If you build a PLL with a low offset op amp (not the usual
ones with 10 mV offset spec) and then use a pot to tweak
out the residual phase detector output offset voltage, you
will see the sidebands on the VCO null out at a certain setting
of the pot. At this setting, you will typically be in or near
the dead zone. Although the spurious sidebands are impressively
low, the loop bandwidth will now be unpredictable, which
may de-optimize the phase noise. Also, there may be a lot
of low frequency residual FM on the VCO because the loop
is acting like a bang bang loop. Many engineers have never
seen this happen, because it is unlikely to happen by accident.
BTW, the pot setting won't hold very well over temperature.

Rick N6RK

"Avery Fineman" wrote in message
...
In article , "W3JDR"

writes:

Dead-zone = phase noise. Very little dead-zone = very little phase noise.
Bigger dead-zone = bigger phase noise.
You can interpolate the rest for yourself.

I have to disagree with some of that.

First of all, a "dead zone" or the almost-exactly-in-phase condition,
occurs at only one VCO frequency where the control voltage sets
up the frequency for that in-phase condition.

Yes, at that exact frequency, there COULD be some phase noise.
But, the phase noise may NOT be from this "dead zone" effect.
Phase noise can come from MANY different sources. If it occurs
well away from the in-same-phase "dead zone" then the phase
noise is NOT caused by any "dead zone."

The relative phase between signal and reference inputs to a PFD
correspond to the VCO control voltage (times the charge-pump or
integrator circuit constants). Signal and reference phases when
in lock will always be offset from one another, one leading and one
lagging. A good loop will show a constant offset of phases even
when both inputs hold a constant phase.

Len Anderson