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Design >> RF Design >> Second Order Passive Loop Filter for PLL
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Message started by cheap_salary on Dec 20^th, 2015, 8:23am

Title: Second Order Passive Loop Filter for PLL
Post by cheap_salary on Dec 20^th, 2015, 8:23am

See attached figure, there are two structures as second order passive loop filter for charge pump base PLL.

How should I choose one of them ?
Just from component's value ?

Title: Re: Second Order Passive Loop Filter for PLL
Post by loose-electron on Dec 28^th, 2015, 4:10pm

Preference (a)

Why?

It gives a single capacitor from the control voltage to the ground, which allows for the best HF filtering of switching noise.

The non ideal nature of the capacitor makes me go in that direction. Two devices in series will have greater parasitic inductance.

If everything is ideal, parasitics are ignored and non ideal charge pump switching is a non issue, then it does not matter.

However the reality is that they are all important.

Title: Re: Second Order Passive Loop Filter for PLL
Post by nrk1 on Dec 28^th, 2015, 10:40pm

To have the zero well below the unity loop gain frequency, C2 >> C1 in your first circuit and C1 >> C2 in the second. The values will come out to be about the same. The first one is more common. There is always some capacitor at the output node and it can be absorbed into C1 in the first circuit. Also, it would be better to put C2 below the resistor or somehow ensure that there isn't a lot of parasitic capacitance from the junction of C2 and the resistor which degrades stability.

Title: Re: Second Order Passive Loop Filter for PLL
Post by cheap_salary on Dec 29^th, 2015, 7:22am

[Example]Kvco=22MHz/V, Icp=5mA, Ndiv=300, fc=10kHz, PM=45deg

T1=2.24e-7
T2=3.84e-5
T3=6.59e-6

Passive Filter-1
C1=0.0385uF
R2=207ohm
C2=0.186uF
C1+C2=0.2245uF

Passive Filter-2
C1=0.224uF
R2=142ohm
C2=0.0465uF
C1+C2=0.2705uF

Passive filter-2 requires large capacitor than Passive filter-1.
And we can not use grounded capacitor in passive filter-2.
So I don't think there is any merit to use passive filter-2.

Neverthless, these two structures are introduced in literature.

See following.
http://edadocs.software.keysight.com/display/ads2011/Open+and+Closed+Loop+Simulation+of+PLL
This ADS example use Passive Filter-1 now.
However Passive Filter-2 was used previously in same example.

See the followings.
http://edadocs.software.keysight.com/display/ads2009/Phase+Noise+Simulation
http://edadocs.software.keysight.com/display/ads2009/PLL+Simulation+of+DECT+Radio+System
Here Passive Filter-2 is still used in these.

Title: Re: Second Order Passive Loop Filter for PLL
Post by loose-electron on Dec 30^th, 2015, 6:32pm

45 degrees phase margin?

Over process variance that may be a problem.

For a lowest phase margin corner I am good with it.