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Old May 27th 04, 04:31 PM
Deepthi
 
Posts: n/a
Default Phase frequency Detector

Hi!
I need help understanding a conventional phase/frequency detector.I
consists of 6 two input NAND gates and 3 three input NAND gates.It
compares the phase and generates UP and DOWN signals.I was wondering
why the dead zone is high specially when there is a large reset delay
path.
Deepthi

  #4   Report Post  
Old May 28th 04, 03:26 PM
Rick Karlquist N6RK
 
Posts: n/a
Default

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.

Rick N6RK

"Avery Fineman" wrote in message
...
In article ,
(Deepthi) writes:

Hi!
I need help understanding a conventional phase/frequency detector.I
consists of 6 two input NAND gates and 3 three input NAND gates.It
compares the phase and generates UP and DOWN signals.I was wondering
why the dead zone is high specially when there is a large reset delay
path.
Deepthi


The "conventional phase-frequency detector" I know is the basic
circuit of the Motorola MC4044 package. That one is explained in
detail - in the form of a timing chart of ALL gate states with little
arrows indicating which gate acts on other gates - in the
September, 1982, issue of HAM RADIO Magazine in the "Digital
Techniques" column titled "Inside A Phase-Frequency Detector
(MC4044)." The particular timing diagram is rather straightforward
waveform diagrams rather than the symbolic logic-state graphics
others have used. I am the author of that column.

The "dead zone" you mention is due to differential gate delays
and can be minimized with high-speed logic families. It has
several causes depending on whether the signal input is leading
or lagging the reference input. The waveform diagram lets you
select either one and, with a schematic, see the path that causes
the differential gate delay.

Len Anderson
retired (from regular hours) electronic engineer person



  #5   Report Post  
Old May 28th 04, 03:26 PM
Rick Karlquist N6RK
 
Posts: n/a
Default

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.

Rick N6RK

"Avery Fineman" wrote in message
...
In article ,
(Deepthi) writes:

Hi!
I need help understanding a conventional phase/frequency detector.I
consists of 6 two input NAND gates and 3 three input NAND gates.It
compares the phase and generates UP and DOWN signals.I was wondering
why the dead zone is high specially when there is a large reset delay
path.
Deepthi


The "conventional phase-frequency detector" I know is the basic
circuit of the Motorola MC4044 package. That one is explained in
detail - in the form of a timing chart of ALL gate states with little
arrows indicating which gate acts on other gates - in the
September, 1982, issue of HAM RADIO Magazine in the "Digital
Techniques" column titled "Inside A Phase-Frequency Detector
(MC4044)." The particular timing diagram is rather straightforward
waveform diagrams rather than the symbolic logic-state graphics
others have used. I am the author of that column.

The "dead zone" you mention is due to differential gate delays
and can be minimized with high-speed logic families. It has
several causes depending on whether the signal input is leading
or lagging the reference input. The waveform diagram lets you
select either one and, with a schematic, see the path that causes
the differential gate delay.

Len Anderson
retired (from regular hours) electronic engineer person





  #6   Report Post  
Old May 28th 04, 08:43 PM
Avery Fineman
 
Posts: n/a
Default

In article [email protected]_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
  #7   Report Post  
Old May 28th 04, 08:43 PM
Avery Fineman
 
Posts: n/a
Default

In article [email protected]_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
  #8   Report Post  
Old May 28th 04, 11:20 PM
Steve Nosko
 
Posts: n/a
Default

"Avery Fineman" wrote in message
...
In article [email protected]_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


Len - Avery, whomever,
Our experiences differ. When designing PLL synths back then for two-war
radio, we always saw the dead zone. In a type 2 loop (hope I'm remembering
my control theory correctly) the extra integration allows the VCO to float
around within the dead zone, causing a low freq rumble at times. You could
watch the phase wandering around on a scope on the two PD inputs. We would
force some small leakage current just to hold it up against one side of the
dead zone. Perhaps the types of requirements causes the difference. We
were in the audio range with the PD reference freq and lock times in the
tens of ms. if I recall correctly.
If I recall, the Fairchild chip did a better job of matching the delays.
The small overlap causing a narrow pulse to occur seemed like a small
issue - not much energy at the ref freq for some applications. Our
synthesizers were of such requirements that there was a very tight balance
between lock time and spurious. The loop filtering took much care to get
the lock time and keep reference spurs down.
I designed and built what I still believe is the first 2-meter synth hand
held in 1973. 8.0 ma current max drain (varied 'tween 6.5 and 8 across the
band), 70dB spurious (the radio originally was 43 dB). 5kc resolution. In a
Motorola HT-220. Still have it. A year later I got tired of doing the
dip-switches and designed a keyboard entry system. That fella with the
HT-220 site didn't put it on. He does have Dale Heatherington's (sp) though
(got a couple of them also).
--
Steve N, K,9;d, c. i My email has no u's.


  #9   Report Post  
Old May 28th 04, 11:20 PM
Steve Nosko
 
Posts: n/a
Default

"Avery Fineman" wrote in message
...
In article [email protected]_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


Len - Avery, whomever,
Our experiences differ. When designing PLL synths back then for two-war
radio, we always saw the dead zone. In a type 2 loop (hope I'm remembering
my control theory correctly) the extra integration allows the VCO to float
around within the dead zone, causing a low freq rumble at times. You could
watch the phase wandering around on a scope on the two PD inputs. We would
force some small leakage current just to hold it up against one side of the
dead zone. Perhaps the types of requirements causes the difference. We
were in the audio range with the PD reference freq and lock times in the
tens of ms. if I recall correctly.
If I recall, the Fairchild chip did a better job of matching the delays.
The small overlap causing a narrow pulse to occur seemed like a small
issue - not much energy at the ref freq for some applications. Our
synthesizers were of such requirements that there was a very tight balance
between lock time and spurious. The loop filtering took much care to get
the lock time and keep reference spurs down.
I designed and built what I still believe is the first 2-meter synth hand
held in 1973. 8.0 ma current max drain (varied 'tween 6.5 and 8 across the
band), 70dB spurious (the radio originally was 43 dB). 5kc resolution. In a
Motorola HT-220. Still have it. A year later I got tired of doing the
dip-switches and designed a keyboard entry system. That fella with the
HT-220 site didn't put it on. He does have Dale Heatherington's (sp) though
(got a couple of them also).
--
Steve N, K,9;d, c. i My email has no u's.


  #10   Report Post  
Old May 28th 04, 11:41 PM
W3JDR
 
Posts: n/a
Default

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.

Joe
W3JDR


"Avery Fineman" wrote in message
...
In article [email protected]_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





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