Temperature compensation of crystal oscillators using microcontroller- spl mu CTCXO

1994 IEEE INTERNATIONAL FREQUENCY CONTROL SYMPOSIUM
TEMPERATURE COMPENSATION OF CRYSTAL OSCILLATORS
USING MICROCONTROLLER - pCTCX0
Dejan Habid and Dragan VasiljeviC
Faculty of Electrical Engineering, University of Belgrade
Bulevar Revolucije73, P.O.Box 816, 11 000 Belgrade, Yugoslavia
to accelerate production, computer aided methodsmay be
used in compensation network element selection process
Abstract
[1,21.
miThe design of compensation
circuit
in
crocontroller temperature compensated crystal oscillator
(pCTCXO) and designofproduction
line (ACCL) for
oscillator automatic calibrationare presented.
The communicationproperty of pCTCX0is
introduced in order to provide automatic calibration. The
shortened successive approximation algorithm gives short
calibrationtime and reliableconvergenceofoscillator
adjustment by the ACCL system.
The pCTCXOisrealized as alow-costdevice
encapsulated In a single cubic inch volume. The O.5ppm
accuracy of frequency in -40 "C/+85 "C temperature
range IS obtained by a%bit A/D converter,10-bitD/A
output and 5 12 bytes ofsoftware placedin anEEPROM .
Including the standard
laboratory
frequency
counter, temperature chamber and PC host computer the
ACCL system requires only the additional analog multiplexer and interfaceboard. The calibrationsoftware of
250 Kbytes is executed by the host computer. The ACCL
system can compensate many oscillators simultaneously
without selection of components and without human control in only one temperature run during production and
exploitation.
Advances in integrated circuit technology introduced
digital
temperature
compensation
techniques
(DTCXO). In comparison to analog TCXOs, they provide
higher accuracy for wide temperature ranges, and simpl@theadjustmentprocedurewithfewertemperature
runs.
The initial DTCXO solutions were based on a
look-up table saved in a nonvolatile memory incorporated
in the compensationcircuit[3,4,5,6]. An almostlinear
temperature sensor generates voltage whch is then converted by an M D converter having an output which serves
as the memory address. The selected memory location
contentcorrespondstotherequiredvaractorvoltageto
compensate oscillator frequency drift [1,2]. A D/A converter converts back the digital memory contents to the
analog varactor voltage.
The microcomputer compensated crystal oscillators (MCXO) wereobtained in further developmentof
digital compensation. Nonvolatile memory is replaced by
microcomputer which improves compensation flexibility.
The A/D convertergivestemperature
in digitalform.
This data are used by a microcomputerto generate varactor voltage using a small look-up table and interpolation
calculation [7].
The above described digital compensation techniques are similar to the analog compensation producing
a temperature dependent voltage for varactor. The dualAnalog temperature compensated crystal oscillamode
MCXO oscillator is introduced to improve compentors(TCXO)haveatemperaturesensingcompensation
sation
accuracy [8,9,10]. This circuit is based on the dinetwork generating a control voltage applied to a varactor
rect digital frequency synthesis. The difference between
placed in series with oscillator's c y t a l . A compensation
scaled overtone and fundamental oscillating frequency is
networkprovidesfrequencypulling
to canceloscillator
used
as the gate time for period measuring counter which
frequencytemperature drift. Compensationnetworkadcounts scaled overtone frequency. The obtained number
justment requires three to five temperature runs. In order
represents actual oscillator temperature. Using oscillator
Introduction
587
0-7803-1945-1/94 $ 3 . 0 0 0 1994 IEEE
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:16:59 UTC from IEEE Xplore. Restrictions apply.
(temperature dependent voltage generation approach) is
shown in Figure 1. The temperature sensor voltage is digitized by an A/D converter. The CPU accepts temperature
data, finds the corresponding data from the look-up table
taken during calibration, and makes the interpolation to
calculate the oscillator's VC0 input voltage required to
obtain nominal output frequency. The voltage is converted back toanalog form by a D/Aconverter.
frequency to temperature dependence
data taken during
temperature calibration, the microcomputer calculates the
number of pulses to be deleted from the output signal in
order to obtain constant frequency [S]. The dual mode
MCXO realization produces high precision compensation.
