Clock oscillator achieves 0.2 ppm frequency tolerance

Depending on temperature range, the SiT7910 Super TCXO from SiTime can attain frequency stability of less than 0.1 ppm.

Clock oscillator circuits serve two functions. They provide a clock timing signal for digital circuits such as microprocessors and they keep track of time, often for time-stamping of events. While oscillators for digital circuits typically produce signal frequencies of, say 156.25 MHz, oscillators for time tracking operate at 32.768 kHz because that’s easily divisible to 1 sec.

SiTime has released the SiT7910 MEMS-based temperature-controlled oscillator (TCXO), which runs at 32.768 kHz. What makes the SiT7910 different is its frequency tolerance: ±0.2 ppm over a temperature range of -55°C to 105°C (±0.1 ppm from -40°C to 85°C). SiTime claims that the SiT7910 is the first TCXO to achieve less than 1 ppm of timing accuracy.

“Most 32.768 kHz oscillators have 1 ppm precision,” said Piyush Sevalia, SiTime executive vice president of marketing in a conversation with EE World. The SiT7910 primary market is aerospace and defense. The device’s tight timing accuracy lets wireless communications equipment on the ground connect to satellites faster because the tighter tolerance cuts connection time.

“A tighter tolerance also saves power,” added Sevalia. He cited an example of turning on and off a Bluetooth or other radio. With a tight timing tolerance, a radio turns on later and turns off earlier, thus reducing power consumption by increasing off time.

SiT7910 frequency variation versus temperature. — The plot shows the SiT7910 frequency variation under 0.1 ppm versus temperature.

The SiT7910 compensates for temperature changes through a linearization algorithm. It’s internal registers store third-order coefficients that linearize frequency over its entire temperature range. According to Sevalia, the algorithm takes a raw accuracy of 50 ppm to 75 ppm and reduces it to 0.2 ppm.

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