PLL phase coherence - RF Cafe Forums
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Post subject: PLL phase coherence Posted: Mon Jan 23, 2006
Just wondering if anyone has any methods
of getting 2 PLLs, which are running off the same reference, to have
the same RF phase?
Posted: Mon Jan 23, 2006 4:00 pm
Everthing 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.
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.
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
RFout is in the 1-5GHz range, PFD's of the order of
Post subject: PLLPosted: Wed
Jan 25, 2006 8:59 pm
Leakage 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
So I agree with your pessimism, "guest who asked".
Good Luck anyway!
guest who asked
subject: 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.
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 question.
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?