The_design_of_a_microcontroller_temperature_compensated_crystal_oscillator__spl_mu_CTCXO_and_automatic_compensation_line

Prévia do material em texto

IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 42, NO. 4, JULY 1995 509
The Design of a Microcontroller Temperature
Compensated Crystal Oscillator ( CTCXO)
and Automatic Compensation Line
Dejan Habić, Aleksandra Pavasović, and Dragan Vasiljević
Abstract— A new concept in the design of the compensa-
tion circuit in microcontroller temperature compensated crystal
oscillators (�CTCXO) and the corresponding production line
(ACL—automatic compensation line) for fully automatic oscil-
lator calibration are presented.
The communications ability of �CTCXO is introduced to
provide automatic calibration. The shortened successive approxi-
mation calibration algorithm gives a short calibration time and a
reliable convergence of oscillator adjustment by the ACL system.
The �CTCXO is realized as a low-cost device encapsulated in
a single cubic inch volume. The �0.5 ppm frequency accuracy
in the �40�C/+85�C temperature range is obtained by a 9-bit
A/D converter, 10-bit D/A and 512 bytes of software placed in
an EEPROM.
The ACL system, in addition to the standard laboratory fre-
quency counter, temperature chamber and a PC host computer,
requires only two additional boards: an analog multiplexer board
and an interface board. During production and recalibration, the
ACL system can compensate many oscillators simultaneously in
a single temperature run, without requiring component selection
and human supervision.
I. INTRODUCTION
IN ANALOG temperature compensated crystal oscillators(TCXO), a varactor provides frequency pulling to reduce
oscillator frequency versus temperature drift. A temperature
sensing compensation network is used to generate the varactor
voltage. Compensation network adjustment requires three to
five temperature cycles, depending on frequency stability. In
order to accelerate production, computer aided methods may
be used in the selection process of compensation network
elements [1], [2].
Advances in integrated circuit technology introduced digital
temperature compensation techniques (DTCXO). In compari-
son to analog TCXOs, they provide higher accuracy for wide
temperature ranges, and simplify the adjustment procedure
resulting in a smaller number of required temperature cycles.
The initial DTCXO solutions were based on a look-up
table saved in a nonvolatile memory incorporated in the
compensation circuit [3]–[6]. An almost linear temperature
sensor generates a voltage that is applied to an A/D converter.
The A/D output is used as the memory address. The selected
memory location content corresponds to the varactor voltage
necessary to compensate oscillator frequency drift [1], [2]. A
Manuscript received August 8, 1994; revised December 12, 1994.
The authors are with the Faculty of Electrical Engineering, University of
Belgrade, Bulevar Revolucije 73, P.O. Box 816, 11 000 Belgrade, Yugoslavia.
IEEE Log Number 9411948.
D/A converter converts the digital memory content back to
the analog varactor voltage.
In the microcomputer compensated crystal oscillators
(MCXO), a microcomputer is added to the nonvolatile
memory, thus improving compensation flexibility. The A/D
converter gives temperature in digital form. These data are
used by the microcomputer to generate the varactor voltage
using a small look-up table and interpolation calculation [7].
A variation of this technique is a dual-mode MCXO [8]–[10]
in which the temperature sensing is done by the crystal itself,
and the frequency adjustment is based on the direct digital
frequency synthesis and not on frequency pulling.
The purpose of this paper is to introduce the concept of
a microcontroller compensated crystal oscillator ( CTCXO)
suitable for fully automated production. The oscillator com-
pensation circuit is accomplished by a microcontroller. It
is a small, low-power and inexpensive device with many
resources and the ability to communicate, which is necessary
for oscillator calibration by an automatic compensation line
(ACL). The proposed concept provides unsupervised, fully au-
tomated temperature compensation of sealed crystal oscillators
without component selection in a single temperature run. Many
oscillators may be simultaneously calibrated in production
or recalibrated for customers. The proposed compensation
concept supports both digital compensation approaches: the
dual mode MCXO and the classical MCXO with temperature
dependent voltage generation for the varactor diode.
This paper describes the design of the CTCXO and the
ACL system. The design of the CTCXO is elaborated in
the second section. The automated production line ACL is
presented in the third section. Finally, experimental results are
reported in the fourth section.
II. DESIGN OF THE CTCXO
The CTCXO was designed with the goal of achieving a
fully automatic temperature compensation of already sealed
units assembled from standard components.
The CTCXO block diagram is shown in Fig. 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 formed during calibration, and
makes the interpolation to calculate the voltage required at the
oscillator’s VCO input to obtain the nominal output frequency.
