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Visitor jdarko
Visitor
217 Views
Registered: ‎12-17-2018

MMCM/PLL ideal VCO operating frequency

I am instantiating an MMCM/PLL and I am trying to figure out the ideal VCO operating so I can calculate the M, D, and O to generate  a desired output frequncy (I am using only one clock_out port).

For the zynq-7020 with speed grade -1C: MMCM_FVCOMIN = 600 MHz, MMCM_FVCOMAX  = 1200 MHz, PLL_FVCOMIN = 800 MHz, PLL_FVCOMAX = 1600 MHz

How is the ideal operating of the frequency of the VCO determined? Looking at the clocking wizard, the clock wizard reports a different VCO frequency depending on the desired output frequency.

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4 Replies
Scholar drjohnsmith
Scholar
197 Views
Registered: ‎07-09-2009

Re: MMCM/PLL ideal VCO operating frequency

Your trying to out do the wizard ?
good luck,

Ideal frequency of the VCO in the chip , is un known outside of the IP ,
You could try talking to your local FAE and probably setting up an NDA, but that can be long and involves your lawyers,

If you want to set set up your own M, N and O, then general rule is start mid range,

On a more general level, not xilinx specific,
your into the world of analog..

VCO jitter tends to be constant across its tuning range,
so the faster it runs the more output divider you can achieve , to get a lower jitter out,

But the higher the VCO, the more power it is liable to take,
Your also into the loop filter parameters,
these are going to be influenced by the M/N values, and affect how the input jitters is attenuated.

My humble advice is , if your asking, then you are far from being able to do as well as the Wizard,


<== If this was helpful, please feel free to give Kudos, and close if it answers your question ==>
Visitor jdarko
Visitor
186 Views
Registered: ‎12-17-2018

Re: MMCM/PLL ideal VCO operating frequency

Oh no. Not trying to out do the wizard.

My team is trying to parameterize Xilinx's MMCM and PLL with generics, so I need to set up my own M, N and O. I read in UG472 that "The starting M value is first determined. This is based off the VCO target frequency, the ideal operating frequency of the VCO." So I was wondering what the ideal VCO operating frequency was/if it was published.

Good to know. Thanks.

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160 Views
Registered: ‎01-22-2015

Re: MMCM/PLL ideal VCO operating frequency

@jdarko 

"The starting M value is first determined. This is based off the VCO target frequency, the ideal operating frequency of the VCO."
Your quote comes from pg 77 of UG472(v1.14), where it also says, “The goal is to make D and M values as small as possible while keeping ƒVCO as high as possible.” 

So, the ideal operating frequency for the VCO is fVCOMAX.  

As mentioned by drjohnsmith and by pg248 of UG949, setting the VCO frequency as high as possible “generates less clock uncertainty due to reduced jitter and phase error”.

However, other restrictions that we place on MMCM operation will usually cause fVCO to be less than fVCOMAX.  -as you will find when working with the MMCM equations in chapter 3 of UG472.

Mark

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Visitor jdarko
Visitor
121 Views
Registered: ‎12-17-2018

Re: MMCM/PLL ideal VCO operating frequency

Okay so the ideal frequency is fVCOMAX but due to restrictions it's actually less and how the clocking wizard calculates the actual VCO frequency it uses is part of Xilinx's secret sauce.

Thanks.