Acoustic Wave Filters: SAW & BAW

A electronic filter is a device or combination of devices that are engineered to block signals in specific and (hopefully) controlled frequency ranges.The simplest filters are common board components. Inductors naturally block high-frequency signals and capacitors naturally block low-frequency signals. (For the record, resistors have no natural frequency-selective properties.) Other board components do such as diodes can also be used in filters but usually with other components. So lets get away from passive filters, L filters T and π filters and into the more interesting filters you might find under the chassis. I'm also going to skip M and K filters because I wrote them and George Campbell up previously. Today is about SAW filters.


Surface Acoustic Wave (SAW) filters are electromechanical filters used in radio. In these filters, electrical signals are converted to a physical wave piezoelectric crystal. This conversion causes a time delay as the wave propagates through the unit. At the output it is converted back into an electrical signal. SAW filters are limited to frequencies up to 3 GHz. They are commonly used as band-pass filters in superheterodyne receivers and 2G wireless devices.

They were first developed by Prof. Edward "Ted" George Sydney Paige a British physicist. His interest began in college at Reading University. His doctorate was an investigated into the optical properties of wavelengths in crystalline quartz caused by x-rays. His interest in piezoelectric crystals didn't end there. At the Radar Research lab at Malvern he lead a team of physicists that included Graham Marshall, Meirion Lewis and Dennis Maines. Together they developed the SAW filter in 1968. This was somewhat presaged by the development of pseudo-SAW filters. In 1965 R. White and F. Voltmer at the University of California developed a thin-film interdigital transducer (IDT) There are also Temperature Compensated SAW filters (TC-SAW.)  These have a coating of the IDT structures with layers that increase stiffness at higher temperatures. They are used in the middle ground as a cheaper component than a BAW filter (below) and more robust than a standard SAW filter.

Bulk Acoustic Wave (BAW)  filters are also electromechanical filters. BAW filters can be implemented as ladder or lattice filters. In fabrication they are isolated acoustically by silicon substrate. They are used typically for their low insertion loss and steep narrow band rejection. These can be configured two ways:

1.  BAW-SMR (solidly mounted resonator) where the resonator is mounted over a stack of alternating materials.
2. BAW-FBAR (Film Bulk Acoustic wave Resonator)  where the resonator is is suspended by an air-bridge.

One notable filter based on the BAW-SMR is the BAW-CRF (coupled resonator filter). This design stacks two BAW resonators separated by passive layers, to enable an exchange of acoustic energy between the lower and the upper resonator. These may soon be used in CDMA devices.  These usually operate at frequencies between 2 GHz to 16 GHz.

M and K

In radio we use a lot of gear that even obsessed audiophiles and stereo-enthusiasts would never have need for.  Among these arcane devices are filters. (Not that there aren't filters already built into some of your devices.) Electronic filters can be broken into many categories such as: analog or digital, passive or active, linear or non-linear, infinite impulse response (IIR) or finite impulse response (FIR). They have a hundred year history and they're often over looked.

The first electronic filters were passive and analog linear which makes sense. These were made with just the early resistors and capacitors, resistors and inductors. These are known as RC and RL single-pole filters respectively. They are still in use today and they are called RC or RL filters for resistor–capacitor and resistor–inductor. (Remember L is for Lenz). Today there are hundreds of electronic filters on the market. I'll get to a post about those early filters but today let's discuss M and K filters which were more or less contemporary to each other.  

Image impedance is connected to impedance matching. It is the impedances which will simultaneously terminate all the inputs and outputs of a network in such a way that the impedances are equal in both directions. This avoids internal reflection loss and optimizes power output. Constant K-filters are the first and simplest of this type. They consist of a ladder network of identical sections of passive components. Constant K filters were invented by George Campbell at AT&T. He first published his work in 1922 but probably invented it as early as 1917. He referred to it as a "wave" filter. The big improvement with a K filter was that that they could be designed for any desired degree of stop band rejection or roll off between pass bands. You can read that here.


Otto Julius Zobel built upon this work and invented the M-type filter in 1920. You'll note that this year is before Campbell published. Zobel and Campbell worked together at AT&T so he had access to Campbells work before the general public. The different between these two image impedance filters was that the M has a sharp filter response which roll off very rapidly at the cut-off frequency. AT&T used this to squeeze the maximum number of intelligible phone calls onto one wire. Zobel later created hybrid filters that combined the strengths of the K and M filters, with had both the fast transition of the M and good stop-band rejection of the K.

George Campbell Filters!

George Ashley Campbell developed the first electrical wave filter in May of 1917. I cannot begin to explain how important this was to radio. Actually I can. That's why we're here. Before Campbell nobody filtered much other than water and moonshine.

He actually has 2 separate patents on the electric wave filter. He invented the principle and the first practical application of the following:
1. The low‑pass filter
2. The high‑pass filter
3. The band‑pass filter

These were rudimentary, they consisted of simple inductors and capacitors. That first one is now called a "ladder filter." His 2nd patent was more about it's application to the filtration in telephone amplifiers. He'd actually ben using them for years by the time he bothered to patent them. It was his employers at the American Telephone and Telegraph Company that realized this might be important. He'd go on to have a 40 year career in radio engineering.

George graduated from MIT in 1891, and went on in 1893 to get a master’s degree from Harvard University. He went to work for the American Telephone and Telegraph in 1897. American Telephone and Telegraph of course went on to be known as AT&T. More here.

He has been multiply awarded for thsi and other accomplishments in radio engineering. In 1936, the Institute of Radio Engineers awarded him its Medal of Honor. The American Institute of Electrical Engineers awarded him the Edison Medal. He retired from AT&T in 1935 and died in November 1954.