# how to draw waveform of QPSK?

Discussion in 'Homework Help' started by siddharth mittal, Nov 5, 2008.

1. ### siddharth mittal Thread Starter Member

Oct 14, 2008
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i was studying QPSK......i understand the formation of in-phase(odd bits) and quadrature(even bits) component

but i m unable to make the QPSK waveform from the waveforms for odd and even bits(11 00 01 10 )...

i have attached the waveform diagram also.......

pls help!!!

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3. ### siddharth mittal Thread Starter Member

Oct 14, 2008
23
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i tried wikipedia also........even the diagram that i attached is taken from there.....but still i didn't understand how QPSK is formed from in-phase and quadrature waveforms

4. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Hi Siddharth:

I think if you draw what you have with some grid lines, you'll see that indeed you have correctly drawn all four possible phases.

Eric

5. ### KL7AJ AAC Fanatic!

Nov 4, 2008
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Hint: It may also be easier to see if you continue the waveforms with a dashed line beyond the discontinuities. You will come up with four sine waves, each shifted by 90 degrees.

Of course, it's even easier to see this with an oscilloscope in the X-Y mode.

eric

Apr 5, 2008
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7. ### siddharth mittal Thread Starter Member

Oct 14, 2008
23
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I really thanks to u people for the help.......but my problem is still not solved

Actually i don't know when to shift the wave and how much shift is to be given?

pls. help!!!!!11

8. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,181
410

Hi Siddharth:

You want to do the phas shifting right at the zero crossing if at all possible. This results in the least amount of bandwidth. However, not all digital data will allow that...so the phase reversals will just have to happen when the bit shifts from a 1 to a 0 or from a 0 to a 1. The shift will ALWAYS be 180 degrees on each carrier....that's the only option you have!

Eric

9. ### KL7AJ AAC Fanatic!

Nov 4, 2008
2,181
410
If you look at the possible locations on a phasor diagram, your I signal will give you either 0 degrees or 180 degrees. The Q signal will be either 90 degrees or 270 degrees. When you ADD all possible points with vector addition, you get 45 degrees, 135 degrees, 225 degrees, and 315 degrees, or one point in the middle of each quadrant.

Do you have access to an oscilloscope? It's really a lot easier to understand if you actually build an IQ modulator.....(extremely simple!) and watch what happens under each condition.

eric

10. ### siddharth mittal Thread Starter Member

Oct 14, 2008
23
0
hi!

actually i m a student....so don't have access on oscilloscope for it now

i just want to understand the theoritical aspect of it......

pls. see the figure that i attached in my first post......it would really b very heplful if u tell me the phase shift at each crossing(11 00 01 10 )