0885–3010/95$04.00  1995 IEEE
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply. 
510 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 42, NO. 4, JULY 1995
Fig. 1. �CTCXO block diagram.
The voltage is converted back to analog form by a D/A
converter.
Temperature compensation is achieved by the classical
digital compensation method with a minimum number of
components. The method is very flexible allowing a wide
choice of temperature sensors, control algorithms, components,
crystal units, etc.
The general purpose 8-bit microcontrollers have all the
necessary resources for the realization of the described com-
pensation circuit. The CPU provides programmability and
calculating power. The look-up table is saved in an EEPROM.
Analog quantities are coupled to the microcomputer through
A/D and D/A converters. Finally, the microcontroller’s ability
to communicate allows calibration of many oscillators by a
fully automated ACL line. Oscillator identification is obtained
by a hardware key connected to the microcontroller’s digital
input port. The hardware key may be a part of the CTCXO
itself, or it may correspond to the oscillator socket on the
measuring board.
The CTCXO has two operating modes: calibration mode
and autonomous mode. The autonomous mode is the primary
mode of operation and the oscillator is always operated in
this mode during normal use. The oscillator is operated in the
calibration mode only for temperature compensation while it
is inserted in one of the slots of the ACL line.
The simple software that controls autonomous operation of
the CTCXO includes 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
the quality of components (A/D, D/A, EEPROM size, crystal
hysteresis, etc.).
Communication between the CTCXO and the ACL system
during calibration is realized through an asynchronous serial
interface and is controlled by the ACL calibration software.
In order to experimentally test the above concepts and the
ACL line, three sample 0.5 CTCXOs have been built. Target
frequency stability was ppm in the temperature range from
40 C to 85 C. The design of the CTCXO will be reported
through a short description of its building blocks.
Voltage Controlled Crystal Oscillator (VCXO)
The VCXO part of the CTCXO is a Colpitts oscillator
with an AT cut crystal, previously designed for a commercial
TCXO, since its characteristics were already perfected for
the TCXOs. The crystal unit’s frequency-temperature curve
is smooth, otherwise the frequency stability would be de-
graded due to interpolation.Its hysteresis is adequately small
0.1 ppm) in comparison with the desired overall frequency
stability (0.5 ppm).
Oscillation frequency is 8 MHz, the output voltage is
HCMOS compatible, and the oscillator power consumption is
9.5 mA at 9 V DC. Care was taken to separate the power lines
for the digital part (compensation circuit) and the VCXO part
in order to reduce the digital noise affecting the output signal.
Both the supply to the oscillator circuit and to the sensor and
A/D circuits are filtered.
Temperature Sensor
The temperature sensor should be small and inexpensive.
Temperature precision is not required, but repeatability is. The
required temperature sensitivity is determined by the oscillator
frequency stability. Linearity is not crucial.
Many kinds of commercial temperature sensors more or less
satisfy the above stated requirements: platinum (Pt), thermistor
(NTC), silicon (Si) and quartz (Xtal) sensors.
A silicon sensor was chosen for its small dimensions and
low price. Its temperature coefficient is 0.75 C in the
temperature range from -55 C to 150 C. Nominal resistance at
room temperature is 25 C K Linearity is good,
especially if temperatures at the extremes are excluded. The
sensitivity of the sensor is increased by using it in a current
mode circuit, with a BJT simple current source. Although its
current has the same temperature dependence as the transistor
base to emitter voltage this temperature dependence
is irrelevant, since any current changes with temperature are
taken into account during oscillator temperature compensation.
For proper operation of the circuit the only relevant parameter
is the repeatability of the circuit output with temperature that
is achieved with this circuit.
The choice of the current value is a compromise between
large sensitivity and small dissipation. The value chosen was
1 mA, which results in a variation of circuit output voltage of
about 3.75 V, while keeping the dissipation and self-heating
of the sensor low (4 mW on the sensor).
In order to keep the difference in temperature between
the sensor and the crystal to a minimum, they are mounted
side by side on the printed circuit board. Unfortunately, the
temperature time constant is much smaller for the sensor then
for the crystal unit (10 s versus 70 s), affecting the oscillator
dynamic stability.
Microcontroller Block
The microcontroller block is the main part of the
CTCXO’s temperature compensation network. Low-cost
microcontroller chips are used in order to keep the price of
the oscillator low. The microcontroller block performs the
A/D and D/A conversion, calibration, and it generates the
temperature dependent varactor voltage.