The purpose of this paper is to introduce a concept of microcontroller compensated crystal oscillator
suitable for fully automated production. The oscillator
compensation circuit is accomplished by a microcontroller (pCTCX0). It is a small, low-power and cheap device
with rich resources and property to communicate, which
is necessary for oscillator calibration by an automatic
computer controlled line (ACCL). The proposed concept
provides unsupervised fully automated temperature compensation of clystal oscillators without component selection in only one temperaturerun. Many oscillators may be
calibratedrecalibrated
simultaneously in productiodexploitation.The
proposedcompensationconcept
supports both digital compensation approaches: dual
mode MCXO and classical, temperature dependent voltage generation for V C 0 input control.
The general purpose 8-bit microcontrollers have
all necessary resources for the realization of the described
compensation circuit. The CPU renders programmability
and calculations. The look-up table is saved in anonvolatile memory. The analog values are coupled to the
microcomputer by A/D and D/A converters. Microcontroller resources such as frequency dividers,
timedcountersand arithmeticdlogic functions may be
used to realize dual mode MCXO in combination with
external mixers and filters. Finally, microcontroller ability to communicate allows calibration of many oscillators
in a single temperature run by a fully automated ACCL
line. Oscillator identification is obtained by a hardware
key connected to the microcontroller's input digital port.
The pCTCXO circuit has two operating mode:
calibration mode and autonomous modes.
T h s paper describes the design of pCTCXO and
ACCL system. Temperature compensation circuit of the
pCTCX0 iselaborated in the firstsection. The automated
production line ACCL is presented in the second section.
Finally, experimental results are reported in the third section.
The simple software which controls autonomous
operation of the pCTCX0includes routines for control of
the A/D and D/A subsystems and for search and interpolation of data from the look-up table. Compensation accuracy is determined by quality of components.
Design of uCTCXO
Communication
between
the
pCTCX0
and
ACCL system during calibration is realized through an
asynchronous serial M232 interface. Calibration software
The pCTCXO temperature compensation circuit
pGGG-1
HARDWARE KEY
TX FiX GND
t l l
EMPERATWE
SENSOR
RAM
VC0
INPUT
EEPROM
MICROCONTROLLER
Fig.1 pCI'CX0 compensation circuit.
588
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:16:59 UTC from IEEE Xplore. Restrictions apply.
is a part of the automated production line ACCL and not
of the pCTCXO itself.
Design of automated uroductionline ACCL
The ACCL system is shown in Fig.2. The host
computer controls the temperaturechamber
and frequency counter operation through a standard E E E 488
bus interface to provide temperature adjustment and frequency measurement. The interface board is used as an
adapter between the hostcomputer'sISAbus
andthe
asynchronous serial m 2 3 2 interface intended for communication with pCTCXOs and the analog multiplexer.
Three wire serial connection allows simple handling of a
large number of pCTCXOs. The intelligent analog multiplexer provides connection of oscillators signal output to
the frequency counter.
pTq
EEE 488 Bu
Host
Computer
oscillator is connected tothe frequency counter using
analogue multiplexer.
The calibration process is open when the host
computer gives information about desired temperature to
the chamber using 488 bus. By the same bus host computer reads frequency counter. When oscillators frequencies are stabilized, the host decides that the oscillators
transient thermal regime is finished and the temperature
is held constant. Then
the
adjustment of
selected
pCTCX0 is started. Using asynchronous serial interface
the host computer reads the A/D converter and attempts
to pull the oscillator frequencyto the nominal value
through adjustment of the D/A converter input in thefirst
iteration. Now the 488 interface is activated again. The
hostcomputer reads the frequency counter. When the
oscillator frequency is stabilized, the host computer compares it with the target frequency. If the difference is satisfactorily small, the adjustment is finished. If not, the
host computer changes the D/A converter input and the
Frequency Counter
SIGNAL INPUT
1
I
Intelligent Analog
Multiplexer
1 2 3 4
...
OUT
pCTCX0
No. 1
Interface
Board
As232
J-wi,e
1 -
Asynchronous
Interfate R5232
I
I
255
OUT
.. .
pCTCX0
No. 255
R5232
t
TemDerature Chamber
I
I
Fig.2 ACCL system.
The ACCL system operates under control of the
calibration software executed on the host computer. The
calibration process includes three tasks: communication,
calibration and programming.
