The iPhone Forever Changed the RF Filter

My thanks to an anonymous viewer of the channel for suggesting this topic. ❤ You know who you are.

As an electrical engineer, RF is, generally speaking, the blackest art in the whole discipline. The tech that goes into making the RF front-end bits in modern smartphones is on a whole other level, though. In addition to the craziness that is the filters you show here, I'd be fascinated to see your take on the incomprehensible antenna designs that somehow tune and work reasonably efficiently on six or more bands, or the power amplification and oscillator circuitry that operates clear through UHF and a big chunk of the microwave band.

My first engineering job was designing the "packages" for BAW resonators. Your ability to digest and present highly technical and niche information like this is bordering on savant-like.

Saddest thing is not a lot of people will realize the amount of effort that went into making a single RF filter, let alone the whole phone.

Thank you for not having background music

Thanks for shining a light into this little understood corner of the semiconductor industry. Just a small correction. When someone in this industry says that a part cost $1.40 when purchased in quantities of 50 million, that does not mean that you get 50 million parts for $1.40. That means that you still have to pay $1.40 per part as long as you buy 50 million of them. So 50 million parts will cost you about $70 million. It is definitely incorrect to say that you can fill up a bucket with these parts for less than the cost of the bucket. It will be easily hundreds of thousands of dollars to fill up that bucket. I do hope you make a correction just so your viewers do not start to mistakenly short RF semiconductor stocks.

I would never have guessed that wireless communication goes through an acoustic stage like this!

Really interesting and not too niche for me at least. I remember when SAW filters suddenly became common in TV IF stages - ISTR it was not so much about performance as cost. They replaced several LC circuits which all needed to be hand-tuned in production, saving on parts cost, board size and manufacturing time. The IF stage became small enough to fit inside the tuner can, avoiding the need to have additional shielding for a separate IF stage.

Maybe you could do a video on the history of antenna designs, ending with cellphones or satellites. I researched this a few years ago and was surprised to learn the size and shape of the antenna in the iPhone and how such antennas were developed.

This was my dad's field of expertise while working for GTE in the late 70's, after designing their 5 micron lithography library. He did a lot of SAW filter stuff for the military, including the radios used on the space shuttle. In particular, he told me how difficult it was to convince telecom companies back then to adopt any of the stuff they were making, including integrated circuits, because they were staunchly traditional and insisted on building their networks out of old-fashioned discrete components. Phone companies were always really bad at adopting new technologies, and for years, only the military was buying this stuff. Eventually, GTE made a partnership with the Canadian telecom company Mitel, and things started moving into the consumer sector, but it wasn't too much longer before they went into market decline and got sold. GTE was one of the cutting edge giants back in the day that hardly anyone knows about today. Amazing how far things have come, and how many companies (including Apple and Microsoft) totally missed the boat on the Internet revolution many decades ago.

You stopped so early! The story is so much more fun with OFDMA and MIMO thrown in.

Really well done! I should mention that a 40-band phone may not have 40 filters since the baseband bandwidth has also increased. Now, some filters may encompass multiple bands. Its a complicated trade off that considers RF interference, cost, complexity, regulations, and performance.

It's quite the coincidence that you posted this video today. T-Mobile US is about to decommission their GSM1900 network on April 4th 2024. Meaning you can no longer connect to a network with any phone made prior to 2015-ish, which absolutely breaks my heart. This was the last remaining operational 1g, 2g and 3g network in the United States, and the longest continuously running cell network in US history. Meaning that even an old Nokia 2190 from 1994 still connects to the network today and works just as it did in 1994 (so long as you have an older pre-5g sim card). I still enjoy using old phones, like the HTC Dream (Google G1) and old Nokia/Ericsson phones, but come mid next week, that's all gone forever. Truly sad....

I used lots of ceramic SAW filters for analog TV and radio receiver for IF frequency (455 kHz, 10.7 MHz, 45.75 MHz) from Murata. Before SAW filter, we used bulky ferite core coil in square metal can. One good thing about SAW filter, other than high-Q and stability, is it does not need center frequency adjustment, which need time-consuming manual labor. things of that old days with IF coils. But BAW is new to me. I didn't work for mobile RF.

Please help to clarify the following statement: "Nearly 15 years later, each filter cost between $1.40 to $1.60 when bought in volumes of 50 million units. You can fill a bucket with these filters for more than what the bucket cost." How much does the bucket cost? How many filters does it take to fill a bucket?

You can just say “saw” and “baw”. Everyone else does. Source: work in this specific industry. Also 50M parts for $1.40 is a whopper untruth. I think you confused bulk pricing per unit with order pricing. A product run capable of covering an entire generation of smart phone wouldn’t cover the cost of a single photomask for said filter if $1.40 bought 50 million units.

As a ham operator, I feel seen! Lol. Since a lot of hams experiment in the HF bands, they still tend to use coils and capacitors, but, as software defined radios allow small packages to transmit and receive higher frequencies, I'm quite sure that the latest ham radios that do VHF, UHF and higher, are making use of some of the filters in question. The Icom 905 radio allows for Satcom/GHz experimentation, so a bunch of the filters you mentioned will be in that equipment as well. Great video! Thanks for the coverage!

This brings back happy memories of my days in the piezoelectric technology department at Bell Labs in North Andover, MA in the 1980's and 1990's. Lucent eventually sold off the whole business to Vectron in New Hampshire. One of the things not mentioned in the video is why SAW and BAW filters have become less popular in mass-market consumer electronic devices these days. And that is because these RF filters don't integrate well with silicon integrated circuits and that increases cost. One is forced to go off-chip for part of the transceiver chain and return back on chip. Instead, low/zero-IF architectures are used. But these filters can offer very high selectivity because the inherent high-Q of quartz on which they are built in applications that require them. So they'll always have a place in electronics. Even cheap 32KHz watch crystals can keep accurate time to within seconds/year.

I used to build and tune microwave waveguide filters, from 1900MHz to 11GHz. Those filters are waaaaaay bigger than filters found in your cell phones. This topic was interesting to me, Thanks.

I had some RF classes in college (EE), but didn't end up specializing in that field. Even our RF prof often called it dark sorcery. This was a very fascinating and welcome trip down memory lane, great video!