The frequency stability of a CTCXO depends mainly on
the precision of the A/D and D/A converters. The relative
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply. 
HABIĆ et al.: DESIGN OF MICROCONTROLLER TEMPERATURE COMPENSATED CRYSTAL OSCILLATOR AND AUTOMATIC COMPENSATION LINE 511
frequency error due to conversions is [11]:
(1)
where:
is the maximum temperature deviation, i.e. operation
temperature range 85 40 125 C);
is the maximum slope of the frequency-temperature curve
of the crystal (1.8 ppm C);
is the number of bits of the A/D converter;
is the number of bits of the D/A converter; and
is the difference between maximum and min-
imum relative crystal frequency in the operating range (50
ppm).
The corresponding values for the sample CTCXOs are
given in the parenthesis.
The first term in (1) is due to A/D conversion, and the
second is due to D/A conversion. For wide temperature ranges,
the first term is much larger then the second. Thus, the error
mostly depends on the choice of the A/D converter. To achieve
the desired frequency stability of 0.5 ppm, the A/D converter
should be at least 8-bit.
The microcontroller CPU’s main duty is the look-up ta-
ble search and calculation of the interpolation values. Since
any CPU can perform these duties, the simplest (and least
expensive) component can be used, that is, an 8-bit CPU is
adequate. The only requirements for the CPU are that it should
have a timer circuit and the capability of 8-bit multiplication
and index table search. The CPU’s speed is not critical, since
thermal processes are relatively slow.
The size of the EEPROM depends on the size of the look-up
table which in turn depends on the compensation algorithm and
required oscillator stability. Adequate room should be reserved
for the code for the autonomous mode operation, which is also
placed in the EEPROM.
The Motorola 68HC11, 8-bit microcontroller was used in
the sample CTCXOs, since it best satisfies all of the above
requirements. EEPROM and RAM are both 512 byte wide.
EEPROM programming is software controlled and it is used
for storage of the look-up table and autonomous mode code.
An eight-bit A/D converter is also a part of the chosen
microcontroller. Timer functions are used to generate real time
interrupts and impulse waveforms in pulse-width modulation
D/A conversion. The dissipation of the entire compensation
circuit is 10 mA at 5 V DC.
Communication Resources
The microcontroller serial communication interface (SCI), a
full duplex asynchronous system with ability to be hooked up
into a network, is used as the CTCXO’s communication port.
The serial interface operates independently of the CPU and can
be set into interrupt or polling mode. The communication rate
and other communication parameters are software controlled.
The serial interface enables linking of many oscillators to a
simple serial bus and thus the calibration of many oscillators
simultaneously.
Fig. 2. ACL system block diagram.
The oscillator address is set on one of the input ports (PORT
C) of the microcontroller. The addresses are in the range from
0 to 255, enabling the linking of up to 254 oscillators to
the bus (Fig. 2). Before each exchange of data between the
controller and the oscillator, the addresses are exchanged. The
oscillator with the matching address receives the data, the other
oscillators wait for the next address.
A/D Converter
The A/D converter is a ratiometric converter with successive
approximation algorithm used for conversion. Adjustment and
operation of the A/D conversion are software controlled. The
converter can also be turned on and off, which is used to
decrease power dissipation.
In order to decrease the frequency error due to A/D con-
version which would be 0.43 ppm according to (1)
for an 8-bit converter, an additional bit of conversion is
added by splitting the conversion voltage range into halves
using a resistor voltage divider and an analog multiplexer.
The highest bit of the 9-bit A/D value is then given by the
analog multiplexer control signal value. Since the range of the
A/D input voltage is 3.75 V, the frequency error due to A/D
conversion is ppm.
D/A Converter
A PWM (pulse-width modulation) D/A converter has been
implemented using two timers for setting the switch-on time
and the switch-off time The resultant DC voltage
is given by the equation:
(2)
A ten-bit D/A converter was achieved by programming
the 16-bit counters into 10-bit counters. The corresponding
frequency error (1) is 0.02 ppm. The period was set to
2 msec to make filtering easier. The output of
the PWM D/A converter is fed through a passive RC second
order filter with 1Hz cut-off frequency and 40 dB/decade
attenuation slope. The filter is terminated by a resistor needed
to provide a path to ground for the discharge of the filter
capacitors. The maximum voltage at the converter output is
4 V, and the minimum voltage is 0 V. The voltage value at
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply. 
512 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 42, NO. 4, JULY 1995
the D/A output does notdepend on the CPU frequency and
its variations.
The Look-up Table
The look-up table holds pairs of temperature-voltage values.
