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Design >> Analog Design >> analog PLL and digital PLL
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Message started by sivacharan on Feb 19^th, 2007, 4:49am

Title: analog PLL and digital PLL
Post by sivacharan on Feb 19^th, 2007, 4:49am

what is the main difference between analog and digital pll??
is it the PFD that matters??
if the PFD compares two pulses then it is digital PLL regardless of the vco output nature. is this correct??

thanks.

Title: Re: analog PLL and digital PLL
Post by mg777 on Feb 19^th, 2007, 10:32am

At first glance I would dismiss it as the usual analog-digital gerrymandering.

However, one meaningful digital PLL example I can think of is in frequency synthesis e.g; use of pulse swallowing against a stable clock reference. Whenever you wiggle a VCO, VCXO, TCXO... all that's analog.

So, perhaps the right definition is whether the block is part of the boundary scan chain :-)

M.G.Rajan
www.eecalc.com

Title: Re: analog PLL and digital PLL
Post by adesign on Feb 20^th, 2007, 3:31am

The major difference comes in Analog and Digital PLL when loop-filter is considered.
In Analog PLL, the loop filter is analog in nature and is comprised of passive/active devices.
In Digital PLL, the loop filter is implemented using gates/flops.

Title: Re: analog PLL and digital PLL
Post by vivkr on Feb 20^th, 2007, 7:09am

Hi,

As mentioned by adesign, the loop filter is digital and that is the big difference. An advantage of
using the digital filter is the ability to design a stable loop with fast acquisition. Basically, you
can use various algorithms for lock acquisition.

A further advantage is that you can operate the PLL with much lower input frequencies without
requiring a huge capacitor for the loop filter. Standard charge-pump PLLs have loop bandwidth
about 5 -20 times lower than the input frequency.

Regards
Vivek

Title: Re: analog PLL and digital PLL
Post by fonseca.ha on Mar 6^th, 2007, 5:04am

I would say that the main difference comes from the VCO implementation.

A digital PLL has a numerically controlled oscillator and an analog PLL an analog controlled one.

As I see it the filter being digital or not comes from the above,

Regards,
Humberto