EN IEC 60679-2:2026

SLOVENSKI STANDARD
01-december-2026
Piezoelektrični, dielektrični in elektrostatični oscilatorji ocenjene kakovosti - 2. del:
Smernice za uporabo oscilatorjev (IEC 60679-2:2026)
Piezoelectric, dielectric and electrostatic oscillators of assessed quality - Part 2:
Guidelines for the use of oscillators (IEC 60679-2:2026)
Piezoelektrische, dielektrische und elektrostatische Oszillatoren mit bewerteter Qualität -
Teil 2: Leitfaden für die Verwendung von Quarzkristalloszillatoren (IEC 60679-2:2026)
Oscillateurs piézoélectriques, diélectriques et électrostatiques sous assurance de la
qualité - Partie 2: Lignes directrices pour l’utilisation des oscillateurs (IEC 60679-2:2026)
Ta slovenski standard je istoveten z: EN IEC 60679-2:2026
ICS:
31.140 Piezoelektrične naprave Piezoelectric devices
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EUROPEAN STANDARD EN IEC 60679-2

NORME EUROPÉENNE
EUROPÄISCHE NORM October 2026
ICS 31.140
English Version
Piezoelectric, dielectric and electrostatic oscillators of assessed
quality - Part 2: Guidelines for the use of oscillators
(IEC 60679-2:2026)
Oscillateurs piézoélectriques, diélectriques et Piezoelektrische, dielektrische und elektrostatische
électrostatiques sous assurance de la qualité - Partie 2: Oszillatoren mit bewerteter Qualität - Teil 2: Leitfaden für
lignes directrices pour l'utilisation des oscillateurs die Verwendung von Quarzkristalloszillatoren
(IEC 60679-2:2026) (IEC 60679-2:2026)
This European Standard was approved by CENELEC on 2026-10-01. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, the
Netherlands, Norway, Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Türkiye and the United Kingdom.

European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2026 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 60679-2:2026 E

European foreword
The text of document 49/1540/FDIS, future edition 2 of IEC 60679-2, prepared by TC 49
"Piezoelectric, dielectric and electrostatic devices and associated materials for frequency control,
selection and detection" was submitted to the IEC-CENELEC parallel vote and approved by
CENELEC as EN IEC 60679-2:2026.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2027-10-31
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2029-10-31
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national committee. A
complete listing of these bodies can be found on the CENELEC website.
Endorsement notice
The text of the International Standard IEC 60679-2:2026 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standard indicated:
IEC 62884-1:2017 NOTE Approved as EN 62884-1:2017 (not modified)
IEC 62884-2 NOTE Approved as EN 62884-2
IEC 62884-3 NOTE Approved as EN IEC 62884-3
IEC 62884-4 NOTE Approved as EN IEC 62884-4
IEC 60679-1:2017 NOTE Approved as EN 60679-1:2017 (not modified)
IEC 60068 (series) NOTE Approved as EN 60068 (series)
IEC 60068-2-14 NOTE Approved as EN IEC 60068-2-14
IEC 60068-2-27 NOTE Approved as EN 60068-2-27
IEC 60068-2-6 NOTE Approved as EN 60068-2-6
IEC 60068-2-7 NOTE Approved as EN 60068-2-7
IEC 60068-2-13 NOTE Approved as EN IEC 60068-2-13
IEC 60068-1 NOTE Approved as EN 60068-1
IEC 60068-2-2 NOTE Approved as EN IEC 60068-2-2
IEC 60068-2-30 NOTE Approved as EN IEC 60068-2-30
IEC 60068-2-1 NOTE Approved as EN IEC 60068-2-1
IEC 60068-2-78 NOTE Approved as EN IEC 60068-2-78
IEC 60068-2-52 NOTE Approved as EN IEC 60068-2-52
IEC 60068-2-10 NOTE Approved as EN 60068-2-10
IEC 60068-2-21 NOTE Approved as EN IEC 60068-2-21
IEC 60068-2-17 NOTE Approved as EN IEC 60068-2-17
IEC 60068-2-20 NOTE Approved as EN IEC 60068-2-20
IEC 60068-2-45 NOTE Approved as EN 60068-2-45
IEC 61340 (series) NOTE Approved as EN IEC 61340-6-1:2018/A1 (series)
IEC 60068 (series) NOTE Approved as EN 60068 (series)
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments)
applies.
NOTE 1  Where an International Publication has been modified by common modifications, indicated by (mod),
the relevant EN/HD applies.
NOTE 2  Up-to-date information on the latest versions of the European Standards listed in this annex is available
here: www.cencenelec.eu.
Publication Year Title EN/HD Year
IEC 60679-1 - Piezoelectric, dielectric and electrostatic EN 60679-1 -
oscillators of assessed quality - Part 1 :
Generic specification
IEC 60679-2 ®
Edition 2.0 2026-08
INTERNATIONAL
STANDARD
Piezoelectric, dielectric and electrostatic oscillators of assessed quality -
Part 2: Guidelines for the use of oscillators
ICS 31.140  ISBN 978-2-8327-1418-8