Temperature is the digitized output of the temperature sensor
(voltage or frequency, depending on the sensor type) at a given
temperature. Voltage is the digital word that, when presented to
the D/A converter input, results in the varactor voltage needed
to pull the oscillator frequency back to the nominal value at
that temperature point.
The look-up table should be as small as possible to save
microcontroller nonvolatile memory and calibration time, and
large enough to keep the extrapolation error small enough
not to degrade the oscillator frequency stability. Taking into
account the error due to conversions 0.32 ppm) and the
target frequency stability 0.5 ppm), and leaving room
for other sources of frequency instabilities, the maximum
interpolation error should not be grater then 0.1 ppm
[1], [7].
If the interpolation is linear and the frequency-temperature
curve of the crystal unit is assumed to be of third order, the
maximum interpolation error is [11]:
[ppm] (3)
where and are the temperatures between which the
interpolation is being done, and and
are the quadratic and cubic coefficients of the crystal unit
frequency-temperature curve, respectively.
Using the above equation with worst case crystal
coefficients for the used crystal family, and setting
0.1 ppm, a set of 25 temperature values is obtained:
It should be noted
that is smaller at the ends then in the middle of the
temperature range.
Since temperature values are 9 bits long and voltage values
are 10 bits long, temperature-voltage pairs are three bytes long
(8 bits 8 bits 3 bits), resulting in a 75 byte look-up table.
It should be noted that the above analysis is true only if
the crystal unit’s frequency-temperature curve is ideal (no
hysteresis, no discontinuities, no activity dips). The crystal
units used in our experiments were high quality crystal units,
fabricated for precision TCXOs, with compound unwanted
frequency-temperature curve deviations below 0.1 ppm. If
lower quality crystal units are used or a higher stability
oscillator is required, a higher order A/D converter would
be used, for example a 10-bit A/D converter instead of a
8-bit converter. The mircrocontroller family used in sample
CTCXOs is now available with a 10-bit A/D converter. If
that option is used, with everything else as described earlier,
the error due to conversions would be decreased to ppm.
The CTCXO Software
In order for the CTCXOs to work properly, adequate
software had to be developed. The software can be classified
by its function into two parts:
1) autonomous operation software, and
2) calibration mode software, that is software that enables
communication with the ACL during calibration process.
Autonomous Mode Software
Since both the program and the look-up table reside in the
EEPROM, the program itself can be up to 437 bytes long
(512-75). All variables and results of calculations are stored
in the RAM.
The software enables temperature measuring through a 9-
bit A/D converter, interpolation, and forming of the analog
voltage by the 10-bit PWM D/A converter.
The program starts by initializing all of the microcontroller
resources and all of the variables. After initialization, the
program cycles indefinitely through a loop in which the
following operations are achieved:
The A/D converter is turned on and its output acquired and
used for finding the two nearest temperature-voltage pairs from
the look-up table. The voltage corresponding to the measured
temperature is then found by linear interpolation. The A/D
converter is turned off. The last operation in the cycle is
feeding the calculated voltage value to the input of the PWM
D/A converter.
The control of the PWM generator is done through two
interrupt routines controlled by timers T1 and T2. The routine
initiated by T1 sets a high logic level at the PWM generator
output and adjusts the contents of the timer T2. The routine
initiated by T2 sets a low logic level at the PWM generator
output.
Timer T3 is used for initiating the interrupt routine for
temperature measurement and calculation of the proper volt-
age value. The T3 contents are constant, since temperature
readings are taken at fixed intervals.
Calibration Mode Software
The calibration mode software enables temperature measur-
ing, generation of the analog value of the control voltage, com-
munication with the ACL through the SCI bus and EEPROM
programming.
The program consists of four interrupt routines. T1 and
T2 timer controlled routines are identical to the correspond-
ing routines of the autonomous mode software. The routine
controlled by timer T3 is somewhat changed: When a 9-bit
temperature value is read, it is simply placed into RAM to be
sent to the ACL via SCI bus on request. The corresponding
varactor voltage is found using a successive approximation
procedure described in more detail later.
The fourth interrupt routine is used for communication with
the ACL and adjustment and calibration of the oscillator.
The interrupt request is placed when the CTCXO receives
a character through the SCI bus. If the first bit of the 9-bit
character is a 0, the character received is data, otherwise it
is an address. If an address is received, it is compared to
the PORT C value and if the values match, initialization of
all variables is done and a reply character is sent. If a data
character is received, a check is made to determine whether
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply. 
