Glossary of Oscillator Terminology
Glossary of Oscillator Terminology
Contents
Part 1. Types of Timing Devices
Crystal (X or XTAL)
Crystal Oscillator (XO) or Oscillator
Digitally Controlled Crystal Oscillator (DCXO) or Digitally Controlled Oscillator
Digitally Controlled Temperature Compensated Crystal Oscillator (DCTCXO)
Oven Controlled Crystal Oscillator (OCXO)
Temperature Compensated Crystal Oscillator (TCXO) or Temperature Compensated Oscillator
Voltage Controlled Crystal Oscillator (VCXO) Voltage Controlled Oscillator
Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO)
Part 2. Oscillator Terminology
Absolute Pull Range (see Pull Range)
Activity Dip
Aging
Allan Deviation
Clipped Sinewave Output
CML
Cycle to Cycle Jitter
Differential
DPPM
Duty Cycle
Frequency
Frequency vs Temperature Slope
Frequency Stability
Gain Transfer or Kvco
Hadamard Variance
HCSL
Holdover
Glossary of Oscillator Terminology
Part 2. Oscillator Terminology (continued)
Integrated Phase Jitter (IPJ)
Load
Long-Term Jitter
LVCMOS
LVDS
LVPECL
MEMS
MTBF
Operating Temperature Range
Output Enable
Packaging
Parts per Million (ppm) and Parts per Billion (ppb)
Period Jitter
Phase Noise
Pullability
Pull Linearity
Pull Range – Total Pull Range and Absolute Pull Range
Quality Factor, Q
Retrace
Rise/Fall Time
Single-Ended
SPL
Standby
Start-up Time
Total Pull Range (see Pull Range)
Thermal Hysteresis
Tri-State
VOH/VOL
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Glossary of Oscillator Terminology
1. Types of Timing Devices
Crystal (X or XTAL)
A crystal is a passive resonator that vibrates at a fixed frequency. Crystals are used as external timing
reference for semiconductor ICs with an integrated oscillator circuit (i.e., on-chip generation).
Crystal Oscillator (XO) or Oscillator
An oscillator is an active device that combines the resonator and oscillator circuit into a single package.
Oscillators do not require external components to generate a clock signal. Although in some cases,
power supply decoupling components and/or termination resistor(s) may be required. In some regions,
XOs are referred to as OSC or SPXO (simple packaged crystal oscillator). Typical frequency stability of
XOs ranges from ±10 to ±100 ppm.
The minimum pin count for single-ended oscillators is three pins for power, ground, and the oscillator
output. However oscillators usually have at least four pins to accommodate output enable or other
control functions. Differential oscillators are usually packaged in six-pin packages. Some oscillators
2
which include serial interface control such as I C are packaged in 10-pin or higher pin-count packages.
Frequency stability for XOs usually ranges from ±10 ppm to ±100 ppm and they are usually offered in the
following packages: 7050, 5032, 3225, 2520, and 2016.
Digitally Controlled Crystal Oscillator (DCXO) or Digitally Controlled Oscillator
A DCXO is similar to a VCXO in that both types of devices allow pulling the frequency. In some cases,
DCXOs have the capability to program output frequency to a wider range beyond the limited pull range.
The difference with DCXOs compared to VCXOs, is that frequency is adjusted by writing digital control
2
words over a serial interface such as I C or SPI.
Digitally Controlled Temperature Compensated Crystal Oscillator (DCTCXO) or Digitally
Controlled Temperature Compensated Oscillator
A DCTCXO is a TCXO that incorporates the frequency pulling and programming functionality of a DCXO.
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Glossary of Oscillator Terminology
Oven Controlled Crystal Oscillator (OCXO)
An OCXO provides temperature compensation and ovenization to maintain an almost constant
temperature for the oscillator at as ambient temperature varies. These devices enclose the resonator,
along with temperature-sensing and compensation circuits inside a heated enclosure. This temperature
compensation and ovenization enables the OCXO to achieve very good frequency stability ranging from
0
.05 ppb to 200 ppb. The typical package size of a quartz-crystal OCXO ranges from to 9.7 mm x 7.5 mm
to 135 mm x 72 mm.
Temperature Compensated Crystal Oscillator (TCXO) or Temperature Compensated Oscillator
A TCXO is an oscillator that incorporates temperature compensation to compensate for the frequency
vs. temperature characteristic of the resonator. This compensation enables TCXOs to achieve better
frequency stability than non-compensated oscillators (XOs). Frequency stability of TCXOs ranges from
±0.05 ppm to ±5 ppm. These devices are used in applications where precision timing references are
required such as high performance telecom and networking equipment.