IEC 60679-2:2026-08(en)
IEC 60679-2:2026 © IEC 2026
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Working principle of crystal oscillators . 7
5 Classification of crystal oscillators . 10
5.1 General . 10
5.2 SPXO (clock oscillator). 11
5.3 TCXO . 12
5.4 VCXO . 14
5.5 OCXO . 16
5.6 DCXO (disciplining control crystal oscillator) . 17
5.7 PLXO . 19
6 Frequency stability of crystal oscillators . 20
7 Specification and measurement of oscillator performance . 20
7.1 General . 20
7.2 Environmental effects. 21
7.3 Random frequency variations . 22
7.4 Phase noise . 24
7.5 Differential output of crystal oscillators. 25
8 Key parameters of specification . 25
8.1 General . 25
8.2 Stabilization time . 26
8.2.1 General. 26
8.2.2 SPXO . 27
8.2.3 TCXO . 27
8.2.4 OCXO . 27
8.2.5 DCXO . 27
8.2.6 PLXO. 28
8.3 Frequency adjustment range . 28
8.3.1 General. 28
8.3.2 SPXO . 28
8.3.3 TCXO . 28
8.3.4 OCXO . 28
8.3.5 DCXO and PLXO . 29
8.4 Frequency stability under steady-state temperature conditions . 29
9 Characteristics to be specified in article sheets . 30
10 Usage precautions for crystal oscillators . 33
10.1 General . 33
10.2 Power supply . 33
10.2.1 Internal resistance of the power supply . 33
10.2.2 Noise from the power supply . 33
10.3 Input control voltage . 34
10.3.1 Linearity of frequency change . 34
10.3.2 Polarity of control voltage . 35
IEC 60679-2:2026 © IEC 2026
10.4 EMC (electromagnetic compatibility) . 36
10.5 Selecting bypass capacitors . 36
10.6 PLL oscillators. 37
10.7 ESD (electric-static discharge) . 38
Annex A (informative) Differential output of crystal oscillators . 39
A.1 Clock rate and signal level . 39
A.2 Features of differential output wave . 40
A.3 Advantage and precaution of differential signal . 41
A.4 Advantage of differential output oscillator . 42
A.5 Differential output signal evaluation . 42
Bibliography . 43

