Tough 5G Timing Requires MEMS Clock Sources

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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  
Oscillator Glossary  
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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.  
Oscillator Glossary  
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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.  
Oscillator Glossary  
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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.  
Oscillator Glossary  
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Glossary of Oscillator Terminology  
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.  
Oscillator Glossary  
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Glossary of Oscillator Terminology  
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  
Oscillator Glossary  
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Glossary of Oscillator Terminology  
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  
Oscillator Glossary  
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Glossary of Oscillator Terminology  
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  
distributors, or programmed for lower volumes in the customer’s lab using an oscillator programmer.  
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.  
Oscillator Glossary  
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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.  
Oscillator Glossary  
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Glossary of Oscillator Terminology  
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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