Why the spectrum for a LFM input looks asymmetrical?

I have a model in Simulink R2021b composed by a "Linear FM Waveform” block , and a "Spectrum" block to visualize the signal spectrum. The model runs with no issues, but the spectrum I got looks asymmetrical in base band. This is unexpected because the typical spectrum of a complex LFM waveform is symmetrical in base band.

What is causing this discrepancy?

Answer 1

[MathWorks Support Team]

The spectrum at the output of the block “Linear FM Waveform” may look slightly different due to the following reasons:

-       The "Spectrum" block used is not in the RF Blockset library. Despite the differences of “Spectrum” blocks across the different libraries being minimum, the “Spectrum” block from the “RF Blockset” fits best for communication purposes.

-       One or more properties defined in the “Linear FM Waveform” block may be causing the spectrum asymmetry. For example, Gaussian envelope tends to produce asymmetrical frequency response in baseband, compared to rectangular envelopes. 

 The following actions are recommended to resolve spectrum asymmetry issues:

1.   Use a “Spectrum” block from the “RF Blockset” library. You may notice a slight increase in the block size. Please refer to the following documentation page for more information: https://www.mathworks.com/help/releases/R2021b/simrf/ref/spectrumanalyzer.html
2. Revise the parameter values used to configure the “Linear FM Waveform” block. Please refer to the following documentation page for more information: https://www.mathworks.com/help/releases/R2021b/phased/ref/linearfmwaveform.html