The communication is started whenhostcomputer downloads the program for communication to all
pCTCXOdevices simultaneously. Then hostcomputer
and selected pCTCXO exchange the address defined by
the pCTCXOs hardware key. Signal out of the selected
process is continued until target frequency is achieved.
When adjustment is finished, the host computer defines
the new temperature point, andthe calibration at this
temperature is carried out in the same way. The number
of temperature points is determinedby the uncompensated oscillators frequency to temperature characteristics
(hflf/AT and the desired accuracy of compensation[1,7].
Obviously, the processingtime is proportional to the
number of temperature points.
589
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:16:59 UTC from IEEE Xplore. Restrictions apply.
When calibration is finished, the host computer
programs pCTCXO's EEPROM with contents of look-up
table and program for oscillator operation in autonomous
regime.
defined as won as a new temperature point is selected. In
this way, all pCTCXOs will settle their frequencies simultaneously and adjustment time is shortened.
Two problems had tobe solved during thedevelopment of the described algorithm. The first problem is
the determination of the initialvalue for theD/A input in
order to obtain minimum number of approximation steps
and shortest adjustment time. The right choice presumes
a value as close as possible to the varactor voltage which
gives nominal oscillator frequency. The initial value can
be estimated on the basis of experimentally determined
data on influence of every bit of the D/A converter on the
oscillator frequency change Af for each pCTCX0 family.
These data are placed in the host computer which uses it
together with the Werence hf ' between the measured
operating and nominal frequency in order to estimate the
starting value of the D/A converter input.
ExDeriments
The second problem is how to select D/A input
in the second and other further steps in order to obtain
convergence of the adjustment procedure. The successive
AfJf
The compensation circuit of pCTCX0 device is
realized by a Motorola 68HCll microcontroller and by a
KTY silicon temperature sensor, Fig.1. The passive second order RC filter is added to complete pulse-widthD/A
converter circuit. The pCTCX0 autonomousmodeoperation is controlled by a 512 bytes program including 95
bytes look-uptable.
The frequency stability better than +/-0.5ppm in
the range of operating temperatures of (-40 "C , +85 "C)
is obtained bya 9 bit A/D converter and 10 bit pulsewidth D/A converter, Fig. 3. The look -uptable is obtained
in 30 points for AT cut crystals with uncompensated frequencytotemperaturecurve
with maximumfrequency
deviation At7e+/-25ppm
and maximum slope
(Aflf)lAT=l.Sppm/ "C.
[PPml
0.7
0.5
0.3
0.l
-0.1
-0.3
-0.5
-0.7
-50
Fig.3 Frequency to temperature characteristic of a compensated pCTCX0.
The compensation circuit of thepCTCX0 is
built by general purpose and low-cost components. The
entire pCTCXO is encapsulated in a standard
36mmX27mmX19mm size case. The compensation circuit current consumption is low(lOmAI5V) in awide
range of operating temperatures (-40"C, +85"C).
approximation algorithm, common in ND conversion, is
agood choice due to the reliable and fast convergence.
The proposed procedure represents a shortened successive
approximation algorithm.
Whenmany pCTCXO devices are adjusted simultaneously, the input to each oscillator's D/A should be
590
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:16:59 UTC from IEEE Xplore. Restrictions apply.
The ACCL line includes standard instruments
such as laboratoryfrequency counter and temperature
chamber with IEEE 488 bus. A PCis used as a host computer (Fig.2.). The ACCL line also includes a specially
designed interface board and an intelligent analog multiplexer. A 68HCll microcontroller and a RAM buffer
memory make up the interface board. The analog multiplexer has the same microcontroller and an analog multiplexer tree with 255 inputs. The ACCL system is under
control of a 250 Kbytes s o h a r e under DOS.
The operation of fully automated ACCL system
during calibration is illustrated in Fig.4. The number of
attempts to obtain nominal frequency at a selected constant temperature depends onthe closeness of the varactor
voltage initial value and its final value. The diagram also
illustrates the convergence of the shortenedsuccessive
approximation algorithm.
The ACCLsystem calibration time for three
pCTCXO devices is shown in FigS. The dark rectangle
represents the time interval needed for temperature stabi-
-5
-45
'-
1
-18
lAttempt 1
I
A
I
9
I
approxination algorithm and time spent for oscillator
settling after change of varactor voltage. The main problem in design of ACCL system is to discover efficient
calibration algorithm to save adjustment time.
