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[SOLVED] Which Photo is carrier Leakage ?

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criterion456

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HI,


image001.png
image003.png
image005.png


As shown above, there is carrier leakage caused by a DC offset in I/Q signal.
And, in terms of spectrum, the carrier leakage component is at the center of signal.






Nevertheless,
image007.png


As shown above, in terms of spectrum, the carrier leakage component is between image and signal.


I’m confused.

In terms of spectrum, why are the locations of the 2 carrier leakage components are different ?
One is at the center of signal, and the other one is between image and signal.

Thanks a lot~!!
 

criterion456 said:
As shown above, there is carrier leakage caused by a DC offset in I/Q signal.
And, in terms of spectrum, the carrier leakage component is at the center of signal.
Click to expand...
This image shows a spectrum from a direct conversion modulator.
Nevertheless,
View attachment 142208


As shown above, in terms of spectrum, the carrier leakage component is between image and signal.
Click to expand...
This is from an intermediate frequency SSB modulator.


In terms of spectrum, why are the locations of the 2 carrier leakage components are different ?
Click to expand...
The carrier leakage it always at the LO frequency. For an IF modulator, the signal is modulated into a sideband, away from the LO. In a direct conversion modulator, there are no sidebands, so the signal sits on top of the LO. This is one of the advantages of SSB modulation.
 
You are comparing completely different modulation scenarios.

First picture describes I/Q modulation with baseband signal, second picture is SSB modulation with frequency offset.
 
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