HABIĆ et al.: DESIGN OF MICROCONTROLLER TEMPERATURE COMPENSATED CRYSTAL OSCILLATOR AND AUTOMATIC COMPENSATION LINE 513
the oscillator in question was already addressed, and if yes,
how many bytes of data has it already received since last
initialization of variables (that is, since last being addressed).
If the data character in question is the first byte, then it is
received and processed as the first byte of the varactor voltage
value and the first byte of temperature value is sent to the
ACL via SCI bus. If the data character in question is the
second byte, then it is received and processed as the second
byte of the varactor voltage value and the CTCXO sends the
second byte of temperature value to the ACL via SCI bus.
If the data character in question is the third byte, then it is
received as the check-sum control byte. The CTCXO checks
the received value with the check-sum of previously received
bytes. If the values match, the oscillator answers by sending
temperature check-sum control byte through the SCI bus to
the ACL, otherwise it sends the error character. In either case,
annulment of all variables follows.
Another branch of the fourth interrupt routine deals with
EEPROM programming. If the received byte after the oscil-
lator is addressed is an address consisting of all ones FF),
then the CTCXO’s CPU starts the EEPROM programming
routine and each successively received byte is written into the
EEPROM.
III. DESIGN OF THE CUSTOMIZED PRODUCTION LINE ACL
An automatic line for mass production of precision crystal
oscillators should provide:
1) elimination of human influence in the compensation
process (reducing price and eliminating possible errors);
2) possibility of compensating many oscillators simultane-
ously;
3) elimination of component selection and adjustment of
each oscillator before compensation;
4) flexibility
5) possibility of adding gathered data to the data base for
future reference
6) possibility of recompensating oscillators without reopen-
ing their cases (to correct frequency deviations due to
aging, etc.); and
7) ability to handle low-cost oscillators.
An example of such a line is shown in Fig. 2. The low-cost
oscillators are the oscillators already described in Section II
and the ACL is designed for use in their production process.
The host computer controlsthe temperature chamber and the
frequency counter operation through a standard IEEE 488 bus.
The interface board is used as an adapter between the PC
computer’s ISA bus and the asynchronous serial bus. Three
wire serial bus allows simple handling of a large number of
CTCXOs. The intelligent analog multiplexer linked to the
same bus provides connection of oscillators signal output to
the frequency counter.
The ACL calibration software, executed on the host com-
puter, controls the communication, calibration and program-
ming processes.
The ACL operation starts with the host computer download-
ing the calibration software to all CTCXO devices simulta-
neously.
The calibration process itself starts with the host computer
setting the temperature of the temperature chamber to the
first temperature point through the IEEE488 bus. The tem-
perature of the chamber is held constant until the oscillators’
frequencies are stabilized, and the adjustment procedure for
each oscillator is successively is performed by the following
procedure:
The host computer and the CTCXO exchange the identi-
fication address. The output signal of the addressed oscillator
is then connected to the frequency counter through an analog
multiplexer. The host computer receives the temperature sen-
sor information from the CTCXO’s A/D converter and sends
back a varactor voltage value calculated to pull the oscillator
frequency to the nominal value (first iteration). 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 voltage value and the process is
continued until the target frequency is achieved (second and
further iterations).
When all oscillators are adjusted at one temperature point,
the host computer defines the new temperature point and the
calibration at that temperature is carried out in the same way.
The number of temperature points is determined by the uncom-
pensated oscillators’ frequency to temperature characteristics
and the desired accuracy of compensation, as described earlier.
Obviously, the processing time is proportional to the number
of temperature points and the number of oscillators.
When calibration at all temperature points is finished, the
host computer downloads the look-up table contents and the
autonomous mode program to the corresponding CTCXO’s
EEPROM one by one. The host computer then concludes the
adjustment procedure by shutting down all instruments, writing
the data gathered during the calibration process to the data-
base for future reference, and providing the documentation for
each oscillator in a printed form.
Two problems had to be solved during the development of
the described algorithm. The first problem is the determination
of the initial value for the D/A converter in order to obtain
the shortest adjustment time through a minimum number of
approximation steps in oscillator frequency pulling. The right
choice presumes a value as close as possible to the varactor
voltage that gives nominal oscillator frequency. The initial
value is estimated on the basis of experimentally gathered
data for each CTCXO family. These data are placed in
the host computer that uses it together with the difference
between the measured and the nominal frequency in
order to estimate the starting value of the D/A converter
input.
The second problem is how to select D/A input in the second
and further iterations in order to obtain fast convergence
of the adjustment procedure. The successive approximation
algorithm, common in A/D conversion, is a good choice due
to the reliable and fast convergence. The proposed procedure
represents a shortened successive approximation algorithm.
