How Critical is Gm_critical?

The trend toward increasingly low power consumption for many MCU and RF chipsets has accelerated with the emergence of the Internet of Things (IoT). This trend pressures IC designers to wring out every bit of energy consumption in circuit design. Oscillator design geared toward low power consumption comes with an inherent side effect; the loop is starved of gain also known as the transconductance (gm). Although not of great disadvantage in the digital design domain, analog circuits can be severely affected by reduced power consumption, particularly the Pierce oscillator.

The Pierce oscillator that drives quartz crystals is one of the most common circuits in the electronics industry. Present in every microcontroller, almost all RF chips, and many other ICs, this feedback loop is the true workhorse of timing in almost every device developed today. As most design engineers employ an analysis of negative resistance with respect to the loop dynamics, some tend to ignore a very significant parameter of the Pierce oscillator design: the gm_critical metric.

gm_critical can be defined as the margin above the transconductance (gm) of the amplifier in the Pierce oscillator. In order to maintain stable oscillation over temperature and across all corners, a margin of 3X to 5X is desirable. For example, if the transconductance (gm) of the amplifier in a Pierce oscillator circuit is 0.5µA/V, the critical transconductance (gm_critical) becomes 2.5µA/V (5X 0.5uA/V). This represents the safety margin built into the design of the oscillator loop. To guaranteed stable oscillation for electronic systems across all factors including temperature, drift, and aging effects, the Pierce oscillator must be matched with a crystal that does not exceed its gm_critical metric.

For any given Pierce oscillator, a crystal must not exceed this parameter. A simple calculation yields whether a crystal will violate this critical metric.

gm_critical = 4 x ESR x (2piF)^2 x (C0 + CL)^2

Abracon’s family of newly released IoT optimized crystals are specifically engineered to address the increasing low power consumption trend which engenders the falling transconductance values designers are facing across the industry. These IoT optimized crystals are compatible with the industry’s lowest gm_critical designs. The ABM8W/ABM10W/ABM11W/ABM12W series are guaranteed to operate with gm_critical as low as 3.55mA/V for 50MHz operation. The ABS06W/ABS07W series are guaranteed to operate with gm_critical of 1.1µA/V or higher. Given that many common MCUs, such as the ST Micro series STM32M4F/STM32F3/STM32F4/STM32L4, which, exhibit gm_critical near 1µA/V at 32.768kHz operation, the criticality of using very low plated crystals is huge. Abracon’s new series of IoT optimized crystals currently offer the lowest load capacitance options in the world.

Datasheets: ABS06W | ABS07W | ABM8W | ABM10W | ABM11W| ABM12W

As the transconductance (gm) of most Pierce oscillators continues to dive towards unprecedented lower levels, the quartz crystal has been forced to not only be of smaller size but also offer very low plating load (CL) and low equivalent series resistance (ESR) parametric options. Meeting the lowest gm_critical requirements demanded by energy saving MCUs and RF chipsets depends on maintaining guaranteed low CL, ESR and C0 parametrics. This requires new non-trivial plating designs on crystals that drop both CL and ESR simultaneously and can still guarantee low specifications over temperature and aging effects. New quartz crystal plating designs are enabling the next generation of Pierce oscillators to run on fumes while being able to maintain oscillation. The electronic industry has one very consistent trend directly dependent on time – as technology gets smaller, demand for reduced power consumption gets higher. Addressing the quartz crystal design requirements, contingent on the industry demands, has become truly critical.

For additional information read the IoT Optimized Crystal White Paper here.

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 ISO9001-2015 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.