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Adventurer
Adventurer
863 Views
Registered: ‎11-18-2017

What is a phase alignment between different frequencies?

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Hello.

 

From what I understood, phase alignment is matching the phase of a signal.

For example, in the below figure, clk1, clk2 and clk3 have same frequency.

Additionally, clk1 and clk2 have the same phase(phase aligned, phase difference is 0 degree).

However clk1 and clk3 have different phase(180 degree phase different).

So phase alignment between clk1 and clk3 is aligning the signal so that phase different is 0 degree.phase.JPG

 

 

This is the case in signals with the same frequency but I'm not when it comes to different frequencies.

For example, in the below figure, clk1 and clk2 have different frequency.phase2.png

How can I define the phase difference? How much degree are they different?

And what does phase alignment means in this case?

 

 

The reason I addressed this issue is because MMCM locked signal is related with phase alignment as below figure (from UG572).

I'm not sure what the phase alignment means in the below explanation (phase alignment between who).

mmcm.JPG

 

Thanks for your help.

 

 

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1 Solution

Accepted Solutions
809 Views
Registered: ‎01-22-2015

Re: What is a phase alignment between different frequencies?

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@kimjaewon 

Phase alignment means that when a PLL or MMCM starts producing clock outputs, then the first rising edge for every output clock will occur at the same time. If two clocks (say CLK1 and CLK2) have an integer relationship (eg. 3 periods of CLK1 is equal to 5 periods of CLK2) then the rising edges for CLK1 and CLK2 will again align (every 3 periods of CLK1). If two clocks do not have an integer relationship, then phase alignment for these two clocks is meaningless.

Having phase alignment every time an MMCM or PLL powers-up is important for consistent timing analysis.  Without phase alignment of the clocks, timing analysis would be unable to analyze direct-crossing of data from one clock-domain to another clock-domain. 

Cheers,
Mark

2 Replies
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Contributor
Contributor
836 Views
Registered: ‎11-25-2013

Re: What is a phase alignment between different frequencies?

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I believe in this case, phase alignment might be understood as phase lock. In order to create other clocks, the MMCM must lock ( using some sort of PLL, digital or Xilinx magical ) to the incoming clock. You might google how a PLL works, and how frequency synthesizer use them if you are interested.

810 Views
Registered: ‎01-22-2015

Re: What is a phase alignment between different frequencies?

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@kimjaewon 

Phase alignment means that when a PLL or MMCM starts producing clock outputs, then the first rising edge for every output clock will occur at the same time. If two clocks (say CLK1 and CLK2) have an integer relationship (eg. 3 periods of CLK1 is equal to 5 periods of CLK2) then the rising edges for CLK1 and CLK2 will again align (every 3 periods of CLK1). If two clocks do not have an integer relationship, then phase alignment for these two clocks is meaningless.

Having phase alignment every time an MMCM or PLL powers-up is important for consistent timing analysis.  Without phase alignment of the clocks, timing analysis would be unable to analyze direct-crossing of data from one clock-domain to another clock-domain. 

Cheers,
Mark