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I am currently trying to generate a psk modulated carrier with gnu radio.

Attach is an image of my flow graph. enter image description here

As can be seen I am generating a bit stream using the Random Source module and implementing the modulation using the Constellation Modulator module. I then Low Pass Filter it and move forward to the conversion from baseband to passband generating Fc +- Fbb. I would like to utilize a bandpass filter to deal with the lower carrier frequency. It seems though when I implement the filter I still see both of the carrier frequency.

Why is this? I can't wrap my head around it.

EDIT: I figured out my bandpass filter question. All I had to do was set the FIR Type in the filter to "Complex->Complex(Complex Taps)(Decim)" from "Complex->Complex(Real Taps)(Decim)"

Now for the bandwidth question...

This is is fairly general question (I am assuming), but how does one define bandwidth in this context?

Typically BW with PSK is established at BB with the highest frequency component representing the BW.

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  • $\begingroup$ When you say "deal with the lower carrier frequency" what do you mean by "deal with" and what do you mean by "lower carrier frequency"? The carrier frequency is strictly Fc, do you mean the lower sideband? And when you say both of the carrier frequency do you mean both sidebands? If so, it would further help if you explained why you want only one sideband and what you are trying to accomplish? I suspect your confusion may be in not realizing that both sidebands exist at baseband as well (positive and negative frequencies) and the upconversion is just a frequency translation of the carrier? $\endgroup$ – Dan Boschen Oct 15 at 23:33
  • $\begingroup$ I have edited my post if anyone would like to review the madness. Thanks again. $\endgroup$ – DasBoy Oct 21 at 21:16
  • $\begingroup$ I would define bandwidth as twice the highest frequency component of the PSK waveform at baseband, since that is the RF bandwidth your signal will occupy. $\endgroup$ – Dan Boschen Oct 21 at 23:31
  • $\begingroup$ Okay that makes sense from a theoretical stance. I am still a confused about how one can define the highest frequency component of the PSK waveform at baseband. This would have to be done through the Random Source block by adjusting the maximum? My justification for this is it generates a larger amount of data thus more bandwidth needed at baseband increasing the maximum freq. at baseband. EDIT: I eventually would like to implement a delay of half a symbol period in my flowgraph, but to do so I must understand a few things like bandwidth and such. Thanks again, Dan. $\endgroup$ – DasBoy Oct 21 at 23:58
  • $\begingroup$ I haven't used GNU radio to give best answers, but why do you have two different frequencies for your sine and cosine in your upconverter? They should both be the same frequency. It looks like your modulator has an excess BW of 0.35 (based on the pulse shaping filter used) which I believe means the occupied bandwidth would be the symbol rate (which is half your data rate) * 1.35. $\endgroup$ – Dan Boschen Oct 22 at 0:13

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