When many CTCXO devices are calibrated simultane-
ously, input to each oscillator’s D/A should be set as soon as
a new temperature point is chosen. In this way, all CTCXOs
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply. 
514 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 42, NO. 4, JULY 1995
Fig. 3. ISI board.
will settle their frequencies simultaneously and total adjust-
ment time is shortened.
The ACL uses standard instruments such as laboratory
IEEE 488 bus controlled frequency counter and temperature
chamber. Any IBM compatible PC with ISA bus can be used
as the host computer (Fig. 2.). Nonstandard elements of the
ACL are a specially designed intelligent serial interface (ISI)
board and the intelligent analog multiplexer (IAM) board. The
ISI board is placed in one of the PC slots. The IAM board
is placed inside the temperature chamber, together with the
oscillators. The DOS based ACL software is 250 Kbytes long.
The description of the nonstandard ACL elements and the ACL
software follows.
The Intelligent Serial Interface Board
The ISI board enables communication through a simple
asynchronous serial bus between the PC on one side and the
IAM and CTCXOs on the other side (Fig. 2).
ISI is designed as a microprocessor system shown in Fig.
3. The microprocessor 68HC811E2 serves as a master on the
serial bus (the oscillators and the IAM are the slaves). ISI
software (to be described later as a part of the complete ACL
software package) is placed into a 2KB EEPROM. RAM is
used for temporary placement of calculation variables and data
and its size is determined by the number of slaves. For 254
oscillators, 32Kbytes of RAM are sufficient. The CPU SCI
communication port is used to link the ISI board to the serial
bus. The SCI port is linked to the serial bus through two drivers
(74HC541). The exchange of data between the PC and the ISI
is done through two latches, one that receives an 8-bit word
from the PC which is then read by ISI, and the other for the
other direction of data transfer. A hardware interrupt request
is placed every time data is placed in a latch. The request is
deactivated when the latch is read and the latch is then ready
for receiving the next byte of data.
The Intelligent Analog Multiplexer Board
The IAM board (Fig. 4) enables the link between oscillators’
signal outputs and the frequency counter input. Since it is
Fig. 4. IAM board.
located inside the temperature chamber, it should have a wide
operating temperature range.
The main part of the IAM board is the same microprocessor
used in CTCXOs. It communicates with the PC through its
SCI port in the same way that the CTCXOs do. After being
addressed (its address is 00), it receives data containing the
address of the oscillator whose output should be connected
to the frequency counter. The data is converted into an
address code that is sent to the analog multiplexer through
the microprocessor’s PORT B. The output of the analog
multiplexer is linked directly to the frequency counter input.
The size of the analog multiplexer depends on the number
of oscillators to be calibrated simultaneously, which in turn
depends on the capacity of the temperature chamber. For larger
capacity chambers, an analog multiplexer tree would be used.
The ACL Software
The ACL software is divided into four parts:
1) PC software,
2) ISI software,
3) IAM software, and
4) CTCXO’s calibration mode software.
The CTCXO’s calibration mode software was already
described in Section II. A short description of the other three
parts follows.
PC Software
PC software controls the operation of the temperature cham-
ber and frequency counter, and it supervises the calibration
procedure and the operation of the system as a whole.
The program starts by reading the input parameters for
the calibration process (temperature range, nominal frequency,
frequency stability, etc.) from a text configuration file. It then
initializes and checks the measuring instruments, oscillators,
resources and the ISI board and downloads the calibration
mode software to the oscillators through theISI board. Then
the calibration process begins as described earlier. The oper-
ation of the temperature chamber and the frequency counter
is controlled by the PC software, but the calibration itself is
partially controlled by the ISI software.
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply. 
HABIĆ et al.: DESIGN OF MICROCONTROLLER TEMPERATURE COMPENSATED CRYSTAL OSCILLATOR AND AUTOMATIC COMPENSATION LINE 515
When all oscillators are calibrated at all temperatures, the
autonomous mode software for each oscillator is sent to the
ISI board, followed by the command for programming of the
CTCXO’s EEPROM’s one at a time.
The program ends by reinitializing of all instruments and
writing an output text file containing all relevant information
about the calibration process for future reference, statistics,
and checking.
ISI Software
The ISI software consists of seven routines, each initiated
by a corresponding command received from the PC. The first
one is for sending ISI status to the PC. The second and
third routines are for receiving from the PC the autonomous
mode software (together with the corresponding look-up table
formed during the calibration process) and calibration mode
software, respectively. The fourth routine is for downloading
the calibration mode software to all oscillators. The fifth
routine is for checking the operation of the serial bus and
oscillators.