Conclusions
This paper presents the design of a temperature
compensationnetwork in microcontroller compensated
crystal oscillators (pCTCX0) and the design of production line (ACCL) for automatic oscillator calibration.
The pCTCXO circuit is built by general purpose
low cost components. It is a small, intelligent unit able to
communicate with the outside world.
The proposed concept of ACCL system provides
fully automated, unsupervised temperature compensation
of crystal oscillators withoutcomponentselection in a
single temperature run. Many oscillators may be calibratedsimultaneously in productionand later, during
I
I
1
36
Temperature ["C]
63
90
Attempt 2 Anempl3 Attempt 4 Attempt 5 Attempt 6 Attempt 7 Attempt 8
]
Fig.4 Operation of ACCL systemduring calibration.
lization in the temperature chamber and oscillators. The
gray rectangles designate time consumed by the ACCL
software during execution of the shortenedsuccessive
exploitation. Both digital compensationapproaches are
supported: dual mode MCXO and classical, temperature
dependent voltage generation for VC0 input control. Ef-
59 I
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:16:59 UTC from IEEE Xplore. Restrictions apply.
Adjustment Time [min]
30
c
20
10
0
Temperature ["C]
Fig.5 Time consumption of ACCL algorithm.
ficient shortenedsuccessiveapproximationalgorithm
provides fast and reliable convergence. Upgrading of a
standard nonautomatic calibration system to an ACCL
system is inexpensive, since few lowcost additional
components are needed.
"A Temperature
Compensated
SC
[4]
E.K.Mgue1,
Cut Quartz Crystal Oscillator," in ProceedinPS of
the 36th Annual Freauencv Control SvmDosium ,
1982, pp. 576-585.
[5]
Takehiko Uno and Yoshio Shimoda, "A New Digital TCXO Circuit Using a Capacitor-Switch Array," in Proceedinns ofthe 37th Annual Freauency
Control Svmmsium, 1983, pp. 434-441.
[6]
Z.Aleksid and D.Vasiljevid, "Digital Temperature
Compensation of Crystal Oscillators Using Temperature Switches," in Proceedings of the 40th AnnualFreauencvControl
SvmDosium , 1986, pp.
340-343.
[7]
TMyayama, Y.Ikeda and S.Okano t'ANew Digtally Temperature Compensated Crystal Oscillator
for a Mobile Telephone System,"in Proceedinm of
the 42nd Annual Freauencv Control SvmPosium ,
1988, pp. 327-333.
[S]
A.Benjaminson and S.C.Stallings, "A Microcomputer-Compensated Crystal Oscillator Using A
Dual Mode Resonator,I' in Proceedinm of the 43rd
Annual Freauencv Control Svmwsium , 1989, pp.
20-26.
Currently, we are investigating further calibration algorithm improvements to shorten the calibration
time even more.
References
M.E.Frerking, "Method of Temperature Compensation," in Proceedings of the 36thAnnual Frequency Control Svmposium , 1982, pp. 564-570.
V.Candelier, G.Caret and A.Debaisieux, "Low
ProfileHighStabilityDigitalTCXO;Ultra
Low
Power Consumption TCXO,"in Proceedings of the
43rdAnnualFreauencvControl
SvmDosium ,
1989, pp. 51-53.
RRubach, "Dual
Mode
Digitally Temperature
Compensated Crystal Oscillator," in Proceedings
of the36th Annual Freuuencv Control Svmpo1982, pp, 571-575.
m,
592
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:16:59 UTC from IEEE Xplore. Restrictions apply.
[g]
RL.Filler,
J.A.Messina
and
V.J.Rosati,
"Frequency-Temperameand Ageing Performance
of Microcomputer-CompensatedCrystal Oscillators," in Proceedings of the43rdAnnualFreauencv Control Svmwsium , 1989, pp. 27-33.
[lo] M.Bloch,
M.Meirs,
J.Ho, J.R.Vig and
S.Schodowski,"LowPowerTimekeeping
," in
Proceedings of the 43rd Annual Freuuencv Control
Svmmsium , 1989, pp. 34-36.
593
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:16:59 UTC from IEEE Xplore. Restrictions apply.