I did as you said.

Did you add Q-component and I-component signals together?

No, I just plotted the spectrum of either Q or I component. should I add them before transfer to analog signal for spectrum analysis?
thank you

 

[cwjcwjcwj]

Yes. You should. Let me know if that doesn't work.

 

[kydong]

Dear cwjcwj,

I tried, but the result I got is almost the same. I dont think that we should add the real and image component of OFDM signal before plotting spectrum ...

any idea to help me?
thanks very much,

 

[cwjcwjcwj]

1). Do you have a block diagram of how the OFDM signals generated?

2) Do you have a simulation model (Matlab,etc)? If so, try FFT the OFDM signals and have a look on the spectrum.

 

[kydong]

1. I designed it on Matlab simulink, after that, I use the Synphony tool for hardware design and created verilog code.
2. I also checked it on Matlab Simulink. the spectrum of OFDM is flat... not make a angle as viewing on spectrum analyzer.

I wonder maybe the number for FFT in spectrum analyzer is too low? or the buffer number of samples, for example.