Voltage Controlled Crystal Oscillator (VCXO) or Voltage Controlled Oscillator
VCXOs incorporate a control voltage pin that controls the output frequency around the nominal
frequency. The extent of frequency control is called the pull range which typically ranges from ±50 ppm
to ±200 ppm but can extend to ±3200 ppm for SiTime VCXOs. VCXOs are often used in discrete jitter
attenuation and clock recovery applications.
Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) or Voltage
Controlled Temperature Compensated Oscillator
A VCTCXO is a TCXO that incorporates a control voltage pin to allow the output frequency to vary
around the nominal frequency. The frequency tuning range for a VCTCXO is typically ±5 ppm to ±25
ppm. Some vendors refer to these devices as TCVCXOs.
Note regarding SiTime MEMS-based oscillators
While all of SiTime’s devices use MEMS resonators and not quartz crystal resonators, SiTime does not
replace the “X” in the above acronyms with “M” (for MEMS) because these product categories and
acronyms have been established in the industry for decades and are associated with certain timing
functions. As SiTime devices offer the same or better functionality as quart-based products, it causes
less confusion to continue with the same well-known product classifications and acronyms.
Oscillator Glossary
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Glossary of Oscillator Terminology
2. Oscillators Terminology
Absolute Pull Range
See Pull Range
Activity Dip
Activity dips result from mechanical coupling of the principal resonance mode to one or more interfering
modes that exist but are not electrically excited by the sustaining circuit. Resonance frequencies of
these modes shift as the environmental temperature changes. At some temperatures, the frequency of
the interfering mode(s) may come close to the frequency of the desired mode, causing the main mode
to loose energy. This, in turn, causes an increase in the resonator equivalent resistance which manifests
as a shift in output frequency.
This shift is usually a rapid jump in the frequency over temperature characteristic. After the frequency
jumps, the smooth frequency curve continues on a similar trajectory as before, but it is shifted up or
down due to the jump. This rapid frequency change can cause system problems such as PLL unlock or
packet loss. Quartz-based resonators are susceptible to activity dips. However, SiTime MEMS-based
resonators are free of activity dips.
Aging
Aging is the change in oscillator frequency, measured in ppm over a certain time period, typically
reported in months or years. This change in frequency with time is due to internal changes within the
oscillator, while external environmental factors are kept constant.
Note regarding SiTime MEMS-based oscillators
SiTime MEMS oscillators aging data is provided for up to a 10-year period. SiTime oscillator aging is
significantly lower (better) than quartz oscillators because MEMS resonators are vacuum sealed in
silicon using a process that eliminates foreign particles that can affect aging.
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Glossary of Oscillator Terminology
Allan Deviation
Also known as short-term frequency stability, Allan deviation (ADEV) is the measure of oscillator stability
in the time-domain. It represents a frequency change over an interval of time called averaging time.
Allan deviation is calculated as the root mean square (RMS) change in successive frequency
measurements. The averaging time typically ranges from milliseconds to thousands of seconds
depending on the target application. The formula for Allan deviation is shown below, where the y values
represent the values of fractional frequency deviation between adjacent clock cycles and M is the
sample size.
Allan deviation is used for clock oscillators because it converges for more types of oscillator noise
compared to standard deviation. Allan deviation converges for white phase modulation, flicker phase
modulation, white frequency modulation, flicker frequency modulation, and random walk frequency.
Allan deviation does NOT converge for flicker walk frequency modulation and random run frequency
modulation.
Clipped Sinewave Output
Clipped sinewave is a common single-ended output format often encountered in TCXO (temperature
controlled oscillator) or OCXO (oven controlled oscillator) devices. The main feature of clipped sinewave
output is very slow gradual rising and falling edges that resemble portions of the sinewave, hence the
name. Slow rise/fall times have several benefits including reduced energy of high-frequency output
harmonics that are undesirable in RF applications. This helps achieve good signal integrity with fewer
restrictions in the layout rules. The drawback is slightly lower jitter performance at high frequencies
compared to LVCMOS output.
The diagram below shows a typical clipped sine waveform and the significantly slower rise and fall times.
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CML
Current mode logic (CML) is a common oscillator differential output format. It is an open drain type
output which means the driver only drives low and that external pull-up resistors are required to pull
the clock signal high during the high portion of the clock period. Two voltage swings are commonly
supported, 450 mV and 850 mV. The diagram below shows a typical 450 mV waveform. CML is
commonly used in telecom infrastructure applications such as wireless base stations.