The sixth and longest routine (ISICAL) is for calibration
of the oscillators. Its operation is interactive with the PC
software. ISICAL starts with sending the command to the IAM
telling it which oscillator’s output to connect to the frequency
counter. Then the PC is informed that it can start with the
frequency measurement and it sends the frequency value to the
ISI board. ISICAL then calculates the needed voltage value to
pull the frequency back to the nominal value and sends it to the
oscillator under calibration. The process is repeated until the
target frequency is achieved. The corresponding temperature-
voltage pair is saved in the ISI RAM and the calibration of
the next oscillator begins. The routine ends when all oscillators
are at their nominal frequencies.
The last routine is for downloading the autonomous mode
software to the oscillators. The downloading process is done
for each oscillator separately, since the look-up tables differ
from oscillator to oscillator.
IAM Software
At the start of the IAM main program the initialization of
all resources and variables is done. The program then waits
for an interrupt request that is automatically activated each
time a character is received through the serial bus. If the
character is an address and it is equal to $00 (IAM address),
the appropriate variable is set. If the character is data, and
the IAM was already addressed, the character is transferred
to PORT B, thus selecting the applicable oscillator’s output.
The variables are then reset and the next interrupt request is
monitored.
IV. EXPERIMENTAL RESULTS
A frequency stability better then 0.5 ppm in the range of
operating temperatures from 40 to 85 was obtained
with a 9-bit A/D converter and 10-bit pulse-width D/A con-
verter (Fig. 5). The 25 point look-up table was obtained for
AT-cut crystals with uncompensated frequency to temperature
Fig. 5. Frequency to temperature characteristic of compensated �CTCXOs.
Fig. 6. Convergence of the frequency offset during calibration.
curves with a maximum frequency deviation 25
ppm and a maximum slope 1.8 ppm .
The entire CTCXO is encapsulated in a standard 36 mm
27 mm 19 mm size case.
The crystal units used in our test oscillators were high
quality crystal units, with compound unwanted frequency-
temperature curve deviations (hysteresis, activity dips, etc.)
below 0.1 ppm. If lower quality crystal units are used or
a higher stability oscillator is required, a higher order A/D
converter would be used. There was no need to leave room
for the errors due to the trimming of oscillators to nominal
frequency, since the oscillators are sealed before the com-
pensation process, thus the trimming is not necessary at the
time of production. Obviously, the trimming effect due to
trimming necessary to correct frequency drift due to aging will
degrade the stability of oscillators as in all other compensation
methods. If this degradation of frequency stability can not be
tolerated, correction of the frequency drift due to aging can be
done by recalibration instead of by trimming capacitor.
Frequency excursions of one of the sample CTCXOs
during calibration is presented in Fig. 6. The number of
attempts to obtain nominal frequency at a selected constant
temperature depends on the closeness of the varactor voltage
starting value and its final value. The diagram also illustrates
the convergence of the shortened successive approximation
algorithm.
The ACL system calibration time for three CTCXO de-
vices is shown in Fig. 7. The dark rectangles represent the time
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply. 
516 IEEE TRANSACTIONS ON ULTRASONICS, FERROELECTRICS, AND FREQUENCY CONTROL, VOL. 42, NO. 4, JULY 1995
Fig. 7. The ACL algorithm’s time consumption.
interval needed for temperature stabilization of the temperature
chamber and oscillators. The white rectangles designate time
consumed by the ACL software during execution of the short-
ened successive approximation algorithm. The gray rectangles
represent the time spent for settling after change of varactor
voltage. The main problem in the design of a ACL system is to
find an efficient calibration algorithm to save adjustment time.
The phase noise of the CTCXO output signal is high
( 60 dB/Hz, 100 Hz) due to low rejection of harmonics
in the PWM output voltage used for D/A conversion. This
shortcoming may be avoided by using a classical resistive
network D/A converter if the oscillator is used in applications
requiring low phase noise.
V. CONCLUSIONS
This paper describes a new concept in the design of a tem-
perature compensation network in microcontroller compen-
sated crystal oscillators ( CTCXO). Oscillators are designed
in such a way as to make compensation procedure as simple
as possible. A new concept of a production line for automatic
oscillator calibration is also described. The oscillators under
calibration form an integral part of the ACL, and they perform
a part of the compensation procedure.
