AUSTIN, TX – Abracon, LLC (Abracon), a leading global manufacturer of frequency control, timing, synchronization, RF, connectivity, and power component solutions, announces the release of a new application note titled "Benefits of Measuring Crystals In-Circuit". Read the complete application note here.
Below is an excerpt:
Basic Oscillator Structure
The basic structure of an oscillator comprises of two elements – an amplifier and a frequency-selective network.
There are two ways to clock a system: use a fully integrated crystal oscillator, or mate the crystal directly with the on-chip oscillator. Using an XO tends to increase power consumption and system costs. By mating a quartz crystal with the embedded Pierce oscillator circuit inside the MCU, system power consumption and costs are reduced.
The majority of embedded oscillator circuits use the Pierce oscillator, a configuration which comprises of a simple inverter amplifier as the inverting gain element within the loop.
In most cases, the amplifier unit is internal to the MCU and the frequency-selective network is external to the MCU. The key component in the external network is the quartz crystal. Associated loop capacitors and a series (current limiting) resistor (Rs) are also used.
Quartz Crystals used in this approach are coined as Parallel Plated Crystals, with standard values such as 10pF, 12pF, 18pF, etc. This implies that the final oscillation frequency will be within the tolerance of the standalone quartz crystal when the closed-loop effective capacitance is exactly equal to the quartz plating capacitance
As a feedback system, the oscillator demands extensive analysis along with a thorough understanding of circuit board and layout parasitics to optimize the loop and ensure operation over all conditions. Designers often optimize the crystal oscillator performance via trial and error, which saves time analyzing and modeling the components and circuit board. Tight time-to-market and scheduling constraints have led to more trial and error rather than bottoms up analysis. The result is non-optimal coupling between the crystal and the Pierce oscillator.
Optimal coupling guarantees the crystal is neither over-driven nor under driven. Overdriving the crystal, especially in today’s low power varieties can lead to stress fractures and overall reliability concerns during operation. Under driving may lead to failure on startup or an eventual decay of the oscillation. Additionally, there are frequency accuracy concerns. MCUs with Pierce oscillators that offer configurable transconductance, can also optimize for lowest power consumption. Without careful analysis and verification, design by trial and error can lead to any of these concerns.
Growing technology trends including green energy initiatives and the explosive growth of IoT centric solutions, are driving the need to accurately define the frequency-selective network in order to achieve the best possible accuracy in frequency domain, while ensuring robust oscillator loop performance.
Measuring in-circuit crystal performance eliminates the problem by converting the unknowns into well knowns. Characterizing the complete frequency-selective network including board parasitics minimizes uncertainty and maximizes confidence in long term system reliability.
The frequency-selective network includes the following passive components selected and optimized by the designer:
- Quartz Crystal
- Loop Capacitor, C1
- Loop Capacitor, C2
- Current Limiting Resistor, RS (if applicable)
There are three key oscillator performance parameters dictating the values of the passive components. These parameters include frequency accuracy of the oscillator loop, drive level seen by the crystal, and the closed-loop Gain Margin of the oscillator...
About Abracon, LLC
Founded in 1992, and headquartered in Spicewood, Texas, Abracon is a leading global manufacturer of passive and electromechanical timing, synchronization, power, connectivity and RF solutions. Abracon offers a wide selection of quartz timing crystals and oscillators, MEMS oscillators, real time clocks (RTC), Bluetooth modules, ceramic resonators, SAW filters and resonators, power and RF inductors, transformers, circuit protection components and RF antennas and wireless charging coils. The company is ISO9001-2008 certified with design & application engineering resources in Texas and sales offices in Texas, California, China, Taiwan, Singapore, Scotland, Israel, Hungary, UK, and Germany. Abracon’s products are offered through its global distribution network.
For more information about Abracon, visit www.abracon.com.
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