Figure 1 – Swinging pendulum motion . 8
Figure 2 – Colpitts oscillator circuit . 8
Figure 3 – Equivalent circuit of a Colpitts oscillation circuit . 9
Figure 4 – Basic diagram of a crystal oscillator . 10
Figure 5 – Frequency-temperature characteristics of typical AT-cut crystals with
different orientation angles . 11
Figure 6 – Frequency-temperature characteristics of typical tuning-fork crystal . 12
Figure 7 – CMOS oscillator circuit and equivalent circuit . 12
Figure 8 – Block diagrams of various TCXO . 14
Figure 9 – VCXO oscillator circuit and equivalent circuit . 15
Figure 10 – VCXO with added circuits for improved bandwidth and linearity. 15
Figure 11 – Disciplining control crystal oscillator diagram . 17
Figure 12 – DCXO operating state transition diagram . 19
Figure 13 – Crystal oscillator type and frequency stability . 20
Figure 14 – Typical output power spectrum of a crystal oscillator. 21
Figure 15 – Dependence of the short-term stability (Allan Variance) on the sample
averaging time for a typical crystal oscillator . 24
Figure 16 – Typical frequency stabilization behaviour of an OCXO following initial
switching on . 29
Figure 17 – Output waveform . 34
Figure 18 – Phase noise characteristics . 34
Figure 19 – Example of positive polarity VCXO input circuit . 34
Figure 20 – Control voltage-frequency change characteristics (positive polarity) . 35
Figure 21 – Control voltage-frequency change characteristics (negative polarity) . 35
Figure 22 – Example of input circuit for negative polarity VCXO . 35
Figure 23 – Simulation of frequency response of bypass capacitor . 36
Figure 24 – Distance between crystal oscillator and bypass capacitor . 37
Figure 25 – Jitter amplification image by PLL cascade connection . 38
Figure A.1 – Clock rate vs. signal level (constant edge rate) . 39
Figure A.2 – Clock waveforms and specifications . 40
Figure A.3 – Effect of noise (single output) . 40
Figure A.4 – Effect of noise (differential output) . 41
Figure A.5 – Common mode noise . 41
IEC 60679-2:2026 © IEC 2026
Figure A.6 – Eye pattern (eye diagram). 42

Table 1 – List of typical oscillator parameters . 26
Table 2 – OCXO stabilization parameters . 27
Table 3 – Check list of recommended parameters . 30

IEC 60679-2:2026 © IEC 2026
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
Piezoelectric, dielectric and electrostatic oscillators of assessed quality -
Part 2: Guidelines for the use of oscillators

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
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preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
with the IEC also participate in this preparation. IEC collaborates closely with the International Organization for
Standardization (ISO) in accordance with conditions determined by agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
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any IEC Publication and the corresponding national or regional publication shall be clearly indicated in the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
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6) All users should ensure that they have the latest edition of this publication.
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Publications.
8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). IEC takes no position concerning the evidence, validity or applicability of any claimed patent rights in
respect thereof. As of the date of publication of this document, IEC had not received notice of a patent, which
may be required to implement this document. However, implementers are cautioned that this may not represent
the latest information, which may be obtained from the patent database available at https://patents.iec.ch. IEC
shall not be held responsible for identifying any or all such patent rights.
IEC 60679-2 has been prepared by IEC technical committee 49: Piezoelectric, dielectric and
electrostatic devices and associated materials for frequency control, selection and detection. It
is an International Standard.
This second edition cancels and replaces the first edition published in 1981. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) some new contents that reflect the latest manufacturing technologies;
b) added contents about the operating principle of crystal oscillators;
c) added contents about usage precautions for crystal oscillators.
IEC 60679-2:2026 © IEC 2026
The text of this International Standard is based on the following documents:
Draft Report on voting
49/1540/FDIS 49/1544/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.
This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
described in greater detail at www.iec.ch/publications.
A list of all parts of the IEC 60679 series, published under the general title Piezoelectric,
dielectric and electrostatic oscillators of assessed quality can be found on the IEC website.
The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
– reconfirmed,
– withdrawn, or
– revised.
IEC 60679-2:2026 © IEC 2026
INTRODUCTION
Crystal controlled oscillators are commonly used to provide the stable frequencies required for
telecommunications, navigations and data processing systems. Depending upon the frequency
of operation, ambient conditions and specific oscillator design, crystal oscillators are capable
-4 -10
of providing frequency stability varying from 1 × 10 to 1 × 10 .
This document offers a guideline that describes the general properties, performance
characteristics and usage precautions for crystal oscillators.

