PLL Phase Coherence - RF Cafe Forums
Post subject: PLL phase coherence Posted: Mon Jan 23,
2006 1:56 pm
Just wondering if anyone has any
methods of getting 2 PLLs, which are running off the same reference,
to have the same RF phase?
Post subject: Posted: Mon Jan 23, 2006 4:00 pm
else being equal you should be in phase. The higher you go in freq
the less this is true, due to mismatch of the VCO.
Post subject: Posted: Tue Jan 24, 2006 8:24 am
differences in leakage currents between the PLLs?
Post subject: PLL phase matchingPosted:
Tue Jan 24, 2006 3:34 pm
1. You'll want a real third-order loop,
to force the phase error at the detector to zero and keep it there.
See PLL books such as Gardner's classic.
2. The two loops should
be physically identical, and equidistant from the reference and
the point where the output phase difference is measured. (Assuming
a UHF/microwave PLL - you weren't very specific).
guest who asked
Post subject: Posted: Wed
Jan 25, 2006 1:14 pm
thanks for your suggestions, but if there
are leakage currents, then no amount of filtering will remove them.
a 10uA leakage current will flow regardless of whether you use a
2nd, 3rd or 4th order loop filter. The charge pump will then force
a phase error at the PFD inputs...
RFout is in the 1-5GHz
range, PFD's of the order of 1-20MHz.
Post subject: PLLPosted: Wed Jan 25, 2006 8:59 pm
currents are controllable, so they can be reduced - almost always
to negligible levels, given modern FETs. Whether or not you want
to go to the effort is your choice. Still, you're using multiplication
ratios in the 1000-5000 range, so phase error at the PFD would have
to be tiny.
At that point, many PFD's have dead-zone and transition-time
issues. So I wouldn't count on the phase matching too closely.
So I agree with your pessimism, "guest who asked".
guest who asked
Posted: Thu Jan 26, 2006 7:59 am
thanks for replying. yes that's
why im pessimistic. If one part heats up by 10 degrees and the other
doesn't, the leakage current would double in the higher temp one.
Depending on N divide ratios/ base leakage currents, it could translate
to a huge phase difference at RF.
Post subject: Posted: Thu Jan 26, 2006 2:02 pm
I do not
understand very well the involvment of PLL leakage current in the
process of generating a phase error.
From my understanding,
even a second order loop under a fixed reference signal, would give
no phase error if we look at the closed loop phase error transfer
function evaluated at T= ininite.
I the phase is different
the charge pump will generate a pulse every cycle, the only condition
driving to no pulse is a very small jitter around perfect phase.
As far as we are concerned about what is going on in the loop, the
phase must be exact
Under a moving phase reference then only
a 3rd order loop makes the job from what I read but that is another
I see more the other facter around the PFD to Charge
pump section to be contributiing to the error (reference conditionning
circuitry, LO amplificatoin circuitry, and tranmsission lines exlectrical
lenght variation due to whatever factor will indeed deteriorate
the phase error.
I am missing something regarding charge
pump leakage current?