Cycle to Cycle Jitter
Cycle to cycle (C2C) jitter is defined as the variation in cycle time of a signal between adjacent cycles. It is
measured over a random sample of adjacent cycle pairs (JEDEC JESD65B). The suggested minimum
sample size is 1,000 cycles as specified by JEDEC.
See related terms: Integrated Phase Jitter (IPJ), Long-Term Jitter, Period Jitter, Phase Noise
Differential
In contrast to single-ended output, differential output consists of two complementary signals with 180°
phase difference between the two signals. This output type is often used in high-frequency oscillators
(
100 MHz and above). Differential signals usually have lower voltage swing than single-ended signals,
faster rise/fall times, better noise immunity, and are used when better performance or higher frequency
is required. The most commonly used differential signally types are LVPECL, LVDS, and HCSL.
See related term: Single-Ended
DPPM
DPPM (defective parts per million) quantifies how many units may be defective per 1 million units. This
unit of measurement is estimated with certain degree of confidence.
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Duty Cycle
Duty cycle is a clock signal specification that is defined as the ratio in percentage between the pulse
duration in high state to the period of the oscillator signal. The diagram below illustrates duty cycle % =
1
00* TH/Period, where TH and Period are measured at the 50% point on the waveform. Typical duty
cycle specifications range from 45% to 55%.
5
0%
High Pulse
TH)
Low Pulse
TL)
(
(
Period
Frequency
Frequency is the repetition rate (cycle) of the signal output from the oscillator and is measured in Hertz
(Hz) per second. Many applications call for a specific oscillator frequency. Following is a list of standard
frequencies and their typical applications.
Output Frequency (MHz) Application
0
0
0
1
2
8
8
9
.002000
.008000
.032768
.544000
.048000
.000000
.192000
.830400
Frame Clock
BITS Clock
Real Time Clock
Telecom DS1
Telecom E1
Automotive CAN Bus
ISDN
Wireless CMA, UART
1
1
1
1
1
1
1
0.000000
2.000000
2.288000
2.352000
2.800000
3.500000
4.318180
GPS Disciplined Oscillator, Network Time Protocol, test and Measurement
USB/Automotive CAN Bus
DAT Digital Audio
Telecom DS1
Common OCXO Frequency for Telecom
Audio/Video
NTSC Clock, Crystal Reference for PC Motherboard clock
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Output Frequency (MHz) Application
1
2
2
2
2
2
3
3
9.200000
4.576000
5.000000
6.000000
6.562500
7.000000
2.000000
0.720000
GSM/UMTS
Audio, IEEE1394
Ethernet
GSM/UMTS
Fibre Channel
Audio/Video
Wireless IoT
Wireless W-CDMA
Parallel PCI
3
3.33333313
3
3
4
4
4
5
5
6
4.368000
8.880000
0.000000
4.736000
8.000000
0.000000
3.125000
1.440000
Telecom E3
Telecom SONET
WIFI, SCSI, CPU Reference
DS3
USB
Ethernet, General Purpose
Fibre Channel
W-CDMA
6
7
6.66666625
4.17582418
Parallel PCI, PCI-X General Purpose
Video
7
7
7
9
4.250000
5.000000
7.760000
6.000000
Video
SATA
SONET
USB
1
00.000000
06.250000
22.880000
25.000000
PCI Express/General Purpose
Fibre Channel
W-CDMA
1
1
1
1G Ethernet
PCI-X, General Purpose
Video
1
1
33.3333325
48.3516484
1
48.500000
50.000000
53.600000
55.520000
56.250000
Video
1
SATA
1
1
1
W-CDMA
SONET
10G Ethernet
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Output Frequency (MHz) Application
1
59.375000
Fibre Channel
1
1
61.1328125
66.6666656
10G Ethernet (156.25 * 66/64 Line Coding)
Computing
1
68.040678
75.000000
87.500000
00.000000
12.500000
OTU3 (155.52 * 235/236 FEC)
General Purpose
Ethernet
1
1
2
2
General Purpose
Fibre Channel
2
33.3333333
Computing
2
50.000000
66.666665
00.000000
11.040000
12.500000
22.265625
SAN/General Purpose
Computing
2
3
3
3
3
SATA/General Purpose
SONET
Ethernet
Ethernet (312.5 * 66/64 Line Coding)
Computing
3
33.3333331
3
50.000000
25.000000
14.400000
25.000000
37.500000
44.531250
00.000000
General Purpose
Fibre Channel
4
6
6
6
6
7
W-CDMA
Ethernet
Fibre Channel
OTU3 (625 * 66/64 Line Coding), 100GbE, 400 GbE
General Purpose
Ethernet
1
1
250.000000
275.000000
Fibre Channel
Note regarding SiTime MEMS-based oscillators
SiTime oscillators are available in frequencies as low as 1 Hz for low-power devices and as high as
7
25 MHz. The frequency of SiTime oscillators is programmable within this range to 6 decimals of
accuracy. Frequency can be factory programmed by SiTime, programmed by key partners and
Oscillator Glossary
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Glossary of Oscillator Terminology
Frequency vs Temperature Slope
Frequency vs temperature slope, also shown as ΔF/ΔT, is the rate of frequency change due to a 1°C
change in temperature. It quantifies sensitivity of the oscillator frequency to small temperature variations
near the operating temperature point. It is one of the major performance metrics of precision TCXOs that
determines if the TCXO is stable enough to support the needs of the target application. Smaller frequency
vs temperature slope values mean lower frequency variation due to the temperature change in a
confined temperature window. For example, an average system temperature window may be ±5°C.