IEC 60679-2:2026 © IEC 2026
1 Scope
This part of IEC 60679 describes the general properties, performance characteristics and usage
precautions for quartz crystal oscillators. This document mainly describes crystal oscillators,
but some descriptions also apply to oscillators other than crystal units (e.g. MEMS resonators).
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies.
For undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60679-1, Piezoelectric, dielectric, and electrostatic oscillators of assessed quality - Part 1:
Generic specification
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60679-1 apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
– IEC Electropedia: available at https://www.electropedia.org/
– ISO Online browsing platform: available at https://www.iso.org/obp
4 Working principle of crystal oscillators
The operating principle of crystal oscillators is easier to understand when compared with that
of a pendulum. To maintain the swing of a pendulum, the position and time point of maximum
amplitude of the pendulum should be detected, and the operation of pushing the pendulum back
should be repeated so that the time point and position are maintained. The pendulum
corresponds to a crystal unit which comprises an amplifier with a feedback circuit that provides
the detection and push-back force (see Figure 1).
A typical feedback amplifier (Colpitts circuit) is shown in Figure 2, where the signals at both
terminals of the crystal unit are divided at capacitors C , C , one connected to the input side
A B
and the other to the output side. It can be considered that the "detection" in the case of the
pendulum is on the input side of the feedback amplifier and the "push-back force" is given on
the output side. Here, R and R are bleeder resistance to provide a fixed bias to the base
A B
terminal of the oscillation transistor, R is negative feedback resistance connected to the emitter
E
and R is load resistance connected to the collector.
C
Next, in the state of oscillation, as shown in Figure 3, the oscillator circuit side which is viewed
from the crystal unit terminals, can be represented by a series circuit of equivalent input
capacitance C and equivalent input resistance R. In this case, the crystal unit side is
i i
equivalently an inductive effective inductance L and an effective resistance R and the crystal
e e
oscillation conditions are as follows:
Frequency condition:
ωL −=0
(1)
e
ωC
i
IEC 60679-2:2026 © IEC 2026
where
is the inductive effective inductance, and
ωL
e
ωC is the equivalent input capacitance.
i
Amplitude condition:
R ≤ R
(2)
e
i
where
R is the effective resistance;
e
R is the equivalent input resistance.
i
The frequency condition determines the oscillation frequency. For the amplitude condition,
equivalent input resistance on the circuit side shall be negative value in the vicinity of the
oscillation frequency and at the small signal on the start-up, then this is called negative
resistance. This negative resistance can be obtained with active circuits such as transistors and
operational amplifiers. R shall be designed to be larger than R in order to ensure oscillation
i e
at the start-up of the oscillator, meaning that in the steady-state following the end of
R = R
e
i
the start-up and the oscillation becomes the steady-state amplitude.

Figure 1 – Swinging pendulum motion

Figure 2 – Colpitts oscillator circuit
IEC 60679-2:2026 © IEC 2026
Figure 3 – Equivalent circuit of a Colpitts oscillation circuit
The circuit side capacitance C is called the load capacitance (C ) with respect to the unit and
i L
is an important parameter that determines the frequency change amount from the series
resonance frequency, variable sensitivity and frequency stability, etc. The relational equation
is as follows:
Fractional load resonance frequency offset
ff−
L r
D ≈
L
f 
C (3)
r
L
21r +

C
0
where
f
L
is the load resonance frequency;
f
r
is the series resonance frequency.
Pulling sensitivity
d
D −1
L
S ≈
dC
L (4)
C
L
21rC +

