How ceramics filters (quite big) disappeared from moderns mobile phones ? - Page 1
Forum section: electronics / rf-microwave

author: jujun
author_rank: Regular Contributor
timestamp: February 05, 2019, 12:57:11 pm
content: Hello,I just tear down an old Nokia, and found a lot of big interesting RF stuff in it, first it seem that there is a "big" diplexer made with ceramic coaxial TEM resonators.I really don't understand how in a modern smartphone you can avoid a low loss diplexer ? Maybe there is still one but very small ? but how is it possible ? is these RF parts are now inside a chip ? But how a low loss diplexer for a few watts can be that small ? And today with all the differents frequency used, it must need a lot more differents filer, so how is it possible ?Any pointers wellcomeThank you

author: OwO
author_rank: Super Contributor
timestamp: February 05, 2019, 01:06:22 pm
content: A quick search on mouser turns up this:https://www.mouser.com/ProductDetail/TDK/DPX250894DT-5025A1?qs=%2fha2pyFaduhhng04kCaFCQxphXig2dNGY8LJtxNYB9p9WJcRXHqAJJGOmBfTgYs%2fand many many more. It's only 2mm x 2.5mm.

author: jujun
author_rank: Regular Contributor
timestamp: February 05, 2019, 01:10:15 pm
content: Quote from: OwO on February 05, 2019, 01:06:22 pmIt's incredibly small, how dose it work ? In the datasheet they only say "Multilayer".

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 05, 2019, 01:35:45 pm
content: Simple. Analogue phones had simultaneous transmit and receive, istr about 40MHz apart, which is a pretty amazing feat-half a watt tx and -100bBm receive on the same anrenna.With digital. Tx and rx are time-multiplex so you don't need so much filtering

author: jujun
author_rank: Regular Contributor
timestamp: February 05, 2019, 02:40:56 pm
content: The one I was writing about is this one :https://en.wikipedia.org/wiki/Nokia_1610So not that "old". It's a 2G mobile phone. So it's exactly like a SIM900 GSM module for example.But filter are very big inside.

author: jujun
author_rank: Regular Contributor
timestamp: February 05, 2019, 06:34:04 pm
content: I just found this document with all technical details on how it works. There is a big part explaining all the RF stages.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: February 06, 2019, 11:56:17 am
content: The fact that it is no longer full duplex helps a lot. But there is the additional advantage of more powerfull ICs. I think in most modern smartphones, the RX LNA and TX PA stage will be on a separate chip (made in fancy-pants GaAs, SiGe, or even some InP variant) and then the rest happens on one chip. Technologies such as low-IF allow you to do a lot on-chip, and DSP allows us to move a lot of the filtering to on-chip stuff. Instead of a lot of RF, now we just have Antenna -> LNA -> Mixer -> polyphase filter -> ADC. The reason we needed those fancy filters is because when working at 1 GHz, and you need to filter out a 10 MHz channel, you need a very narrow relative bandwidth. This means you need very precise components (shifting the center frequency a few percent could mean your filter no longer covers the rightband -> Impossible on chip, and even hard with discrete without trimming). If you now convert to a 12 MHz carrier first, and then filter that 10 MHz at those frequencies, your filter can be made on chip using switchcap or similar.Keep in mind that this phone was made in 1995 or such. At the time, desktop PCs had 4 or 8 Mb of RAM. The CPUs ran at 33 MHz, so even your entire desktop can't keep up with the slow speed and DSP that goes on in a modern phone.Now building 100 megasample/s ADCs (That are good enough for comms, so 8 bits is already more than enough) is something almost trivial on nanometer CMOS.And as a final note: size is one of the reasons why the 5G folk want to go to higher frequencies. A 1 wavelength resonator at 900 MHz is many centimeters long - at 28 GHz it will be 30 times smaller.

author: djacobow
author_rank: Super Contributor
timestamp: February 06, 2019, 03:51:04 pm
content: Technology for small filters:Surface acoustic wave, bulk acoustic wave, film bulk acoustic resonator, etc. Very interesting technology and quite small.

author: radioactive
author_rank: Regular Contributor
timestamp: February 07, 2019, 04:43:34 am
content: Quote from: jujun on February 05, 2019, 06:34:04 pmI remember looking at / using similar filters back in the early 00's for 900 MHz. I don't think there were many choices that could perform that well with that kind of footprint beside the ceramic mono-block filters. Those specs you found indicate 4 Watts tx. I don't think you will find small saw filters that can handle that power (usually more like 1/4 Watt). Probably also came from the desire to filter both rx / tx in a single filter at the antenna.I haven't been tearing phones apart, so not sure what they look like now. Maybe they just use saw on the rx side and possibly low-level sections in the tx chain, then use a pi filter for the final tx amp to knock down the harmonics... would be my guess.Yes, those new saw filters for cell phones are very small. remove comment about switches, since phone is full duplex. (as mikeselectricstuff pointed out).