Stability is nice. As an engineer you would like to get a super stable reference frequency and just not have to worry about this one variable. And now you probably can, as very stable oscillators that are small, inexpensive, and don't take much power are readily available. So what information do you look for?
Temperature stability: The frequency of all oscillators (quartz and MEMS-based) varies over temperature. This is a crucial performance specification expressed as stability in parts per million (ppm). Reference timing products are often categorized as XOs (oscillators) offer 10 to 100 ppm stability, while TCXOs (temperature compensated oscillators) offer 0.1 to 10 ppm stability.
The stability requirements of most TCXOs dictate compensation by means of a multiple thermistor network with several interdependent variable components thus making the solution of simultaneous equations by computer the only practical approach to temperature compensation.
When an oscillator manufacturer specifies a stability of ±1 ppm over -20°C to of + 70°C, this means a total peak error of 2 ppm over the temperature range, not referenced to the frequency at any specific temperature. If a reference, such as room temperature, is desired with a maximum allowable error of ±1 x 10-6 from that reference, the specification should clearly state ±1 ppm over -20°C to +70°C referenced to the frequency at +25°C. Also, the frequency versus temperature characteristic of a TCXO is not linear.
Power supply and load effects: Secondary to temperature, but a sometimes very important specification that usually runs 0.1 or 0.02 ppm for each.
Aging: In clock oscillators with moderate temperature stability, aging is usually of little consequence. However, in highly temperature stable TCXOs, crystal aging becomes a significant factor in the oscillator's overall frequency error. Therefore, TCXOs employ specially processed crystals in evacuated glass or coldweld holders. TCXO typically have both moderate and long term aging of ±1 ppm per year.
A few example pertinent parts are in order:
Abracon
The Abracon ASTX-H11-16.000MHZ-T temperature compensated crystal oscillator provides ±2.5 ppm stability and costs just $2.20 ea/1,000. Available in a broad frequency range from 0.65 to 55.0 MHz, it comes in a 5.0 x 3.2 x 1.5-mm SMT package. The stability specification is not inclusive of all factors for this device, so a supply voltage change of 5% can add 0.2 ppm and a load change of ±10% can add another 0.2 ppm max. Aging is ±1 ppm in the first year at 25°C. Supply current is 4.8 mA max, supply voltage is 3.3 V, and phase noise is –130 dBc at 1-kHz offset and -158 dBc at 100-kHz offset. This particular part's 2.5-ppm stability specification is over –30° to 75°C; –40° to 85°C is available.
Abracon also offers a real-time-clock with 32.768 kHz TCXO in a 3.2 x 2.5 x 1.0 mm SMT package. The AB-RTCMK series provides a sub-±5.0-ppm drift over -40° to 85°C with complete RTC functionality. Supply current is only 4 µA max from a 3.0-V supply. RTC communication is via I2 C and price is $3.38 ea/3,000.
Vectron International
Vectron's VT-802 is a quartz-stabilized CMOS output, temperature-compensated oscillator (TCXO), operating off either a 2.5- or 3.3-V supply, in a hermetically sealed 5.0 x 3.2 ceramic package. Output frequencies are 2 to 50 MHz, supply current is 2 to 5 mA max, and available stabilities are ±2.0, ±2.5, ±5.0, or ±10 ppm over either -20° to 70°C or -40° to 85°C. Initial accuracy is ±1.0 ppm, power supply stability adds ±0.5 ppm, load stability is ±0.2 ppm, and aging is ±1.0 ppm/yr.
Linear Technology
A different kind of oscillator is the LTC6905-XXX silicon oscillators series from Linear Technology. Their basic frequency accuracy is only ±1% and their temperature stability is ±20 ppm/°C – or 2500 parts over -40°C to 85°C. They have a frequency divide by 1, 2 or 4 built-in, take 8 mA at 100 MHz from a 2.7 to 5.5-V supply and cost $1.41 ea/500.
