Guys,
I managed another comparison test last night.
http://zoolu.co.kr/episodes/6
*. Test tone signal quality
I used a siggen as good as I could get. :-) According to the
spectrum analyzer, it seemed that the siggen did not generate any
noticeable harmonics(1st and 2nd pictures). Actually, I raised the
power level up to 0 dBm when I tested last night. But took the
snapshots with -34dBm level.
*. Signal level
I reduced the signal level down to -34dBm just to avoid any
possible input level saturation. I believe it is low enough. :-)
My questions
1. Still get spurs
As you can see in 3rd and 4th pictures, I still have quite
significant spurs in band. However, when I match the local oscillator
frequency with that of test signal like picture #5, all spurs were
gone away. So, I am still suspicious if those spurs came out of the LO
leakage from the FPGA CORDIC. (I don't believe that happens in the
digital domain. Probably, the digital switching noise spilled over to
the analog part and it was fed to ADC input. That's my best guess.) Is
there any comments, corrections, or suggestions? I probably have to
figure out Kyle Pearson's work on rounding in FPGA that Frank
mentioned before.
2. Automatic signal level scaling
I made comparison between different FFT sizes and the peak level
varied. As you can see, The Peak level was -11dB with FFT size of 8192
and -19dB with 1024 while the absolute signal power level was -34dBm.
The differences are about 8 dB and it explains because the FFT size
scale is 8(=2^3) times. Which part of the code does the automatic
scaling?
Ilkyoung.
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