C

where
C
is the shunt capacitance of the crystal unit;
C
is the motional capacitance of the crystal unit;
C C
is the load capacitance ( on the circuit side);
L i
r
CC
is the capacitance ratio of the crystal unit ( ).
=
=
IEC 60679-2:2026 © IEC 2026
5 Classification of crystal oscillators
5.1 General
In very general terms, a crystal oscillator can be described as consisting of an amplifier or gain
circuit, together with a positive feedback network. A typical oscillation circuit is shown in
Figure 4.
Figure 4 – Basic diagram of a crystal oscillator
Self-oscillation of such a circuit will occur provided the loop gain exceeds unity at some
frequency for which the total loop phase is 2 nπ (n = 0, 1, 2.). The level of the oscillation signal
will depend on circuit gain characteristics, while the oscillation frequency will be determined by
phase considerations. The output signal spectrum will depend upon the level of oscillation,
electrical noise from circuit elements and the bandwidths of the circuits. Generally, the spectral
width is extremely small and, for a majority of applications, only the centre or average frequency
behaviour should be considered. Disregarding for the moment the degradation of a signal purity
by electrical noise, the average frequency of oscillation will always be determined by the loop
phase requirement. Frequency deviations will result whenever any perturbation of loop phase
occurs. These perturbations can be generally considered to be of two types, namely:
a) deviations in electrical circuit parameters;
b) deviations in the crystal unit.
Type a) deviations include such factors as the temperature coefficients of capacitors, inductors,
resistors and transistors; the ageing of these devices; their voltage and current characteristics
and their susceptibility to mechanical disturbance. Whatever the cause, a phase perturbation
( φ ) in the circuit transfer phase will cause a change in the average oscillation frequency:
N
Δφ
N
Δf = f
(5)
2Q
X
where
f
is the nominal oscillation frequency and
Q
is the effective of the narrowband crystal network.
Q
X
Δ
IEC 60679-2:2026 © IEC 2026
Type b) deviations include changes in crystal unit characteristics caused by temperature,
temperature gradient, drive level, "frequency ageing" and mechanical environment (shock,
acceleration and vibration). The crystal oscillation circuit is generally designed to use a gain
circuit having a band as broad as practicable, in order to reduce the network phase sensitivity
as much as possible. The crystal feedback network, on the other hand, is usually designed to
have a transmission band as narrow as can be obtained so as to make the oscillation frequency
depend essentially only upon the crystal unit characteristics. When frequency adjustment is
required, it is preferably accomplished by introducing a variable reactance which changes the
frequency of the crystal network without widening the transmission band. When these general
practices are followed, the oscillator frequency stability will depend primarily upon the
characteristics of the crystal unit, which should be chosen so as to:
c) have a high Q-factor;
d) exhibit a small change of frequency over the intended operating temperature range;
e) have a low drive level dependence (DLD) of frequency and resonance resistance;
f) provide low sensitivity to mechanical shock, acceleration, and vibration;
g) exhibit a small frequency drift due to ageing.
5.2 SPXO (clock oscillator)
Most electronic equipment is required to operate over a range of ambient temperatures, i.e.
−10 °C to +60 °C, or −40 °C to +85 °C. Consequently, the frequency/temperature characteristic
of the crystal unit is usually the most important factor in determining the frequency stability of
simple packaged crystal oscillators (SPXO). For low-frequency crystal units vibrating in flexural
-4
or extensional modes, deviations as great as ±1 × 10 might be expected, while the thickness
-5
shear types (frequencies above about 1 MHz) can be expected to deviate by about ±2 × 10
over a temperature range of −40 °C to +90 °C. When better frequency stability is required, it is
necessary to provide temperature compensation circuitry, or to provide a temperature-stabilized
environment for the crystal unit by means of a temperature control device. Special
characteristics of oscillators of these types are considered in 5.3. The temperature behaviour
of frequency of crystal units is dependent upon the dimensions of the quartz plate, the mode of
vibration, the material and thickness of electrodes, the mounting methods employed and, mainly,
the orientation of the plate with respect to the crystallographic axes. Figure 5 shows the
characteristics which can be obtained for various orientations of an AT-cut crystal unit.

