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Analog Angle Blog

Forget about 10 GHz–let’s aim for 100+ GHz

It wasn’t that long ago that most RF circuits operating even slightly above 1 GHz were mostly the-domain of experimental, scientific, or one-off projects, with just a few specialized exceptions such as costly radar systems. The required components were scarce, if available at all, and hard to use; the designs were complex and limited in performance. Equally frustrating, the tools needed to model and design, and the instrumentation needed for test and evaluation, were also costly, awkward, or not up to the job (pick on or more). Mass-production consumer products operating in that part of the spectrum were rare and expensive.

Times have changed, a lot. The development of cell phones, Wi-Fi, and the similarly situated license-free ISM (instrumentation, scientific, medical) 2.4 GHz band has driven availability of all the pieces needed to live and prosper in this >1 GHz region. Now systems are moving higher, with activity in the 5 GHz Wi-Fi band as well, due to crowding and interference issues at 2.4 GHz. Ironically, this at least partially due to the success of the cellular, Wi-Fi ad ISM initiatives at 2.4 GHz.

No single technical development is the enabling factor for this move to higher frequencies; instead, it's the back-and-forth interplay between demand and resources. Demand drives the need for active analog and passive components as well as associated modeling and manufacturing tools, while advances in the components and tools enable the demand to be realized, and then the virtuous cycle begins again.

Of course, 5 or even 10 GHz is not the end of the RF spectrum. There's significant component and end-product work being done at 60 GHz supporting data transfer rates up to 7 Gbit/s, with standards promoted by Wireless Gigabit Alliance via IEEE 802.11ad. Moving up the spectrum, we're seeing automotive radar operating at 77 MHz which will drive design, production, and cost initiatives.

Still, it's a tough battle with many obstacles. While Maxwell's four equations, Figure 1 , govern the electromagnetic reality regardless of wavelength, the interplay between the EM fields and materials is a complex function of frequency, and often non-intuitive. For example, his equations do not directly indicate that 2.4 GHz would be the resonant frequency of water molecules, and so the critical frequency for microwave-oven magnetrons due to energy absorption. They do not show that a 60-GHz signal can barely penetrate walls but can propagate via reflections from walls, ceilings, floors.

Figure 1

Maxwell's equations, reduced to this well-known set by Oliver Heaviside, are the foundation for all EM theory and practice.

Maxwell's equations, reduced to this well-known set by Oliver Heaviside, are the foundation for all EM theory and practice.

(Side note: while we call them “Maxwell's equations”, they should really be called “the Maxwell-Heaviside equations” as it was really Oliver Heaviside who reduced Maxwell's numerous and extremely complicated expressions to the classic quartet we use today, see “Oliver Heaviside: A first-rate oddity” from Physics Today .)

Even designing and fabricating a coaxial-cable connector is a real challenge as we go past 10 GHz and reach for 100 GHz. A recent article in Microwaves & RF , “Reaching Beyond 100 GHz With Coaxial Connectors,” went through the details. It noted that just a few decades ago, the feeling was that coaxial connectors above around 40 GHz would be unlikely because of mechanical limits, and designs would have to rely on more-conventional waveguides. Yet today, vendors such as Anritsu, Keysight Technologies, and others offer coax connectors and cable assembles which go to 110 GHz.

These 100-GHz connectors and cables are hard to fathom. The cable's outer diameter is on the order of 1 to 2 mm, and the connector's inner mating surfaces are smaller, of course; the required slot on the female connector is cut using a saw blade with a 0.05 mm kerf. Even more impressive, these connectors are not one-off, custom-machined and assembled components; instead, they are designed to be manufactured in a production environment with at least modest volumes. Modeling all the EM-field modes in the connector, connector, and their interface is almost magical as practice meets theory at these wavelengths.

Can mass-market use of 100 GHz be that far away? The future is hard to predict, but apparently, the answer is that it is starting to become a real possibility. As we reach further and beyond 100 GHz, we'll cross into terahertz and even optical realms.

