Required Frequency Stability for Different Applications
● USB low speed (1,5Mb/s) ±1.5% (15 000ppm)● USB full speed (12Mb/s) ±0.25% (2 500ppm)
● USB high speed (480Mb/s) ±500ppm
● Ethernet 10/100/1000Mb/s ±100ppm (±50ppm at 25°C, ±50ppm for other influences)
● Wi-Fi 802.11 a/b/g/n, Bluetooth: 20ppm
With the increasing demand for higher data rates and more efficient use of radio spectrum, the requirements for frequency stability are also increasing. For example, a base station for mobile phones needs 50ppb (part per billion) accuracy.
Oscillators in microcontrollers
Current microcontrollers have built-in factory calibrated RC oscillator and oscillator for external quartz crystal or ceramic resonator.
Internal RC Oscillator
16MHz internal RC oscillator in modern microcontrollers, like STM32L4 family, is factory calibrated and can have total accuracy-2% to 1.5% (-20 000ppm up to 15 000ppm) in the temperature range -40 to 85°C. Drift over time is not specified.
Oscillator with Ceramic Resonator
● Ceramic resonator 12MHz, 3,2x1,3x0,7mm, Murata CSTNE12M0GH5L000R0
● Ambient Operating Temperature Range: -40...85°C
● Frequency Tolerance at 25°C: ±0.07%=±700ppm
● Frequency Stability over Temperature: ±0.11%=±1100ppm
● Aging: ±0.07%/10 years at 25°C
Total frequency deviation for 10 years: 700ppm+1100ppm+700ppm=2500ppm
Oscillator with Crystal
● Standard Ansen crystal 12MHz, 3.2x2.5x0.75mm: ASX3F-12M-FE180E5
● Operating Temperature Range: -40...85°C
● Frequency Tolerance (25°C): ±30ppm
● Frequency Stability over Temperature: ±30ppm
● Aging: ±3ppm/year
● Total frequency deviation for 10 years: 30ppm+30ppm+30ppm=90ppm
Quartz crystal oscillator offers the best accuracy. Practical limit for standard crystal oscillator (XO) is ±10ppm including frequency tolerance at 25°C, frequency stability over temperature, supply voltage change, change of shock and vibration and 1st year aging.
Important Crystal Parameters
Nominal Frequency and Load Capacitance (CL)
The most used topology is Pierce Oscillator. The crystal acts as an inductor.
The load capacity CL closes the parallel resonant circuit. The CL value is specified by the crystal manufacturer. To be accurate, the oscillator circuit must have the same load capacity to which the crystal has been set.
Equivalent Series Resistance (ESR)
The feedback resistor transforms the logic CMOS gate (inverter) to a linear amplifier with a transconductance Gm = - di/du, thus the output current of the amplifier is proportional to the input voltage.
The amplifier must have sufficient transconductance to compensate for losses in the oscillation loop (ESR and current flowing through the CL1, CL2, and the parasitic capacities of the leads to GND) and provide energy to build-up oscillation. Typical Gm is mA/V for MHz range crystals and uA/V for 32.768KHz crystals.
For reliable operation, Gm > 5gm_crit is recommended.
gm_crit = 4*ESR*(2πF)^2*(C0 + CL)^2 where
● ESR is the equivalent series resistance
● C0 is the crystal shunt capacitance
● CL is the crystal nominal load capacitance
● F is the crystal nominal oscillation frequency
Drive Level
The recommended or maximum drive level is specified by the crystal manufacturer, usually in uW. Exceeding this value may shorten the life of the crystal or cause damage by excessive mechanical vibration. Ansen recommends drive level of 100uW. Lower drive level can prevent oscillations, the higher level increases the resonant frequency of the crystals with the AT cut.
You can find more information in STM AN2867 and John R. Vig - Quartz Crystal Resonators and Oscillators For Frequency Control and Timing Applications - A Tutorial.
Why have we decided for Ansen?
Ansen is a leading manufacturer of frequency control products. Their offer includes quartz crystals, crystal oscillators, and ceramic resonators. Through continuous technology breakthrough and high-precision manufacturing technology, Ansen products are not only utilized in consumer electronics and industrial sectors but also designed in automotive fields.
Ansen offers:
1. management system certified to the international ISO 9001, ISO 14001 and IECQ QC 080000 standards
2. excellent reliability, solderability, thermal hysteresis, vibration resistance, and shock resistance
3. low aging and extended temperature range
4. high-precision manufacturing technology
5. automotive-grade products with AEC-Q200 compliant
6. all products are complied with RoHS and REACH request
We included 40 Ansen crystals in HC-49/US (SMD and TH versions) and 3.2x2.5mm SMD packages into our stock offer. We will gradually expand the offer according to your requirements.
All customers logged in to our e-shop can view the prices of crystals.
For further information about Ansen products, please do not hesitate to contact us at ansen@soselectronic.com
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