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Design >> RF Design >> How to determine the delay in the reset pulse of a PFD ?
https://designers-guide.org/forum/YaBB.pl?num=1313401895

Message started by harpoon on Aug 15^th, 2011, 2:51am

Title: How to determine the delay in the reset pulse of a PFD ?
Post by harpoon on Aug 15^th, 2011, 2:51am

A classic way of avoiding the dead band zone in a standard PFD is to insert a delay in the reset line.

Does anyone know how to determine how long/short this delay should be ?

Thanks.

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by raja.cedt on Aug 15^th, 2011, 5:30am

hello,
higher the delay in reset path, will increase the static phase offset. So many people will add enough delay which can ensure no dead-zone. In many cases this will be done by simulations rather than expressions.

Thanks.

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by purplewolf on Aug 15^th, 2011, 5:41am

i would like to know how to plot vout vs phase error to see the dead zone in the first place

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by raja.cedt on Aug 16^th, 2011, 12:37am

hello,
so now i understood your question, you mean to ask how to simulate.

So conect loopfilter cap at the o/p of the charge pump and sweep the phase difference between refclk and fb clk from lag to lead. Then plot the current thorugh the filter cap vs phase difference between refclk and fbclk.

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by harpoon on Aug 16^th, 2011, 1:25am

opps ... it appears my post has been hijacked and relocated.

@raja.cedt, you understood my question correctly the first time.

My questions is what determines the lower and upper limit of this reset pulse delay ? If the delay is too long, what effect does it have on the PLL ?

Thanks.

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by raja.cedt on Aug 16^th, 2011, 2:55am

if delay is too long, then both charge pumps turn on for more time, so more ripple on the control voltage hence ref spurs.......

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by harpoon on Aug 16^th, 2011, 3:58am

so the shorter the better ? i.e. just enough over process, voltage and temp

thanks.

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by raja.cedt on Aug 16^th, 2011, 5:44am

yes..you are correct. Make sure than you don't have dead zone across PVT.

Title: Re: How to determine the delay in the reset pulse of a PFD ?
Post by harpoon on Aug 16^th, 2011, 5:57am

@raja.cedt

thanks for the speedy reply ...