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I perform real experimental tests for optical communications using DCO-OFDM. The transmitted signal is uploaded into an AWG (arbitrary waveform generator), then bias (2.5 V) is added before adopting the signal into the source light which includes a lens. The received signal is also focused by another lens and passed through the APD which is connected to the oscilloscope.

This structure is common and many people are using it; what I am asking about is the connection between the AWG and the oscilloscope. There is a cable of clock reference connecting the AWG to the oscilloscope. When I do not use it (the channel is always estimated and equalized), the constellation of the received signal becomes a circle (similar to the CFO effect). When using that cable, the signal becomes ok. I can not understand the meaning of that effect. Does that mean that when using that cable, we do not have any effect for the sampling offset, while when not used, I must estimate and equalize the frequency offset?

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  • $\begingroup$ Your question needs edits. How is the oscilloscope set up? What is the constellation of? One of the frequency bins of the signal, perhaps? I'm not sure I can see how this will work unless the O-scope can demodulate OFDM -- if that's the case, then it's a very important part of the O-scope setup. Also if that's the case, you should share the make & model of the 'scope, and whatever plugin is demodulating OFDM. I suspect that what's happening is that the O-scope is set up to demodulate, but not synchronize, and in the absence of a trigger you'd get exactly what you're seeing. $\endgroup$
    – TimWescott
    Aug 13, 2022 at 14:44
  • $\begingroup$ @TimWescott the oscilloscope is set up to only visualize the received signal. The constellation is 4QAM. I do the synchronization and demodulation offline by matlab. It means I collect the data from the oscilloscope and and then process it offline. $\endgroup$
    – Sajjad
    Aug 15, 2022 at 11:02

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