Author Topic: Question on PSK  (Read 2001 times)

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Offline DavidDLCTopic starter

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Question on PSK
« on: March 11, 2017, 01:18:32 am »
I don't know about PSK, I was just reading about it, and I have the next question:

How is the change on the bits from 1 to 0 or 0 to 1 synchronized with the sine and cosine wave ?

My question is based on the attached block diagram.

 

Offline ataradov

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Re: Question on PSK
« Reply #1 on: March 11, 2017, 02:22:14 am »
It does not have to be synchronized. And in general, it is impossible to synchronize them.
Alex
 

Offline DavidDLCTopic starter

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Re: Question on PSK
« Reply #2 on: March 11, 2017, 02:35:06 am »
Alex, thanks for your answer

So is the change in phase at zero crossing ?

If yes, how is this achieved ?
 

Offline ataradov

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Re: Question on PSK
« Reply #3 on: March 11, 2017, 02:39:32 am »
So is the change in phase at zero crossing ?
No, phase changes when it needs to change. It does not affect anything. Things don't look good, but it does not matter for transmitters and receivers.

Plus, transmitters are rarely done this way, with a single multiplication. Typically you form baseband I/Q signal at some low frequency in a digital domain, pass it through the DAC, and then use quadrature mixer to bring it up to to the final RF frequency. This messes things up a lot towards the end anyway.

PS: And since the fist stage is digital, you can pick a baseband frequency that will work nicely.
« Last Edit: March 11, 2017, 06:13:41 pm by ataradov »
Alex
 

Offline w2aew

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Re: Question on PSK
« Reply #4 on: March 11, 2017, 06:12:23 pm »
Alex, thanks for your answer

So is the change in phase at zero crossing ?

If yes, how is this achieved ?

The baseband signals are filtered, so the phase change occurs slowly, across many, many RF cycles. If you made the phase change instantaneously at the carrier zero crossing, you'd occupy a very large bandwidth (bad).
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Offline DavidDLCTopic starter

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Re: Question on PSK
« Reply #5 on: March 25, 2017, 07:01:19 pm »
I cannot visualize your comment w2aew. Can you explain a little bit more, maybe some pictures or drawings will help.


David DLC
 

Offline ataradov

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Re: Question on PSK
« Reply #6 on: March 25, 2017, 07:10:20 pm »
Here is a screenshot from a real radio modem model.

As you can see, "bits" are not sharp squares, but a filtered signal. If they were sharp, they would occupy infinite spectrum, and FCC will be all over you.
Alex
 

Offline Howardlong

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Re: Question on PSK
« Reply #7 on: March 25, 2017, 08:26:39 pm »
Furthermore, it might be easier to consider how BPSK works first rather than QPSK.

The baseband filters applied to PSK are a whole area of comms engineering. There is the classic engineering trade off between several sometimes conflicting requirements including SNR/CNR, intersymbol interference and spectrum use (maximum energy within a channel with minimal spectrum regrowth). Which filtering is used is determined by the application, which in turn drives the priorities of the aforementioned parameters.

 


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