Second Order Passive Loop Filter for PLL
How should I choose one of them ?
Just from component's value ?
Some amount of input resistance must be present, for the structures to work. It is not evident from the diagrams. Is it in the formulae?
Anyway, in the upper network...
R2 increases the time constant of C2, and it also makes the rolloff curve less steep.
In the lower network...
C1-C2 create a capacitive transformer. R2's influence becomes a chore to figure out.
Wrong.
This is a filter where an input is current and an output is voltage.
So it is characterized by Trans Impedance, F(s) in figure.
See page-174(CHAPTER 21 EQUATIONS FOR A PASSIVE SECOND ORDER LOOP FILTER.) of http://www.unhas.ac.id/rhiza/arsip/r...deansbook4.pdf
[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
I take it the input current is finite, thus there is some input resistance. I guess it's not absolutely necessary to know the resistance.
I lack knowledge of the criteria that would tell which of the two filters is preferable. So I said, why not try a simulation? It reveals that both filters affect square waves approximately the same way... that is, making transitions less abrupt, creating the familiar waveform.
To see the obvious manner in which their behavior might differ, a sine sweep is useful, to bring out their rolloff curve. (And just because I was curious to see what these different capacitor combination filters might do, that a simple RC low-pass network doesn't do.)
Notice C1 has a sharper cutoff curve. C4 has a bandpass curve. These might serve some purpose for the PLL function, I don"t know.
Do you understand charge pump type PLL surely ?
These two structures give completely same transimpedance, F(s)=Vo/Ii.
So simulation results are completely same.
Your parameter's values give differerent transimpedance.
Again do you understand PLL surely ?
Completely irrelevant.
For people who can't understand current charge pump type PLL, my filters are used as (b) in attached figure.
Of course it would be wonderful if there is a forum member who (a) has direct experience with this same current charge pump type PLL, and (b) is reading here today, and (c) sees your thread, and (d) feels like replying.
Although I have not worked with a project as you are asking about, I could surmise that square waves were applied. In simulation the two filters act similarly, since they are both integrators. No big difference in behavior. And my sinewave sweep (although irrelevant to a charge-pump, or PLL), is the kind of test to bring out noticeable behavior, if it's there to notice.
Anyway, now that I think about it, I believe the resistor is variable. In other words, use a potentiometer to fine-tune performance on just one of the two capacitors. It changes the overall RC time constant slightly. It's easier than trying to find a single capacitor with some precise value. My suggestion is counter-intuitive, but it works if you don't try to push it to an extreme.
Your simulation is as good as mine. Why don't you try it? I believe you'll observe the same effects.
I don't understand what this thread is about? The equations in post #1 clarify that both circuits are functionally equivalent, you can use one or the other. So what's exactly the problem?
See #4 in this thread.
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.
I would use Filter 1 only. That is because you can change C1 and pretty much ONLY change the POLE frequency, and you can change C2 and pretty much only change the ZERO frequency. That is helpful when tuning it up.
See following.
http://edadocs.software.keysight.com...ulation+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...ise+Simulation
http://edadocs.software.keysight.com...T+Radio+System
Here Passive Filter-2 is still used in these.