I have just started studying the techniques of digital modulation. The book i am reading only has mentioned the psd of bpsk, bfsk , qpsk and msk. I would like to know, how the psd is actually calculated and I would also like to know, why the angles used in qpsk are (2m+1)pi/4 why cannot be one with a 89 degree and another with 91 degree instead of 90 degree and 90 degree. By these degree I indicate the angle between to signal vectors.

  • \$\begingroup\$ Can you link to where "instead of 90 degree and 90 degree" is coming from? Have you grasped the concept that seperating binary codes with a phase "gap" that is detectable in a reasonable time span is what is needed? \$\endgroup\$ – Andy aka May 23 '13 at 22:01
  • \$\begingroup\$ no i didnot learn it. but i would like to know. and the 90 degree is in the signal space representation, the orthonormal co-ordinate thing \$\endgroup\$ – shortCircuit May 23 '13 at 22:08
  • \$\begingroup\$ i have learned aliasing , intersymbol interference but i think its not what you are saying.. \$\endgroup\$ – shortCircuit May 23 '13 at 22:11
  • \$\begingroup\$ I'm sure there will be better answers but to "detect a change of something that is down to modulation you need something bigger than a 1 or 2 degree phase change in a carrier especially on high speed data links. \$\endgroup\$ – Andy aka May 23 '13 at 23:23
  • \$\begingroup\$ 89 degree >> 1 or 2 degree . Unfortunately Google is bad at electronics. perhaps if i could post an image then i would be more clear. \$\endgroup\$ – shortCircuit May 23 '13 at 23:26

Either a sine or a cosine has a 0 at multiple of pi/4. So one or the other is zero (say I is 0, Q is the one you want. This allows you to just read the value; the one that isn't zero (Q here) is that value.

If you read the value at (say) 89 or 91 degrees, the one (I) wouldn't be quite 0. And the one you wish to read (Q) isn't quite its full value. This makes it harder to get the right values. And this reading isn't easier.

  • \$\begingroup\$ i wished to upvote, but i donot have 15+ rep... \$\endgroup\$ – shortCircuit May 24 '13 at 7:36

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