IDT
The IDT 4H series are differential MEMS oscillators that feature 0.15-ps typ phase jitter performance and frequency fine-tune capability. The extremely low phase jitter and adjustable frequency significantly reduces bit error rate in 10-GbE Ethernet switches and routers. The device is available with 100 to 212.5 MHz outputs. The 4HF125000Z4s frequency stability is ±50 ppm or ±25 ppm over -40 to 85˚C and the oscillators are available in multiple package sizes including the small 3.2 x 2.5 mm SMT. They operate from 2.5 or 3.3 V and require 100 mA typ. I could find no stock on this family and no price information.
The IDT 4H series MEMS oscillators
Pericom
The WT325 series clipped sign wave oscillator from Pericom comes in frequencies between 10 and 40 MHz with stabilities of ±0.5 to ±2.5 ppm over temperature ranges up to –30° to +85°C. It seems to be readily available only in the 2.0 ppm over -30° to +85°C version (I) with a cost of $2.40 in 3,000 quantities. The device comes in a 3.2 x 2.5 x 1.2-mm package and takes only 1.5 to 2.0 mA max from a 1.8 to 3.3-V supply. You need to add 0.1 ppm for a ±5% supply voltage change and 0.2 ppm for a 10% load change to the stability number, and aging is 1 ppm max for the first year. Phase noise is specified as -128 dBc/Hz at a 1-kHz offset and 26 MHz.
Silicon Laboratories
Silicon Laboratories offers the low-cost CMEMS Si501, Si502, Si503 MEMS-based oscillators with 32-kHz to 100-MHz frequencies and ±20-, ±30-, and ±50-ppm stability options. They are specified over –20° to 70°C or -40° to 85°C and come in standard 2 x 2.5-, 2.5 x 3.2-, and 3.2 x 5-mm packages.
CMEMS technology enables 10-year frequency stability including solder shift, load pulling, VDD variation, operating temperature range, vibration, and shock. The Si501 is single-frequency with output-enable, the Si502 is dual-frequency with OE and frequency-select, and the Si503 is quad-frequency with frequency-select. Period jitter is only 1.1 ps rms. They operate from 1.8, 2.5, or 3.3 V, take 1.8 mA typ, and cost from $0.44 ea/10,000.
SiTime
The SiT5001 MEMS TCXO and VCTCXO comes in 1 to 80-MHz versions and features stabilities of ±1 to ±5 ppm in commercial and industrial temperature ranges. Initial tolerance is ±1 ppm and first year drift is ±1.5 ppm.
This device features rms period jitter of 2 ps max, and it comes in four package sizes as small as 2.5 x 2.0 mm and operates at five specified supply voltages from 1.8 to 3.3 V. It takes 33 mA max.
Pletronics
The TCE4 series has stabilities of ±0.5 to ±2.5 ppm over the –30° to 85°C temperature range with a frequency range of 10 to 52 MHz. The oscillator has phase noise of –136 dBc/Hz at 1 kHz.
Its stability vs. power supply and load is ±0.2 ppm for each and aging is ±1 ppm/yr. Operating voltage can be specified from 1.7 to 3.7 V, taking 2.0 mA typ, and it comes in a 2.5 x 3.2-mm LCC ceramic package. Cost is around $3.04 ea/1,000 for a 40 MHz, 3.3 V, 0.5 ppm, -30° to 85°C device.
Fox Electronics
The FOX914B series from Fox Electronics will give you stability of ±2.5 ppm over -30° to 85°C, while costing only $2.39 each in lots of 1,000. Supply voltage is 2.5, 2.8, or 3.3 V, and the device takes a supply current of only 2.0 mA typ .
To get the whole picture for the 5.0 x 3.2-mm SMT-packaged device, you have to add a ±1-ppm initial tolerance, and the typical ±0.2 ppm each for supply voltage and load change. First-year aging adds another ±1 ppm.
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