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some questions on QPSK and transmitters

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whlinfei

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Hi,

I am doing Power amplifier design.
In many PA papers, QPSK is considered as a non-constant envelope signal.
But from its waveform, it seems it's constant-envelope.

Some books say the filtered QPSK is non-constant envelope.
Even in that case, how is the signal demodulated given that the demodulator simply times the singal with sine and cosine, without any use of amplitude information?

Thanks in advance.

Best Regards,
whlinfei
 

The amplitude changes when the phase changes. You need to use MSK to get constant amplitude.
 

Hi,

thank you.

But if I use a switching amplifier with a output filter, I also can get a non-constant envelope signal due to the shape change at the symbol transition.
I am a bit confused about why we have to use a linear amplifier to amplifier QPSK signal while we can just deploy a filter to get the amplitude variation?

Thank you.

Best Regards,
whlinfei
 

flatulent said:
The amplitude changes when the phase changes. You need to use MSK to get constant amplitude.
Click to expand...

Hi ,
Please I asked to explain this to me why the amplitude change in QPSK case ? ,,,,,, I notice that when I worked it practically & i didn't read this in books ,,,,,,,,,,,,,, Please explain to me this point if that possible

With my deepest thanks
Rawan Ayyoub
 

The diagrams that you see in textbooks almost always show an instantaneous phase change in QPSK. However, it would require an infinite bandwidth to achieve that phase change the way it is shown in textbooks (constant envelope). In reality all systems have some filtering to restrict the bandwidth, and this causes a non-constant envelope.
 

Firstly thanks a lot for replay .
However, I worked in this subject and this information is the first once i hear it , so please if you can expansion to this point , if you have any links or picture to explain it.

With my deepest thanks.
Rawan Ayyoub
 

See the attachment, section 4.5 on page 25 for a good basic description of the effect of filtering on QPSK.
 

Attachments

  • Agilent - AN1298 Digital modulation primer.pdf
    631.6 KB · Views: 86

ahhaaa , thanks for your cooperation , but i need more info exactly about QPSK as non-constant envelope ,,,,, I don't understand it obviously ,,,,,,,,,,,,, Thanks for your time .

Thanks in advance :)
Rawan Ayyoub
 

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