see also:

• radio receivers
• software defined radio (SDR) receivers
• radio antennas
• radio basics
• electromagnetic interference (EMI)

• the environment is full of complex and mixed radio wave transmissions and this all adds to noise when you are trying to receive signals on one frequency band
• radio frequency filters (RF filters) have become critical components of most of our communication devices, particularly radios and mobile phones
• there are many types of RF filters, each with their own characteristics
• RF filters have blocking actions within the range 100kHz and 10GHz (higher frequency filters are microwave filters)
• an RF filter receives a radio input and processes it with those “pass band frequencies” being sent to output hopefully with minimal change (although there will always be some losses which are termed “insertion loss”), whilst blocking other frequencies to as close as possible to zero output
• blocked frequencies are usually measured as those in which the output is -3dB or lower compared to the input
• band pass RF filters are measured by:
  • their Quality Factor (Q factor) which is the central frequency / bandpass width ie. the smaller the bandwidth passed the better quality factor of the filter
  • their insertion loss (the reduction in db of the desired band pass frequencies)
• main types by function are:
  • band pass - pass frequencies at a particularly central frequency and adjacent frequencies (the “pass band”) but block everything else outside this bandwidth
  • high pass - pass frequencies above a particular frequency
  • low pass - pass frequencies below a particular frequency
  • notch filters - do the opposite of a band pass - ie. they block a specific frequency range such as AM radio band or FM radio band - this is very handy if there are strong transmitters nearby which would otherwise overload your receiver resulting in many artefactual signals and creating excessive noise
• main types by technology
  • Surface Acoustic Wave (SAW) filter - usually only effective below 2GHz
  • Bulk Acoustic Wave (BAW) filters such as FBAR - used in 4G LTE and in 5G smartphones
  • older designs:
    • coil and capacitor based filters
    • ceramic filters

• AM block filters
  • eg. Nooelec Flamingo AM
• FM block filters
  • eg. Nooelec Flamingo FM

• eg. RTL-SDR high pass filter

• *Airband filter
• etc

• sources: https://www.youtube.com/watch?v=L8jmHtfVmPY
• 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
• 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: 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:
    • 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
• 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
• 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
• 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