Sleeping Crystals
and
DLD ( Drive Level Dependency )
Copyright, 2000
©
Hong Kong X’tals Limited.
http://www.HongKongCrystal.com
Arthur Lee
Technical Director
1
What is Sleeping Crystals ???
Sleeping crystal is a well known phenomenon in
the Crystal industry :
The Crystal will NOT start, but after the crystal is
started by some mechanical or electrical
excitation, it works fine for some time, then,
after some unknown time ( usually with power
removed), it sleeps again --- not functioning.
The time that it begins to sleep again is
unpredictable, could be minutes or months.
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2
Sleeping Crystals make
ambiguous failures !
This phenomenon is one of the reasons
why when a crystal user finds a
faulty crystal, returns it to the crystal
maker, but the failure analysis
reports says the crystal is good.
Most likely the sleeping crystal was “woken up” by either the
mechanical shock / vibration during transportation, or being
tested at a relatively high drive level during failure analysis. (
note : there are many other reasons for these send-faulty, return-good situations, sleeping
crystal is one of them ).
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3
I have been using crystals for years.
Why I haven’t ever seen a Sleeping
crystal.
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4
Slow Starting Crystals
= Sleeping Crystals !??
Decades ago, electronic circuits worked at higher voltages
and lower frequencies. Hence, crystals operated at higher
drive levels ( say, 1000 W or above ). The high drive
levels of these circuits woke up the crystal every time it
was switched on, hence, in the olden days, this sleeping
phenomenon was not a big problem. However, the high
drive level of these circuit still require some time ( from
mSec to Seconds ) to wake up the sleeping crystal.
That is why some engineers call these crystals “Slow
Starting”.
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Nowadays application are more
demanding !
 The lower voltage of application circuits usually result in lower
drive levels,
 The higher operating frequencies, hence more phase lag in circuits
usually result in lower drive level ( due to lower load capacitance
CL ),
 Applications now demand faster start up time, say for example :
VGA display cards, USB devices with hot plug feature, PC card
with hot plug feature, etc.,
 Battery operated devices with power saving mode demand very
fast start up time, say for example : PDA, mobil phones, cordless
phones, and pagers that switch on and off in milli-seconds cycles
so as to save battery power.
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Why do Crystals sleep ?
The root cause of sleeping crystals is contamination in
the various production stages.
Contamination of quartz crystals is not only limited to
dirt particles, but traces of moisture, gases, oils, etc are
also considered as contamination. Hence, the cost of
controlling contamination in all the production stages
is high, and it is some times very difficult to locate the
source of contamination.
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The Structure of a
Quartz Crystal
Cover
Quartz Wafer
( Blank )
Any
Silver Electrode contamination
will affect
on both sides
performance of
the crystal,
Silver Epoxy
including :
Sealed with
Clean, Dry,
inert gas
Base
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Contamination
on the quartz
surface and,
inside the
structure.
8
Hot topic in the industry
Today, sleeping crystals are still a hot topic
in the industry and a lot of research work
is still being done by a lot of crystal
makers.
However, not all crystal makers know how to
avoid producing sleeping crystals. That is
why some of them do not like to talk about
this topic.
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Is there a cure for Sleeping Crystals ?
Unfortunately there is no cure, a sleeping crystal
will sleep again, even after excited / woken up.
The sleeping state is the crystal’s natural stable
state, it will return to this stable state after
external excitation is removed for some duration.
There is no way that a Sleeping crystal can be
cured and become a non-sleeping crystal again.
There is no way to remove contamination after the
manufacturing process.
There is NO cure !
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High Driving
-- by Crystal Makers ??
Some crystal makers “high drive” their crystals in
their production ( driving the crystal with a very
high drive level ) so as to wake up all the
crystals before outgoing test hence, achieving a
higher output yield. This means delivering
sleeping crystal to users, but these crystals will
sleep again eventually.
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11
High Driving
– wrong specification !?
For the same reason, you should specify the operating drive
level of your circuit correctly, so that good crystal makers
will not test ( drive ) the crystal inappropriately. Say for
example, if your circuit operates the crystal at 50 W, but
you specify 1mW ( by copying from old specification
sheets ), then you are instructing your crystal makers to
wake up ( high drive ) all the sleeping crystals during their
production, and final outgoing test.