Figure 5 – Frequency-temperature characteristics of typical
AT-cut crystals with different orientation angles
Figure 6 shows the characteristics which are obtained with tuning fork crystals, e.g. at the
common clock frequency 32,768 kHz.
IEC 60679-2:2026 © IEC 2026
Figure 6 – Frequency-temperature characteristics of typical tuning-fork crystal
The frequency change as a function of time, usually referred to as "frequency ageing", is
primarily determined by the crystal used. A typical ageing rate for 10 MHz fundamental mode
-6
crystals is 2 to 5 × 10 per year while overtone units and well-aged units exhibit values lower
than this. The ageing is usually unimportant in applications using SPXO, since it is small
compared with the frequency deviations caused by temperature variations. The oscillation
circuit of the SPXO is a feedback amplifier for operating the crystal unit, and the CMOS
oscillation circuit used in crystal oscillators for clocks, etc. is mostly an equivalent circuit as
shown in Figure 7, and the circuit side can be represented by internal resistance R and a
d
square wave voltage supply that varies from 0 V to V .
DD
C and C are capacitance;
A B
R is the damping resistance.
d
Figure 7 – CMOS oscillator circuit and equivalent circuit
For the frequency stability of the SPXO, the characteristics of the crystal unit used are almost
the same as the characteristics of the oscillator. The frequency stability is said to be determined
by the crystal unit's characteristics, among which, frequency/temperature characteristics and
ageing are usually predominant.
5.3 TCXO
To obtain better stability than that possible with the SPXO, temperature compensated crystal
oscillators (TCXO) can be used. Improved stability in TCXO will be obtained at the expense of
added circuit complexity and increased cost.
IEC 60679-2:2026 © IEC 2026
TCXO are oscillators that provide temperature compensation for the frequency/temperature
characteristics, which fluctuate the most in the characteristics of the frequency stability, so that
the frequency fluctuations are even lower than those of theoretical frequency/temperature
characteristics of crystal units. New communication systems for the transition to high frequency
bands and the effective utilisation of frequencies need high frequency stability over a wide
temperature range. TCXO are used in many mobile communication systems as reference
oscillators suitable for this purpose. They can be broadly classified into the following three
categories according to the frequency-temperature compensation method and required
performance.
a) Direct analogue type [Figure 8 a)]
As the temperature compensation circuit is connected directly to the crystal unit, resistance
component of the circuit is added to the crystal resonance circuit, resulting in a decrease in
effective Q and a large change in effective Q depending on temperature. The temperature
compensation circuit section requires a compensation circuit design to flatten the frequency
characteristics of the crystal unit. Generally, a frequency stability of about ±1,5 to
-6
±3,0 × 10 (−40 °C to +85 °C) can be obtained.
b) Indirect analogue type [Figure 8 b)]
This is an oscillator designed so that the reference voltage supplied from a stabilized power
supply is used to create a compensation voltage by a temperature compensation circuit
consisting of resistance and a temperature sensors - usually thermistors - and the
compensation voltage is supplied to the voltage-controlled crystal oscillator VCXO, resulting
in flat characteristics in frequency fluctuations due to changes in ambient temperature. To
determine the element values of the circuit that compensate for the frequency/temperature
characteristics being the cubic function of the crystal unit over a wide temperature range, it
is necessary to use a computer to obtain a multi-point approximate solution with temperature
-6
as a function. The compensation accuracy is limited to ±0,5 × 10 (−20 °C to +70 °C),
although it is much better than the former. In recent years, using IC circuit technology, the
appearance of single-chip TCXO ICs incorporating a temperature compensation circuit
which is configured to obtain a higher-order temperature compensation function by
combining differential amplifiers and an oscillation circuit, has made it possible to
manufacture smaller TCXO, contributing significantly to the miniaturization of mobile
-6
terminals. Generally, a frequency stability of about ±0,5 to ±2,0 × 10 (−40 °C to +85 °C)
can be obtained. It should be noted that frequency hysteresis can deteriorate the frequency
stability.
c) Indirect digital type [Figure 8 c)]
This is a method that digitally performs fine compensation control for minute temperature
changes in order to increase frequency stability. Typical of this method is to digitally convert
the temperature information obtained by the temperature sensor and record the
corresponding voltage information such the frequency deviation becomes zero in a storage
medium such as ROM. Then, during operation, the VCXO is controlled by reading the
voltage information corresponding to the temperature information from the storage medium.
The theoretical limit of frequency stability is determined by the maximum temperature
coefficient of the crystal unit with respect to the increment of ambient temperature and the
voltage conversion accuracy of the D/A or A/D converter. Generally, a frequency stability of
-6
about ±0,05 to ±0,5 × 10 (−10 °C to +60 °C) can be obtained. It should be noted that
frequency discontinuities can occur if the ambient temperature fluctuates between the
discrete temperature steps. Furthermore, frequency hysteresis can deteriorate the
frequency stability.
IEC 60679-2:2026 © IEC 2026
a) Direct analogue type b) Indirect analogue type c) Indirect digital type

