The kHz quartz crystals, also known as clock quartz or tuning fork crystals, are suitable, e.g., for the discrete design of an oscillator with a controller or RTC IC (real-time clock). The key parameters of such a design are the negative resistance (oscillator gain) and the load capacitance of the overall circuitry necessary to satisfy the phase and amplitude condition (k · v > 1) of a self-oscillating oscillator.
The supply situation for kHz quartz crystals has been plagued by shortages over the last two years. Suppliers who produce quartz crystals themselves are generally still able to deliver, while many others who buy in the raw material have had to increase their delivery periods from around 16 weeks to up to two years. The fact that many well-known suppliers have removed kHz quartz crystals from their portfolios over the years further complicates the situation.
In-house production increases reliability in terms of quality and delivery time
One supplier that produces its quartz crystals completely in-house is Epson. They are produced in the company’s own autoclaves from self-grown synthetic quartz crystals. Epson is, therefore, not only able to ensure availability but also reliable, consistent quality.
In the meantime, Epson focuses entirely on the SMD design for kHz quartz crystals, available in particularly small designs with welded metal covers (FC3215AN, FC2012AN, and FC1610AN series). Plastic encapsulated cylinder housings are also still part of the portfolio. However, they are no longer recommended for new projects, as the trend in production planning and capacity expansion continues to move toward smaller designs.
For the FC2012AN and FC3215AN series, Epson has been able to lower the maximum equivalent series resistance (ESR) to an attractive level of 50 kΩ, which counters the usual increase in ESR with smaller sizes. For these and the FC-12M and FC1610AN series, delivery times have now been reduced to 18 weeks (as of September 2022).
MHz quartz crystals
The insatiable appetite for ever greater volumes of data and thus higher transmission speeds inevitably leads to increased demand for higher frequencies. MHz quartz crystals are also available in increasingly smaller packages (FA-128, FA-118T, and FA1210AN series) as development of this technology also follows the goals of increasing efficiency and minimizing costs.
QMEMS, a wafer-based process, combines the high stability and precision of quartz crystals with MEMS technology to achieve high performance in compact packages. In the photolithographic process, the thickness shear mode oscillators (AT cut) become washed, cut chips with a fine structure. These chip shapes have a larger electrode area and thus lower electrical resistance values and come in small designs. Special further processing using high fundamental frequency (HFF) technology gives them high mechanical stability even with low crystal thickness. The result is fundamental wave oscillators with frequencies of up to 500 MHz.
Often, the current market delivery times for MHz quartz crystals are two years or more. However, for some series (e.g. FA-128, FC-12M, and FA-20H) and technical parameters, Epson is able to deliver the products after approx. 18 weeks (as of September 2022).
SPXOs – programmable oscillators
SPXOs (simple packaged crystal oscillators) are the simplest crystal oscillators without compensation or temperature control. They consist of a quartz crystal unit and an oscillating circuit.
For models that exhibit very low jitter (phase noise) even at high frequencies, Epson uses only quartz crystals as the fundamental wave oscillator in addition to specially developed low-noise oscillator ICs, either using HFF quartz crystals or with a PLL (phase-locked loop; Fig. 3).
With this self-developed oscillator IC compensating for the frequency or temperature coefficient of the HFF quartz crystal, the new SG2520 series achieves low phase noise and improved frequency stability of 25 ppm (SG2520_EGN/VGN) and 20 ppm (SG2520_EHN/VHN) at –40 to +125 °C.
PLL technology ensures programmable frequencies of between 0.67 MHz and 170 MHz. This allows the modules to be set to a specific frequency (with six decimal places) as required, which significantly improves availability. The SG-8018 series also has a low deviation of 50 ppm at –40 to +105 °C, including aging stability over ten years. Even more accurate is the SG-8101 series with a tolerance of 15 ppm, thanks to integrated temperature compensation. Due to its programmable spread spectrum capability, the SG-9101 series is particularly well-suited for reducing EMI problems. It is available with operating voltages ranging from 1.62 to 3.63 V in various package sizes.
In recent years, the demand for SPXOs has risen sharply. Suppliers like Epson with their own development and production operations obviously have a competitive advantage. Due to the high level of demand, it is recommendable to plan projects in good time.
TCXOs – temperature-compensated crystal oscillators
Temperature-compensated crystal oscillators (TCXOs) are available for sensitive applications that require stable frequencies even with large temperature fluctuations or over a wide temperature range (Fig. 4). By providing its own ICs, Epson can also produce the ASICs for its TCXOs and optimally match the individual components, such as the control voltage generator, correction circuitry, and oscillator core. In addition to TCXOs with CMOS outputs, Epson’s portfolio primarily includes those with clipped sine wave outputs. As the name suggests, their sinusoidal signal is clipped and thus generates significantly fewer harmonics
in relation to CMOS, resulting in better EMI behavior. Offering the highest stability and low phase noise, the TG2016SxN and TG-2025SxN series are ideal for use in wireless communication devices.
SPXOs and TCXOs have been particularly affected by supply chain disruptions. And the massive fire that destroyed the semiconductor factory of Asahi Kasei Microdevices (AKM) in 2020 is still aggravating this situation. The Japanese IC manufacturer had a market share of between 80 and 90% in temperature-compensated crystal oscillators (TCXOs). The result of the fire was a global shortage of all oscillator types. Since Epson has equipped almost all TCXO lines with ICs developed and produced in-house for several years, the supplier has been able to meet many targets. In the meantime, Epson has almost doubled its production capacity, so that delivery times of approx. 18 weeks (as of September 2022) or faster are now the norm.
RTCs – real-time clocks
A discrete design with oscillators enables customized solutions but also requires an understanding of the individual components. Hence, errors often occur that result in a lot of time being spent on design corrections. It is actually much easier to use ready-made real-time clocks (RTCs). Epson offers complete modules including quartz crystals, semiconductors, oscillators, and other components for many additional features (wakeup timer, alarm, timestamp, battery charging function, flexible pin assignment FOUT/timestamp) in a package with I2C protocol, but also with 3- or 4-wire SPI. This not only reduces development time, but usually also space requirements and power consumption.
Highly accurate RTCs also include a DTCXO (digital temperature-compensated crystal oscillator). The deviation of the output frequency is compensated with the help of this digital circuitry. For example, RX8900CE and RX8804CE achieve a maximum deviation of 9 s/month at –40 to +85 °C and 21 s/month at –40 to +105 °C, respectively.
The latest RX8901CE series achieves a stand-by current of typically just 240 nA despite temperature compensation up to 105 °C and also includes a maximum number of additional functions for the most versatile applications.
As demand for RTCs continues to grow to meet the increased temperature requirements, standby times, and functionality of many applications, it is recommended to plan and phase projects carefully. This helps to avoid delivery times of six to twelve months (as of September 2022).
For more information and a direct ordering option, please visit our e-commerce platform at www.rutronik24.com.
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