This is a fairly general question I feel like.

So, the scenario I am presenting is this:

I am generating random data using the random source module and feeding that data into the Constellation Modulator, thus producing a Basebase modulated signal. This takes us to my first question, why is the signal from (-f to +f) and not just (0 to +f)? Attached is a picture.

enter image description here

Next question is this, if I implement a low pass filter from (0 to f) f being a frequency relatively close to 0 Hz, why do I not see the rejection following with the roll off? Also, why is the negative frequency component still there?

enter image description here

I am teaching myself quite a bit of dsp and gnuradio, so any constructive feedback would be great!

If you look at the picture that actually has some decent pulse shaping, it seems to me that the cutoff frequency is far to high as well, but it's only at this range that I get decent pulse shaping.



  • $\begingroup$ Why do you expect the spectrum to be zero for $f<0$? $\endgroup$ – MBaz Oct 9 '19 at 23:19
  • $\begingroup$ If your modulated signal at baseband is a complex signal (as opposed to a real signal) then the negative and positive exponential frequency components will not be the same. For the case of real signals the positive and negative frequencies are complex conjugate symmetric (same magnitude opposite phase) so the redundant negative frequency need not be displayed but they still exist. This more detailed answer may help you better understand the concept of negative frequencies and what you are doing with the Constellation Modulator: dsp.stackexchange.com/questions/41073/ $\endgroup$ – Dan Boschen Oct 10 '19 at 1:57
  • 1
    $\begingroup$ Regardless if the baseband signal is real or complex, by showing the positive and negative frequencies at baseband as you did here, we see what the spectrum will look like when it it up-converted to any particular carrier frequency (assuming a linear distortion-free up-conversion process)- so this is the common approach to plot the spectrum of your baseband waveform even if your signal is completely real (such as a BPSK modulation). $\endgroup$ – Dan Boschen Oct 10 '19 at 2:00

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