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MEMS_Oscillators.php   17216 Bytes    24-07-2016 21:31:14


MEMS Oscillators


Evaluated with the SiT8008/SiT8208/SiT8209/SiT3808




SiT8008 Testboard


The MEMS Testboard. With a buffer amplifier and a low drop voltage regulator.




✈ Circuit




The circuit is straightforward. (as usual). The SiT8008 is a Low Power type. We have chosen the largest size (7 x 5 mm) to allow for easy handling. A resistor divider at the output of the oscillator scales down the voltage, reduces the load and matches the input of the following amplifier. This approach was chosen to increase the isolation and reduce the effects of pulling. (in case the output gets shorted ;-) A voltage regulator (IFX25001MEV33HTSA1) produces a clean 3.3 V supply. Only for the oscillator. The buffer amplifier (GALI-6+) is powered by 5 V. The circuit consumes 80 mA when powered with 5 V. Output power is + 6 dBm (fundamental).






✈ Technology




A common way to classify oscillators is by the type of resonator (XCO, VCXO, RC, DRO, ...). In our case MEMS, which stands for Micro Electro Mechanical System. Our resonator is therefore a mechanical resonator in the shape of a tuning fork. What is new is its size and material. First, it has been shrinked into the sub-millimeter range and second, it is manufactured on (in) the silicon die - like other cmos circuits. This has a lot of advantages. As those very small resonators are fully encapsulated in silicon, they are extremely stable and highly durable. The resonators are protected under a layer of silicon and can withstand a very high pressure such as 100 bar. (Conventional low temperature packaging, that uses ceramic packages or wafer bonding, leaves volatile organics and water residues in the package which cause mass loading of the resonator and frequency drift.) A PLL then does the frequency synthesis. We assume, that it is a fractional-n type.


Tuning Fork, 1 kHz, 1941, General Radio
Source: JuiceKing, EEVblog
Tuning Fork, MHz•GHz, 2016, Discera/SiTime
Source: codeboy2k, EEVblog




✈ Programming




Programming can be done just once (OTP). Therefore this is a "set-and-forget" approach. The proprietary programmer works beautiful, but € 250 is slightly above the homebrewers budget (think we). So you may use the programming service from e.g. digi-key - or wait, till someone shows up with an Arduino/Genuino based solution :-)


SiTime TIME MACHINE II


When programming those little pll's you have to think in part-numbers. But thats not a big deal, as they are only a string of characteristics. You may also use the part number generator, which breaks it down to setting (some) ticks and enter a numerical value for the frequency.




✈ Datasheets & Application Notes








✈ Performance




Last but not least a quick look at the performance. The question is : can it replace a crystal oscillator in rf circuits ? The manufacturer probably would say 'Yes, we can' because SiTime offers a broad range of types, which seem to be optimised for almost any application. You may choose between LVPECL and LVCMOS, singled ended or differential. The user may also select a Frequency Stability (±20, ±25, ±30, ±50 ppm). Besides that there are Low Power, Ultra Performance, High Performance, High Temperature, Automotive, VCXO and Spread Spectrum types available. We measured some of the 7 x 5 mm types which have been programmed to 10.000000 MHz in order to compare it with our HP Agilent 58503B GPS Time/Frequency Receiver as well as a Efratom LPRO-101 (from the bay) and a standard 10 MHz crystal oscillator.


SiT8008AC-81-33N-10.000000
SiT8008AC-81-33N-10.000000, high impedance load (380 Ω)




SiT 8008Low Power Programmable Oscillator
Range1 MHz ... 110 MHz
Power5.0 V, 90 mA (incl. amplifier)
MarkingB0AY6
SiT8008 at 10.000000 MHz
Low Frequency Noise


SiT 8208Ultra Performance Oscillator
Range1 MHz ... 80 MHz
Power5.0 V, 110 mA (incl. amplifier)
MarkingB0DF4
SiT8208 at 10.000000 MHz
Low Frequency Noise


SiT 8209High Frequency, Ultra Performance Oscillator
Range80.000001 MHz ... 220 MHz
Power5.0 V, 110 mA (incl. amplifier)
MarkingB06WN
SiT8209 at 97.550000 MHz
Low Frequency Noise


SiT 3808High Performance MEMS VCXO
Range1 MHz ... 80 MHz
Power5.0 V, 110 mA (incl. amplifier)
MarkingB08SM
Remarks• Vin connected to Vdd results in f = 10.000604 MHz
• We assume this is a SiT8208 with a Varicap build in.
SiT3808 at 10.000000 MHz
Low Frequency Noise




Possible "Reference" Oscillators


In order to to place those measurements in relation to other synthesizers, we measured the spectra of some "professional" synthesizers. See screenshots below. One was persuaded to retire afterwards :-)

Marconi 2022 Synthesizer
Stanford Research DS345


Rohde & Schwarz, SMC 100 A
Hewlett-Packard hp 33250


Rubidium Cavity Standard, from here
TXC 7W-10.000MBB-T, XCO


Hewlett-Packard hp 58503B, GPS/OCXO
... we work on that ...




✈ Conclusion




The Type SiT8008 is not suiteable for rf mixing applications, but it mayst serve as a clock source for a dds. Due to its 1 Hz resolution, a frequency of 67.108864 MHz is attainable and can be used to clock an AD9834 (75 MHz max.)




✈ Share your thoughts


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t1 = 3876 d

t2 = 560 ms

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