Figure 8 – Block diagrams of various TCXO
5.4 VCXO
In many applications such as for the generation of narrow-band frequency modulation (FM)
telecommunication signals, frequency shift-keying (FSK) data streams, or for obtaining an
output frequency phase lock (PLL) to an external reference signal, it is convenient to be able
to control the frequency of a crystal oscillator by means of a variable voltage applied to the
device. This function is achieved in much the same way as the frequency of the TCXO is
adjusted by the temperature-dependent voltage function generated by the thermistor network.
The principle differences are that for the former, the variable-capacitance diode voltage leads
are accessible to the user, no thermistor network is required and the range over which the
oscillator frequency can be tuned is made relatively large. The operating principle is similar to
that of the SPXO, but the basic circuit is as shown in Figure 9, with a variable-capacitance
diode connected in series with the crystal unit as an additional circuit and a terminal supply
voltage to it from an external source. The frequency control range and operating point are
determined by the voltage vs. capacitance characteristic of the variable-capacitance diode and
the input capacitance on the circuit side, including this diode. If an inductance is added in
L
series with the crystal unit to widen the frequency control range or to improve its linearity, the
input capacitance described above is the same result as having a negative capacitance value
in series which is equivalent to the reactance of the coil and can be designed using the
−C
T
following Formula (6). Namely, this can be calculated as:
Input capacitance of circuit:
CC
Ti
C =−
(6)
IN
CC−
i T
IEC 60679-2:2026 © IEC 2026
L is the effective inductance;
e
R is the effective resistance;
e
C is the equivalent input capacitance;
i
R is the equivalent input resistance.
i
Figure 9 – VCXO oscillator circuit and equivalent circuit
The bandwidth of the modulation signal to which the VCXO responds depends on the radio
frequency bandwidth of the crystal feedback network, just as it depends on the basic bandwidth
of the variable-capacitance diode drive circuit. The modulation linearity (i.e. as a function of
modulation voltage) depends on the combination of variable-capacitance diode characteristics
and crystal unit parameters. To achieve good linearity and/or to increase the frequency
deviation range, it is often necessary to use a more sophisticated feedback network, for
example as shown in Figure 10.

Figure 10 – VCXO with added circuits for improved bandwidth and linearity
VCXO can be roughly classified into the following two types:
a) those in which linearity of input voltage vs. frequency change is important, such as FM and
FSK modulation;
b) those for obtaining an output frequency phase locked (PLL) to an external reference signal.
IEC 60679-2:2026 © IEC 2026
As FM communication systems become more sophisticated, the former is suitable for
communications with a wide modulation frequency index, such as digital signals, and can be
made into an FM modulator with high frequency stability if a PLL function is added. The latter
is used as a signal source in communication systems and measuring instruments, etc. that
should be phase-locked to a reference signal. In addition, because crystal units are used, the
oscillators have better features in signal purity and phase noise than those using other devices.
5.5 OCXO
-7
The TCXO discussed in 5.3 can provide frequency-temperature stability of about 1 × 10 over
a wide range of ambient temperature. For applications requiring better stability, an
oven-controlled crystal oscillator (OCXO) is used. To achieve higher frequency stability, OCXO
provides high frequency stability by controlling the temperat
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