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Oscillating onions, Batman! Techno Talk Max the Magnificent The thought that we are now capable of creating multi-billion-transistor semiconductor devices with structures whose sizes are measured in billionths of a meter makes my eyes water. I’m too young for all this excitement! I n my previous Techno Talk column (PE, December 2023), I cogitated on the concept of Precision Time Protocol (PTP), a.k.a. IEEE 1588, used to synchronise the nodes forming a packetbased network with an accuracy in the sub-microsecond range. The way this works is that somewhere in the network is a grandmaster clock – which typically obtains its time from some GNSS (global navigation satellite system) source – that propagates its concept of time throughout the network. One thing we didn’t discuss was the fact that each node in the network maintains its own local time-of-day (ToD) value, as part of which it employs an oscillator, but what sort of oscillator might it employ? Oscillating onions, Batman! To be honest, there are more layers to this onion than you might imagine. We start with a resonator, which is a passive device, such as a quartz crystal, that vibrates at a fixed frequency (its resonant frequency). The next step up is an oscillator, which is an active device that combines a resonator with an oscillation circuit to generate a clock signal. The first quartz-based crystal oscillator (XO) was built by Walter Cady in 1921, more than 100 years ago as I pen these words. Now, this is where things get interesting. The typical frequency stability variation over temperature of quartzbased XOs is between ±10 and ±100 parts-per-million (ppm). This isn’t too shabby and will satisfy a wide variety of use cases, but it’s insufficient for many of today’s more demanding applications. The next step up are TCXOs (temperature-compensated crystal oscillators), which typically have frequency stability of ±0.05 ppm to ±5 ppm over their operating temperature range. For those who demand even more, we have OCXOs (oven-controlled crystal oscillators) that achieve high stability by encasing the crystal along with temperature-sensing and compensation circuits inside a heated metal enclosure to create a miniature ‘oven’ with a relatively constant temperature. In this case, we can achieve frequency stability in the range of ±0.5 to ±20 parts per billion (ppb). 8 Ovens don’t cool things down When you think about it, an oven can only heat things up (it can’t cool things down). This means the inside of the OCXO’s oven must be maintained at a higher temperature than the outside ambient temperature (‘duh’). What does this mean in these days of climate change in which a temperature of 40.3°C was recorded at Coningsby, Lincolnshire, on 19 July 2022 (a temperature of 53.9°C was recorded in Death Valley, California, on 16 July 2023)? Well, fear not, because we are talking about oven temperatures around 75°C. If the outside temperature ever exceeds this value, keeping accurate time will be the least of our problems. A rose by any other name The first quartz-based OCXO was created in 1929 and this legacy technology is still ticking along (pun intended) to this day. Having said this, although quartz resonators remain the mainstay of the oscillation industry, devices using other materials – such as ceramic resonators or MEMS (micro-electromechanical systems) – are becoming increasingly common. Theoretically, oven-controlled MEMSbased oscillators should be called OCMOs, but that’s one battle no one in the industry appears prepared to fight. Instead, they refer to these bodacious beauties as MEMS OCXOs, and I cannot find it in my heart to fault them. The reason I’m waffling on about all this is that I was recently chatting with the folks at SiTime. These little scamps have just introduced their Epoch MEMS OCXOs, which are truly OCXOs for the 21st Century. These silicon-based devices – which have a frequency stability of 1 ppb and an internal oven temperature of 95°C – are claimed to be eight-times more consistent, two-times more resilient, use three-times lower power, 30-times more reliable, and 25-times smaller than their legacy quartz-based OCXO counterparts. How low can we go? The term ‘technology node’ (a.k.a. ‘process technology,’ ‘process node,’ or just ‘node’) refers to a specific semiconductor manufacturing process. The first ASIC I designed deep in the mists of time we used to call 1980 was a device at the 5-micron (5µm) technology node. In those days, depending on who you were talking to, the numerical qualifier referred to the width of a track or the length of the channel between the source and drain diffusion regions of a field-effect transistor (FET). I typically think of this number as reflecting the size of the smallest structure that can be created in or on the surface of the silicon chip. Every time we move to a new technology node, we either reduce the area used or increase the number of transistors that can be squeezed into the same area. We also increase the speed of the transistors while reducing the amount of power they consume. We started creating devices at the 1µm technology node circa 1985, where 1µm is 100th the diameter of a human hair (assuming a human hair has a diameter of 0.1mm). At that time, the naysayers started to proclaim that we were reaching the limits of what was possible. But we kept on overcoming problems and coming up with new solutions, and we started to describe nodes in terms of nanometres (nm). I remember the progression well: 800nm in 1987, 600nm in 1990, 350nm in 1993, 250nm in 1996, 180nm in 1999, and 130nm in 2001. Surely this was as low as we could go… but no! We saw 90nm in 2003, 65nm in 2005, 45nm in 2007, 32nm in 2009, 22nm in 2012, 14nm in 2014, 10nm in 2016, 7nm in 2018, and 5nm in 2020. Apple’s latest processor, the M3 is built with 3nm technology – it’s most advanced version, the M3 Max, boasts 92 billion transistors. TSMC, the Taiwanese world leader in chip fabrication plans on introducing its 2nm node in 2025/2026, and pundits are predicting the 1nm node in 2028. (For comparison’s sake, the atomic radius of silicon is 0.132nm, so we are talking about structures just a few times bigger than the atoms used to build them.) All I can say is the thought that we are now capable of creating multi-billiontransistor devices with structures whose sizes are measured in billionths of a metre makes my eyes water. Practical Electronics | January | 2024