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1885: Lord Rayleigh introduced the concept of RF filters via his paper “On waves propagated along the Plane Surface of an Elastic Solid” and introduced a mathematical formula for Rayleigh waves - Surface Acoustic Waves (SAW) (these include seismic waves)
1940's: World War II escalated research into the SAW field of RF filter development to assist with improving signal to noise of radar and developed coil and capacitor based filters
1965: White and Voultmer publish a paper on “Direct Piezoelectric Coupling to Surface Elastic Waves” to create a RF filter by placing a comb shaped input electrode (“Inter-digitated transducers”(IDTs)) on a piezoelectric quartz bar which deforms in response and this mechanical energy sends a SAW to an output comb shaped electrode which converts the SAW back into a electrical signal with the frequency pass characteristics depending upon the width of the gaps in the electrode combs - hence the development of inexpensive SAW based RF filters which, in the 1970's, were manufactured as single layer analog Micro-electro-mechanical systems (MEMS) devices using lithographic etching semiconductor technology and could work in the 10Mhz - 1Ghz range and were initially researched by the military to improve the hand held radio communication devices, and in the late 1970's a SAW RF filter was used on the Voyager space probe as part of its deep space transponder and this worked for at least 50 years.
1973: Motorola's first “mobile phones” used ceramic RF filters which were smaller than coil and capacitor based filters
1975: analog TV manufacturers found these SAW filters as an excellent low cost replacement (~$2 each) of their old coil and capacitor based filters to improve the signal to noise of TVs and reduce static interference which plagued these TVs and production rapidly escalated into the millions making them extremely cheap at only $1.50 each by the late 1980's
1980's: the IDT gaps could be made as small as 300nm wide and for higher frequencies lithium niobate was used instead of quartz
1980's 2G mobile phone standards introduced and smaller RF filters than the ceramic filters were needed but existing SAW filters were not able to go above 2GHz - they initially only used 2 filters - one to send and one to receive for the single band in GSM but then GSM expanded into additional bands and each phone band required additional filters but then the filters started to interfere with each other
1984: Motorola's first commercial 2G cell mobile phone but there were very few cell towers and this required a brilliant new, albeit large, 7“ long antenna (1/2 wave for the 14” wavelength 850MHz band) and a brick sized phone
1989: Motorola's much more compact cellular “flip” phone which halved the size of the phone and had a 7“ retractable helical antenna
1980's: - concept of Bulk Acoustic Wave filters developed in which the energy is send throughout the bulk of the substrate not just along its surface as with SAWs - these bulk waves can travel faster and further without deterioration
1992: Fujitsu develops a low loss band pass SAW resonating filter for portable telephones using a ladder circuit structure which allowed the development of smaller portable phones
1998: Nokia releases 1st Dual Band mobile phone with two antennas hidden inside the stubby physical antenna to allow coverage of 850MHz and the new 1900MHz band
1998: Infineon develops a new RF filter technology - the “Bulk Acoustic Wave” (BAW)
these would be made in two main types, both of a capacitor type design with electrodes on either side of a very thin piezoelectric substrate (usually aluminium nitride or zinc oxide) the thickness of which determines the centre frequency of band pass: (along with the vastly increased number of cellular towers which allowed smaller antennas only 1-2” long sticking out of the phone)
Film Bulk-Acoustic-Resonator (FBAR) - uses a small air gap over a silicon substrate to help isolate signals
has better containment of electric fields cw SAW filters reducing cross-talk
less sensitive to contamination from surface particles as they are a layered system
much harder to make than SAW filters as layers need to be even and requires 9-13 masked layers
Solidly Mounted Resonator (SMR) - uses a Bragg acoustic reflector layer instead of the air gap but are not as good at high frequencies as FBARs
1999: Nokia produces 1st mobile phone without an external antenna, the Nokia 3210 (and added some basic games) - the two quarter wave antennas where two mainly flat pieces of metal making up part of the internal case - a Planar Inverted-F antenna (PIFA) - one of the compromises is that it increases electromagnetic radiation to your head so a metal shield was placed under this to mitigate it
2001: HP's new Agilent offshoot manufactures the 1st FBAR duplexer for bidirectional radio communication which is 1/5th the size of a ceramic duplexer
2004: Motorola releases its unique much thinner Razr V3 flip phone and the antenna was a short copper wire loop placed at the bottom of the phone and away from the head and not requiring the additional shielding of the Nokia
2005: Agilent sells its semiconductor business including its FBARs to Silver Lake which would become Avago
2007: original Apple iPhone released with a quad-band 2G radio using Infineon SAW filters
2008: Apple iPhone 3G released also used SAW filters which were also used in the iPhone 4 but the dual band (880-915Mhz and 1710-1785MHz) front end RF modules were made by SkyWorks
2008: Avago buys Infineon's BAW business to compliment its FBAR business and would become Broadcom
2009: Palm phones reduced the antenna sizes even further by printing them on - primary cellular one at bottom, two at top (GPS and WiFi) which would set the design for the subsequent phones until the iPhone 4 changed it all again
2010: Apple iPhone 4 revolutionizes the antenna by making the metal rim of the phone several antennas separated by a plastic spacer - there were issues with this which were remedied by the iPhone 4S and most subsequent phones adopted this methods although later models also incorporated internal printed ones as well to cater for the many bands which require over 30 antennas
2011: FBAR commercial development driven by the exploding multi-band 4G smartphone bandwidth requirements - Apple Store increases demand for internet bandwidth and the iPhone 5 is released and the 1st iPhone to use the 4G Long Term Evolution (LTE) mobile phone network standard with lower latency and greater bandwidth but required more bands to filter as global coverage required 43 bands to be filtered from 600MHz-3600MHz as well as WiFi and Bluetooth and this was quite costly requiring multiple SAW filters which took up space
2011: adding scandium to Al Nitride FBAR substrate improves performance but as of 2024 not yet optimised
2015: smartphones now mainly using FBARs to allow full spectrum coverage and keep size as small as possible
2023: development of improved SAW Resonating RF filters by placing reflectors of either side of the output IDT - “SAW Resonators”
2024: FBAR manufacturing has largely consolidated into a duopoly of Qorvo (China) and Broadcom (factory in Colorado, US), together they control 95% of the market