Let me try again to explain dead zone.

Many PLL's never experience the dead zone because the loop
filter is constructed using op amps with high (10 mV)
offset voltage specs. This offset forces the loop to
lock up outside the dead zone. If you use a low offset op amp,
and then put in an offset adjust pot to take out any
residual offset from the phase detector, you can observe
the spurious sidebands at the phase detection frequency
null out. However, you will then find that the loop bandwidth
has changed substantially, because you are in the dead zone
region. The VCO will get more phase noise because the
loop wanders around (like a bang-bang loop) in the dead
zone, and/or the change in loop bandwidth has de-optimized
the suppression of VCO noise by the PLL. I have
personally observed this and other engineers I have
mentored have also observed it (after first arguing with
me that it wouldn't happen). By the way, the pot
tweaking to null sidebands doesn't hold over temperature
(no surprise) so it's still bogus even without the dead zone
issue.

To correct previous misinformation about the 11C44: the
gates are not better matched; rather there is an extra pulse
injection circuit as described in Eric Breeze's patent. This
information is from a conversion with Eric Breeze 28 years
ago. The 11C44 hasn't been available for many years but
that was due to mismanagement of Fairchild (which
was bought by National) rather than lack of merit of
the 11C44. (There was a lot of great technology at
Fairchild screwed up by mismanagement).

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