Please note that 1mW is considered as a very high drive level nowadays, due to the
much lower drive levels in application circuits and much smaller crystal sizes than
older packages. If you have difficulty specifying the drive level of your circuit,
send your PCBA to your crystal maker for measurement.
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How to detect Sleeping Crystals ?
The major difficulty in detecting a sleeping crystal is :
once it is woken up, it behaves like a good crystal, and
the time required to wait until it sleeps again is
unpredictable ( could be seconds, minutes, days, or
weeks ).
We have to detect sleeping crystals indirectly, making
use of DLD ( Drive Level Dependency ) of the crystal.
Drive level dependency of a crystal is caused by the same
root cause as Sleeping Crystal, contamination, but less
serious.
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The DLD characteristics of
an ideal crystal
Over a wide drive level range ( decades of drive level ) the ESR and frequency
of the crystal changes very little until the drive level reaches its structural
limit. However, a Sleeping crystal may show the same characteristic after a
large external excitation.
2.5
Normalized
ESR
2
1.5
When Drive Level is below
the structural limit, ESR
changes very little, or no
change.
1
0.5
0
Drive Level
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When Drive Level is higher
than the structural limit, ESR
increases dramatically, this is
normal.
14
The DLD characteristics of some
non-ideal crystals :
Below the structural maximum drive level, the ESR and frequency
of a non-ideal crystal changes a lot ( ESR changes by several
times, Frequency changes by several tens of ppm ) due to
contamination in various production processes.
Normalized
ESR
8
When Drive Level is
higher than the structural
limit, ESR increases
dramatically, this is
normal.
6
4
The four crystals shown
here are bad DLD
crystals due to large
ESR change
2
0
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Drive Level
15
Non-repeatability dilemma
To measure this DLD characteristic, the maximum
measurement drive level specified in the range must be
high enough to excite ( wake up ) the crystal. Hence,
after measuring the first time, the DLD characteristic
of the same crystal changes ( usually appears better ).
And, if the maximum measurement drive level is high
enough, the crystal will be totally woken up, and may
perform like an ideal crystal for some time ( like the
above ideal DLD curve ). Hence, this measurement is
NOT repeatable, and we should only rely on the first
measurement result.
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Important note
If the DLD characteristic of a crystal does not change
with time, nor does it change with external excitations,
then, this crystal is NOT a sleeping crystal ( it may be a
poor DLD crystal, but not a sleeping crystal ).
Depending on the application circuit’s operating drive
level, this poor DLD crystal may or may not be a good
crystal :
if the application circuit works fine with the ESR at the
operating level ( including consideration of start up
speed), this is a good crystal, else,
if the application circuit can not operate with the ESR
at the operating drive level ( including consideration
of start up speed), then this is a failure.
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At 10nW Rs=37
At 100uW Rs=31, non-ideal process,
depending on applications, could be
good or no good
Crystal DLD
Typical Example
Changes due to
Design limit, not
because of poor
process
Kolinker
KH1200
Test data
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Time characteristic of DLD
ESR at low Drive Level,
or DLD-dRs,
or DLD-dFs
The DLD characteristic of a crystal may change with time, and will
also be changed by external excitation.
Excited by external small
stimulation : slightly waken up
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Sleep mode :
stable steady state
Waken mode :
Unstable state
Time
Excited by external larger
amplitude stimulation :
much waken up
19
Gamma ratio test for DLD
When we attempt to test a crystal chosen at random, can we
determine the status of the crystal : sleeping ? waken ? or semiwaken up ? Or, how to device test methods that can better test a
crystal’s Sleeping ( non-repeating DLD ) characteristic? The
answer to the question is not simple.
Some crystal engineers devised a test method focusing on the
ESR ratio instead of the absolute change in ESR, hoping to
solve the problem by comparing the ratio of ESR ( and
frequency ) between several measurements taken at different
drive levels.
