In article B_Htc.4677$pt3.1214@attbi_s03, "Rick Karlquist N6RK"
writes:

There are various fixes for the dead zone problem.
In the mid-1970's, Fairchild (the original company)
sold an "11C44" phase detector that got rid of the
dead zone by injecting a narrow pulse so that the
phase detector pulses would never have to try to
go to zero width. Eric Breeze holds the patent
on this technique; if interested read his patent.
Analog Devices makes that AD9901 phase detector
which gets around the dead zone by first dividing
the frequency by 2. However, it is not suitable
for a frequency synthesizer because of the large
spurious sidebands resulting from this technique.
Motorola had some patents on the circuits in its
MC145159 that dealt with the dead zone and sampling
sidebands. It also used a divide by 2 technique, that
was not documented; (we figured it out by observing
the chip's output). That chip may have been inherited by
On Semiconductor. It was originally developed for
some division of GE.

The "dead zone problem" is less a problem and more a
state of mind. :-)

When implemented with a charge-pump circuit (voltage
& time converter to current) between the PFD and loop
filter, it rather disappears into the woodwork of the whole
PLL. The phase difference between signal and reference
is proportional to the control voltage of the VCO producing
the basic frequency. There is ALWAYS going to be a
signal versus reference phase offset when the entire loop is
in lock so this dreaded "dead zone problem" will only show
up in a very narrow range of controlled frequency.

General intuitive thought on any PLL or other synthesizer
closed loop is that the relative phase between signal and
reference is zero. It isn't. If it was, then the VCO could not
be controlled. As a very rough indicator of VCO frequency,
that signal v. reference offset phase exists for quick scope
checking...when the control voltage range of the VCO is
known. Good for a quick bench check.

In practical terms, that dreaded "dead zone" isn't visible in a
real-world example.

Case in point: 23+ years ago, Rocketdyne Division of
Rockwell International (now a Division of Boeing) was beginning
work on a Deformable Mirror for laser work (they had a sizeable
optics group) that used a 1 MHz signal out of optics to indicate
the light phase error of an optical interferometer. I rigged up a
74H family phase-frequency detector circuit as the heart of that,
an integrator out of that into an A-to-D converter to get a digital
version for computer data manipulation. By all the careful
measurement, the expected dead zone didn't show up on any
graphing and the standard lab time interval counters could
resolve, accurately, 2 nanoseconds using time averaging.
[translates to rather less than a degree of phase error] The
optical physicists had been hopping up and down about "dead
zone" in meetings but the actual circuit performance didn't show it.

One reason for the non-observation of any dead zone is that the
digital gates forming the PFD were so lightly loaded in other-gate
capacitance that their propagation delays all tended to be the
same. Datasheet values of propagation delay of gates are all given
as maximums, rather worst-case things with lots of pFds connected
to outputs, etc. Put on half a prototype board, loaded only by other
gates of the PFD and the resistor input of an op-amp integrator, the
capacitance loading was minimal. [project was successful, and
spawned more work on deformable mirrors]

It can be an interesting academic problem to achieve a zero dead-
dead zone effect in a PFD, but thats about it. When working at
the comparison frequency of less than a few MHz, the PFD dead
zone due to differential propagation delays of the gates disappears
into the woodwork when using 74LS or faster digital families.

There's plenty to be concerned about in any frequency synthesizer
subsystem, but a phase-frequency detector gate structure is a
very minor problem in my opinion.

DDS and fractional-N loops in synthesizers have their own problems
such as spurious output, but those problems can't really be traced
to any PFD dead-zone effect. A PFD is wonderful as a control loop
element in that it can control a VCO (of the loop) from way off the
frequency and bring it into a lock phase range...from either worst-
case start-up frequency. [way back in the beginning of radio time,
lock loops had to use sawtooth sweep circuits to cure that start-up
condition, and couldn't control beyond +/- 90 degrees of phase
shift...PFDs easily handle +/-180 degrees]

Len Anderson
retired (from regular hours) electronic engineer person