As we make the inevitable progress, I'm keeping a poster-size spectrum chart spanning 3 kHz to 300 GHz on my wall for reference, similar to United States Frequency Allocation Chart, Figure 2 ; the version I have also gives some valuable additional perspective with a small bar chart at its bottom reaching through infrared, visible, ultraviolet, X-ray, and gamma rays, all the way to cosmic rays at 1025 Hz.

Figure 2

The wireless spectrum allocation from 3 kHz to 300 GHz shows the severe crowding that moving to 100 GHz and beyond can avoid, while also offering very wide bandwidths (from United States Department of Commerce, National Telecommunications and Information Administration)

The wireless spectrum allocation from 3 kHz to 300 GHz shows the severe crowding that moving to 100 GHz and beyond can avoid, while also offering very wide bandwidths (from United States Department of Commerce, National Telecommunications and Information Administration)

Are you doing any work at 10 GHz and higher? What time frame do you see for mass-market applications at 60 GHz, 100 GHz and beyond?

Also related

Intel 5th Gen vPro Goes 60GHz Wireless

Wi-Fi Alliance Radiates Outward

WiGig Gives a Leg Up on 5G

WiFi Preps for 5G, IoT Roles

5 comments on “Forget about 10 GHz–let’s aim for 100+ GHz

  1. jimfordbroadcom
    February 12, 2015

    Yep, we are working on E-band (71-76 and 81-86 GHz) millimeter wave wireless backhaul, and it is tough going!  I'd like to mention Southwest Microwave as another connector manufacturer making 1.0 mm connectors usable to 110 GHz.  Although we are having our challenges getting a reasonable PCB launch for the Southwest connector.  Free-space wavelength at 86 GHz is less than 3.5 mm (~0.137 inches), and the mechanical tolerances are so tight that the board fab shop is having difficulty finding a way to meet our requirements and still be able to do it in volume.  We'll get there eventually.

    Also boding well for millimeter wave technologies becoming commonplace are the large number of articles I keep seeing in IEEE Communications Magazine about 5G cellular and how it is going to require higher carrier frequencies.  There simply aren't enough Hz to go around at the RF and microwave frequencies we are used to using.  Advances in technology are making it feasible now to use narrow beamwidths and have the transmitter and receiver find each other before transferring data.  In the near future we may look back and laugh at the silly way we used to use omnidirectional antennas and blast RF all over the place instead of focusing it on the intended receiver.  I, for one, am looking forward to working with these new technologies to advance our communications abilities.

  2. MWagner_MA
    February 12, 2015

    I think the speeds will eventually creep up into that arena, however, the quality of the systems and cables will be even more important to work to potential.  Remember the days of the 56K modems that would barely perform better than a 28.8K?  Ahh, marketing.  The consumer will need to much more discerning in the future to avoid being taken by “frivolous specs”.

  3. jimfordbroadcom
    February 12, 2015

    Absolutely!  As a born engineer, I'm all about making stuff work better.  I'm forever having to pull the reins in on gung-ho sales and marketing types!  Don't get me started about the PLAUCS (Politicians, Lawyers, And Used Car Salesmen)!

  4. qbs
    February 13, 2015

    I've done my thesis on 100Gb/s serial link.

    The ft of bipolar transistor is so high we can design very fast circuits (LNA, PLL and so on). Even if the layout is quite hard at such frequencies, some tools help us to design (HFSS and so on).

     

    The real problem is the measurements parts. It's really hard to have oscilloscope and pattern generator above 50 GHz. And you need coaxial wires really well matched. Most of my measurements were with probes on chip and S parameters after a lot of calibration.

     

    But that was a really challenging and rewarding job.

  5. Terry.Bollinger
    February 20, 2015

    The topic of very high frequency comms is fascinating, but I just can't resist a few quick sidenotes on that intriguing sidenote: (1) the general public still has almost no idea who Maxwell was, which is quite sad considering how much his very diverse work contributed to modern science in general and special relativity in particular; (2) more people probably know Heaviside's name if only because of that one line from Cats (yes, that's the same Heaviside, it's a reference to his study of the iononsphere); (3) Maxwell's original theory was entirely written in terms of quaternions of all things; and (4) Heaviside with Gibbs did not just translate Maxwell's theory, they pretty much invented vector analysis as a way of generalizing quaternions to more than four dimensions.

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