In systems that require time and frequency transfer using IEEE 1588, better frequency vs temperature
slope helps improve time error. The unit of measure is in ppm/°C or ppb/°C. Below is a plot of the
SiT5356 Elite TCXO showing the frequency slope from 12°C to 13°C with a value of 0.86 pb/°C. This plot
shows frequency error vs. the nominal frequency instead of absolute frequency, hence the y-axis label
FERROR. The frequency vs. temperature slope is reported as the highest absolute value of slopes observed
over the total temperature rage.
Frequency Stability
Frequency stability is a fundamental performance specification for oscillators. This specification
represents the deviation of output frequency due to external conditions – a smaller stability number
means better performance. The definition of external conditions can differ for different oscillator
categories, but usually includes temperature variation. It may also include supply voltage variation,
output load variation, and frequency aging. Frequency stability is typically expressed in parts per million
(ppm) or parts per billion (ppb) which is referenced to the nominal output frequency.
Gain Transfer or Kvco
Gain transfer or Kvco is a common characteristic of voltage controlled oscillators (VCXOs) that
determines how much output frequency changes in response to a 1-V change in control voltage. This is
useful in calculating the characteristics of closed loops that utilize a VCXO.
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Glossary of Oscillator Terminology
Hadamard Variance
Hadamard variance is the square of the change in three successive frequency measurements. These
measurements are the values of fractional frequency deviation between three adjacent clock cycles and
M is the sample size. Hadamard variance converges for white phase modulation, flicker phase
modulation, white frequency modulation, flicker frequency modulation, random walk frequency, flicker
walk frequency modulation and random run frequency modulation. It is unaffected by linear frequency
drift and well suited for analysis of Rubidium oscillators. Below is the formula for Hadamard variance,
where y represent the values of fractional frequency deviation among three contiguous clock cycles and
M is the sample size.
HCSL
High speed current steering logic (HCSL) is a commonly used differential output format used for PCI
Express, servers, and other applications. As shown below, it has a typical output swing of 700 mV and
swings from 0V to 700 mV.
.
See related terms: LVDS, LVPECL
Holdover
Holdover is a mode of operation used by systems that are synchronized to an external precision
frequency and/or time reference, and that have temporarily lost this reference signal. The local
oscillator should have the capability to maintain, or holdover, stable frequency and/or time within the
defined limits in a system after the loss of the external reference.
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Integrated Phase Jitter (IPJ)
Phase jitter is the integration of phase noise over a certain spectrum and is expressed in picoseconds or
femtoseconds. The diagram below shows an example integration band between f1 and f2 and the area
under this curve is time domain picoseconds or femtoseconds of jitter.
See related terms: Cycle to cycle (C2C) jitter, Long-Term Jitter, Period Jitter, Phase Noise
Load
Within the scope of oscillators, load usually refers to capacitive load – the total capacitance driven by
the oscillator output. Load consists of the input capacitance of the driven IC, trace capacitance, plus any
other parasitics or passive components on the printed circuit board.
Long-Term Jitter
Long-term jitter measures the deviation of clock features from the ideal position over several
consecutive clock cycles. This effectively measures how the duration of a number of consecutive clock
cycles deviates from its mean value.
See related terms: Cycle to cycle (C2C) jitter, Integrated Phase Jitter (IPJ), Period Jitter, Phase Noise
Oscillator Glossary
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