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  • EEVblog #23 – GSM mobile phone audio design

    Posted on August 6th, 2009 EEVblog 21 comments

    GSM mobile phone audio design, RF immunity, and low noise dead bug point-to-point prototyping.

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    21 responses to “EEVblog #23 – GSM mobile phone audio design” RSS icon

    • The techniques Maxim used in the design and layout of their chip is a work of Art that they should share with everybody. It is very easy to reverse engineer ICs, so they can’t keep it a secret. They should use the opportunity to display their know how. What you are showing is what customers are finding out in their labs all over the world. Thanks for putting it on the web.

    • You should really switch to another web hoster… I’m currently downloading your mp4-file at a rate between 80 and 100 kbytes/s…

      • 100KB/s isn’t too bad I suspect, especially if everyone is trying to download at once.
        I’m currently using my personal web host (shared) which I use for about half a dozen different web sites. But I suspect this is not going to work in the long term.
        I’m trial uploading to Blip.TV at the moment (eevblog.blip.tv), so we’ll see how that all goes.
        Still trying to figure out this whole video sharing/blogging/podcasting/iTunes thing!
        Thanks
        Dave.

    • @Marvin Really? Im getting 435KB/s from the east coast of the USA.

    • Eevblog #23 was a good blog, some of us do not design with RF. I had a product that I was part of the international design team for (we all telecommuted, I think the biggest representation of countries in the design team was 2 in the USA.) We were designing a mobile EFTPOS printer, we built it from scratch. We had direct control over the printer mech, IRDA charging of batteries, Blue Tooth, Serial.

      Every part of the design was by a separate person, one designed the printer controller, one the comms manager, another the blue tooth etc. I designed the battery charger (based on an Atmel AVR Tiny 26) it worked great on my work bench,(apart from a few exploding batteries, but that was a problem with the cheap China batteries we tried ;) but in the final product it failed as did some of the other areas, after a few weeks, we traced it to the POS engine, it was a GSM Tx Rx module, with all the smarts, card reader, display and keypad.

      We tracked it down to the POS engine having a higher than normal Tx power level, it was swamping everything, to the point of coming in through the silicon of the Uc’s.

      In the end we had to put RF traps on every conductor that left the printer and a silver coating on the inside of the case.

      Regards,

      Kat.

    • I’ve done some dead bug/manhattan style prototyping for some small RF blocks, but what really got me was that I never thought about each P-N junction being a small diode, rectifying the RF…

      Another good blog post, keep it up!

    • Great post, thanks! One question. Why would you use two capacitors in parallel like in your LC filter? Wouldn’t they behave like a 43pF capacitor (except for parasitic resistivity/inductance)?

      I’d also like to share a link to an article with some related information. It also has some of the diagrams you drew:

      http://www.infoelectronica.ro/date_upload/lucrari/498768(perturbatii%20in%20telefonie%20GSM).pdf?PHPSESSID=4f3ea4a78f00c79ea31cf1e8fdaaf54e

    • Martin,

      It is common to use multiple caps in parallel as each cap will have a reactance at different frequencies, it is not uncommon in Digital circuits to see a 100nf, 10nf and 1nf in parallel on each power pin of a VLSI chip.

      Kat.

    • Kat, I don’t quite follow your reasoning about reactance and frequency.

      But I see now how it may be beneficial to use several caps in order to lower the total parasitic inductance to ground and thus get better attenuation at frequencies where the parasitic inductance is dominant.

      However, I did a simulation for this particular circuit using typical parasitics for 0402 caps and it seems like using 33//10 instead of 43 puts an extra pole very near 1800MHz. Leading to a ~20dB decrease in attenuation at that frequency. That seems bad since this is one of the bands that the filter should block effectively.

      The beneficial effects of distributing the capacitance doesn’t seem to kick in until above 2GHz.

      Maybe I have it backwards, you and other people here are certainly more knowledgeable than me. The simulation schematics and results are here:

      http://imgur.com/3L1wK.gif

    • Another great blog… RF interference certainly raises it’s ugly head everywhere. We have just been upgrading MATV systems, and have come up with interference to AM radio reception. We tracked the fault down to a cheap Chinese plug pack generating all sorts of crap. This plug pack was supplied with our RF amplifier (Kingray DW42) & had the compliant C-Tick on the case of the plugpack. It makes me wonder if it really is compliant, or just bogus. We solved the problem by using a linear plug pack. Also I noticed that linear plug packs are being gradually discontinued, in favour of the switching type, which will apease the greenies, but fill the ether with RF noise. Cheers for now,keep the blogs coming…

    • What brand is your phone ?

      Looks nice !

    • I’m sure you will like this post from a fellow EEblogger:
      http://www.bunniestudios.com/blog/?p=446

      He is a really good writer, and I always enjoy his posts.

    • I enjoy the blog.

      Two things I wanted to comment on, though I am not sure if anyone will notice new comments on an old entry.

      I, like Martin, am not convinced of using multiple bypass capacitors by default – without analyzing it for the particular case in hand. Take a look at this short paper: “Effectiveness of Multiple Decoupling Capacitors”, Clayton R. Paul, IEEE Transactions on Electromagnetic Compatibility, Vol 34, No. 2, May 1992. I am sorry I cannot post it.

      I noticed you used solder wick to connect the bottom layer to the top. I used to do the same thing. (I’m a beginner, so ‘used to’ here refers to just two or three boards.) Solder wick is flat, wide, etc.; better than using a single wire I used to think. And I used to cut traces on the top board like you did. Lately I am realizing that I don’t really know if that serves as a proper ground plane by default. Especially after having read these articles recently:
      http://www.cvel.clemson.edu/emc/index.html

      Thank you for the blog.

    • Great video,

      I had a GSM RF scare years ago. My expensive tube computer monitor (yes many many years ago) was out of its warranty period and the screen was flickering every now and then. I was thinking that I would soon have to replace it. What it turned out to be was a poor selection of a cell phone charging dock placement. :)

    • Talking of using multiple bypass caps, here’s a current AppNotes from Intersil on the subject…

      http://www.intersil.com/data/an/an1325.pdf

      See pages 5 & 6, discusses also the effects of surface mount package sizes on ESL.

    • I’d like to see a demo of you soldering those BGA chips – it would save me a lot of time making prototype boards!

      Those things seem impossibly tiny for that.

    • i want whole information(with ckt diagram) of chip desianing

    • i want to make a mobile phone so i want information of each & every component

    • Thanks for your post about phone..I got a lot of info from this post.

    • Paul Chisholm

      Hi,
      Thanks for the EEVblog.
      This is a little late.
      My experience of the GSM buzz / Bumble bee noise problem is similar to yours that a series inductor then parallel capacitor doesn’t attenuate the RF much.
      I have had success with a PI configuration filter with a parallel capacitor then a series inductor (surface mount ferrite bead) then another parallel capacitor on all PCB interfaces.
      I believe that the antenna impedance is similar to that of a typical surface mount ferrite bead so the series element doesn’t attenuate much without being preceeded by a capacitor to the ground plane.
      My favourite Audio Rectification technique tip comes from the Audio Handbook by National Semiconductor dated June 1976. It lists the various common sense tips but the last is pray.

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