The CTCXO compensation circuit is built using a general
purpose low cost microcontroller. It is a small, intelligent unit
able to communicate with the outside world.
The proposed concept provides fully automated, unsuper-
vised temperature compensation of sealed crystal oscillators
without component selection in a single temperature run. Many
oscillators may be calibrated simultaneously in production
and later, during use. Both digital compensation approaches
are supported: dual mode MCXO and classical, temperature
dependent voltage generation for VCO input control. Efficient
shortened successive approximation algorithm provides fast
and reliable convergence. Upgrading of a standard nonauto-
matic calibration system to an ACL system is inexpensive,
since only two additional low-cost boards are needed. The
described ACL system, with small modifications, can be used
in production facilities for a wide variety of products requiring
individual adjustment.
REFERENCES
[1] M. E. Frerking, “Method of temperature compensation,” in Proc. 36th
Annual Freq. Cont. Symp., 1982, pp. 564–570.
[2] V. Candelier, G. Caret, and A. Debaisieux, “Low profile high stability
digital TCXO; ultra low power consumption TCXO,” in Proc. 43rd
Annual Freq. Cont. Symp., 1989, pp. 51–53.
[3] R. Rubach, “Dual mode digitally temperature compensated crystal
oscillator,” in Proc. 36th Annual Freq. Cont. Symp., 1982, pp. 571–575.[4] E. K. Miguel, “A temperature compensated SC cut quartz crystal
oscillator,” in Proc. 36th Annual Freq. Cont. Symp., 1982, pp. 576–585.
[5] T. Uno and Y. Shimoda, “A new digital TCXO circuit using a capacitor-
switch array,” in Proc. 37th Annual Freq. Cont. Symp., 1983, pp.
434–441.
[6] Ž. Aleksić, D. Vasiljević, A. Pavasović, “Digital temperature compen-
sation of crystal oscillators using temperature switches,” in Proc. 40th
Annual Freq. Cont. Symp., 1986, pp. 340–343.
[7] T. Miyayama, Y. Ikeda and S. Okano, “A new digitally temperature
compensated crystal oscillator for a mobile telephone system,” in Proc.
42nd Annual Freq. Cont. Symp., 1988, pp. 327–333.
[8] A. Benjaminson and S. C. Stallings, “A microcomputer-compensated
crystal oscillator using a dual mode resonator,” in Proc. 43rd Annual
Freq. Cont. Symp., 1989, pp. 20–26.
[9] R. L. Filler, J. A. Messina, and V. J. Rosati, “Frequency-temperature and
aging performance of microcomputer-compensated crystal oscillators,”
in Proc. 43rd Annual Freq. Cont. Symp., 1989, pp. 27–33.
[10] M. Bloch, M. Meirs, J. Ho, J. R. Vig, and S. Schodowski, “Low power
timekeeping,” in Proc. 43rd Annual Freq. Cont. Symp., 1989, pp. 34–36.
[11] M. E. Frerking, Crystal Oscillator Design and Temperature Compensa-
tion. New York: Van Nostrand Reinhold, 1978.
Dejan Habić received the B.E. and M.E. degrees
in electrical engineering from the University of
Belgrade, Belgrade, Yugoslavia in 1989 and 1993,
respectively.
His research interests are in the area of industrial
electronics.
Aleksandra Pavasović received the B.E. and M.E.
degrees in electrical engineering from the University
of Belgrade, Belgrade, Yugoslavia in 1978 and
1986, respectively. She received the M.E. and Ph.D.
degrees in electrical engineering from the Johns
Hopkins University, Baltimore, MD, in 1989 and
1991, respectively.
From 1981 to 1986 she was employed as an R&D
engineer at Mihajlo Pupin Institute, Belgrade. Since
1991 she has been employed as a Consultant at
the Lola Institute, Belgrade, and as an Assistant
Professor at the Faculty of Electrical Engineering, University of Belgrade,
Belgrade. Her current research interests are in the area of integrated circuit
design.
Dragan Vasiljević received the B.E., M.E., and
Ph.D. degrees in electrical engineering from the
University of Belgrade, Belgrade, Yugoslavia in
1974, 1977, and 1984, respectively. Since 1975
he has been employed at the Faculty of Electrical
Engineering, University of Belgrade, where he is
now an Associate Professor. His current research
interests are in the area of crystal oscillators and
digital system design.
Authorized licensed use limited to: UNIVERSIDADE FEDERAL DO RIO GRANDE DO SUL. Downloaded on October 23,2022 at 21:17:33 UTC from IEEE Xplore. Restrictions apply.