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Gamma ratio test for DLD
The test method compares the ratio of ESR, and frequency change between the 1st : 2nd,
and 1st : 3rd measurements. Where :
12 = ESR of 1st measurement / ESR of 2nd measurement <= specified limit
13 = ESR of 1st measurement / ESR of 3rd measurement <= specified limit
( Fr of 1st measurement – Fr of 2nd measurement ) <= specified limit
( Fr of 1st measurement – Fr of 3rd measurement ) <= specified limit
1st m easurem ent
ESR
2nd measurement
3rd measurement
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Drive Level
21
Multiple measurement points
Some engineers further modify this method with multiple
measurement points so as to detect ESR / Frequency changes in
between the minimum and maximum drive levels.
For details, please refer to the IEC standards 444-6. These
methods are still under evaluation in the crystal industry, and
there is still, as yet, no final conclusion, especially when the
discussion has to cover a wide range of applications : military,
professional, industry and commercial.
The Kolinker KH1200  network analyzer provides a convenient
user interface for crystal engineers to further investigate these
methods.
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22
Definition of Sleeping --Relationship of Sleeping and DLD
The ESR and frequency of a commercial grade crystal
change inevitably with drive level. Some may change
by as little as 1 Ohm, while some may change by a few
hundreds or even a few K Ohms, or the ESR becomes
so high that even professional crystal measurement
systems are not able to measure the ESR, and indicate
an “unstable crystal”.
But will a poor DLD crystal sleep ? Or, how poor is the
DLD of a sleeping crystal ? Or, how ….
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Relationship of Sleeping and DLD
Structural Limit
( Design
Limitation )
Other Reasons
( unknown )
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Poor DLD characteristic
Contamination
( Non-ideal
process )
Fail at operating
Drive level
Nonrepeatable
DLD
( Difficult to
measure )
Repeatable
DLD
Sleeping
Fail below operating Slow Starting & Sleeping
Drive Level
Pass both of the
above
Good Crystal, but safety
margin should be set in DLD
specification, to avoid
potential Sleeping crystal
Fail at operating
Drive level
Simple Failure, easy to detect
Fail below operating Slow Starting, easy to detect
Drive Level
Pass both of the
above
Good crystal
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Rough Definition
Roughly speaking, Sleeping Crystals are crystals with Nonrepeatable DLD characteristic and fail to operate in the
application circuit at either:
the operating drive level of the application circuit, or
the drive level roughly 5 to 10 times below the operating.
Practically, this definition is correct, but quite dependent on the
characteristic of the application circuit that it is not easily
understandable by most crystal users.
Unfortunately, there is no clear formal definition for a Sleeping
crystal in the crystal industry.
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Crystal Measurement Precautions
DLD measurement
Measurement results verification
Failure analysis
“Fs stable, FL unstable” crystals
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DLD Measurement Precautions
To further eliminate the possible measurement error due to
equipment ( FL and RL measurement error ) :
Do not measure DLD-dFL, or DLD-dRL … etc
For crystal below 100MHz, measure DLD-dFr, and DLD-dRr,
and
For crystal at 100MHz or above, measure DLD-dFs, and DLDdRs
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Measurement Result verification
Measurement results verification ( including all other
parameters ) :
When verifying measurement results by re-measuring
the crystal, bear in mind that the crystal may have
been woken up after the first measurement. It is a
good habit to leave the crystals for a few days
before any re-measurement.
 Note : The same applies for all oscillators.
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Failure analysis Precaution
When performing failure analysis, bear in mind the
parameters of a crystal with “Poor aging
characteristics” also change with time, but poor aging
crystals could not be woken up by excitation. Of
course, some bad crystal may have both failure
modes : poor aging and poor DLD.

Note : The same applies for all oscillators.
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“Fs stable, FL unstable” crystals
 Some crystals when measured for FL will show
“Unstable”, but when measured for Fs, or Fr will
show stable.
 Most likely, these crystals have DLD problems.
In such an event, use the DLD graphic scan in
KH1200 to find out the DLD characteristic of such
crystals.
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Technical Support from HKC
Send HKC your production failures
Send us your latest PCBA for crystal
specification verification.
Web Page : http://www.HongKongCrystal.com
E-mail : HKXtals@HongKongCrystal.com
FAX : ( 852 ) 24985908
Copyright, 2000
©
Hong Kong X’tals Limited
31