Analog Angle Blog

Is It Time to Cut Back on Study of Classic Analog Filters?

Analog filters using inductors, capacitors, and resistors are critical to most circuits and systems. Whether as passive or active designs, it sometimes seems that they have been studied “to infinity and beyond” via theoretical constructs with very intense math, with practical “application note” designs and BOMs, and even with hands-on circuits with actual physical-construction details. This makes sense because filters play multiple essential roles in the signal path, regardless of application area. Whether low-pass, high-pass, bandpass, or notch, they are necessary even if they don’t appear to add value to the signal as they improve SNR, minimize interference from nearby channels, and attenuate 50/60 Hz pickup, to cite just a few of their many uses.

Still, classic filter theory is a subject which can drive students and engineers to weird states of mind, as they come in mind-boggling and mind-numbing versions and realizations There are so many distinct topologies, such as pi-filter (Figure 1 ), Chebyshev, Sallen-Key, Butterworth, Cauer (elliptical), and Gaussian, to cite a few. Then there are their attributes: first order, second order, roll-off, passband ripple, stopband ripple, phase performance, balanced (differential) – that’s another long list.

Figure 1

Pi Filter (Quora)

Pi Filter (Quora)

(Of course, those are just your classic all-analog filters. There are also quasi-analog switched-capacitor filters which use charge-balancing and clocked switching among multiple capacitors to implement filter functions. These are valuable additions to the filter roster as they are compatible with IC processes and can eliminate need for discrete-component filters in many cases.)

Classic analog filters can be used into the hundreds of MHz and even GHz ranges. However, these lumped-element filters are increasingly difficult to design and then fabricate successfully for these higher frequencies. Parasitics along with component tolerance and drift make them a real challenge, and these filters often need to be individually trimmed to counter their difficult-to-model realities.

If there was no alternative to discrete-component filters, many of the devices with which we are surrounded would be impractical due to size, performance, consistency, and cost. Obviously, these products are very practical, and it’s largely due to using a radically different approach to analog filters: the surface acoustic wave (SAW) and the bulk acoustic wave (BAW) filters (and their film bulk acoustic resonator – FBAR – sibling). SAW and BAW technologies have matured over the past decades to create low-cost, high-performance devices with no relationship to discrete-component analog filters.

Instead, they exploit the well-known, versatile piezoelectric effect, transforming electrical energy into acoustic waves which travel along the surface (SAW) or within (BAW) an engineered ceramic-crystal material (Reference 1 ). SAW devices are viable up to about 1 GHz, while BAW devices conveniently overlap in coverage, starting below 1 GHz and going into the multi-GHz regime. One thing they have in common is that they both obsolete the need for study of classic lumped-element analog-filter design theory and practice.

Figure 3

Basic SAW Filter (SAW, BAW, and the future of wireless Figure 1)

Basic SAW Filter (SAW, BAW, and the future of wireless Figure 1)

Reality is that classic analog filters have a much smaller role in much of today’s design activity, yet they are still taught in detail. I did a non-scientific survey of undergraduate EE courses offered at about a dozen major universities, and all but two listed classic filter design as an available course (although it wasn’t clear if this was a required or optional course). Only two had courses on SAW and BAW devices.

Is this because instructors feel that classic design, with all its math and insight, is still needed? Or is it because they are comfortable teaching it? Or because there is so much vetted material out there at so many levels to easily support such a course? Your guess is as good as mine.

My view is that classic analog filter theory should be now taught as a quick overview/survey course: what filters do, why needed, the different types and key characteristics, and the parameters used to quantify their performance – and with minimal math. Those who eventually need to know more, (or are enamored of the topic) for whatever reason, can easily find it. Instead, focus on the filters of the present and future including SAW, BAW, resonant structure, and even waveguide filters for microwaves (yes, still widely used), see References . Even better would be to look at the filters used for >10 GHz applications such as 5G or 77-GHz car radar.

What’s your view on classic analog filters? Do you love them but are ready to leave them? Do they make you squirm, and you’d like to see them get less-rigorous attention? Or are they a critical building-block along the lines of Maxwell’s equations, one in which every EE needs to be well-versed, beyond just merely acquainted? Or perhaps your view is more cynical, and you feel that filter analysis is really more of a qualifying test to see if the student is really a good fit for the analog-design world?

Related content

EDN, “SAW, BAW and the future of wireless

A major change in smartphone RF filters and front ends as 5G approaches

What Analog’s ‘Imperfections’ Taught Me

Contrary to Rumor, Analog Circuit Design Is Alive & Exciting

Can Analog Circuits Inspire Budding Engineers?

Sensor, signal conditioning define the real performance envelope

Texas Instruments enters the BAW resonator technology realm


Qorvo, “Bulk Acoustic Wave (BAW)

CS ManTech, “Bulk Acoustic Wave Devices – Why, How, and Where They are Going

Broadcom, “FBAR Devices

API Technologies, “Introduction to SAW Filter Theory & Design Techniques

Microwave Journal, “SAW/BAW New Market Entrants Offer New Approaches

7 comments on “Is It Time to Cut Back on Study of Classic Analog Filters?

  1. OldEngineer_47
    March 27, 2019

    My experience has been that most engineers with a BS degree do not have adequate training in filter design. Classic Filter design is usually taught as part of a graduate level class. Only through classic filter design can the engineer determine the order and type of the filter required to meet the system level requirements. 

    I have encounter several so called experts who design their filters by using the optimization tools within RF circuit simulation programs.


  2. Popularityseo
    March 28, 2019

    The experts I know have been taught by themselves…

  3. Steam Kid
    April 2, 2019

     I would say it is not a case of one or the other, classic filter design teaching gives you the understanding of the technical aspects, which surely still applies with SAW and FBAW filters. And can you obtain AW filters which operate down to the lower frquency range which many people still use? i.e. can you get a SAW notch filter for 50/60Hz filtering?

  4. MelBrandle
    April 18, 2019

    I would say that it should be fine to cut back just a little, but not completely. I know that whatever new data that has been made readily accessible thus far is endless, but that doesn't mean that we can simply disregard the early discoveries. We can somehow make them work together cohesively if we want to depending on the requirements.

  5. Andy_I
    May 2, 2019

    I wonder if I entered college at an odd time.  I never had any courses on filter design, and I don't recall there being any in my university's curriculum.  Unlike most of my fellow EE students, I pursued a study that was weighted on the analog end.

    What I learned about the more common classic filters, I got from self-study.

    You mean they had classes about that stuff?

  6. Andy_I
    May 2, 2019

    BTW, where your title says “classic” filters, my first thought was that you meant constant-K and M-derived filters, which I think were some of the real classics.

  7. vansunet
    May 6, 2019

    The experts I know have been taught by themselves…

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