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Comment by xt00 | Hacker News Reader

Only in WiFi bands.. so not super flexible as you would expect from an actual SDR rig.. or maybe I’m missing that it can actually transmit in other bands ?

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RF compliance is really complicated in mobile devices, so there's all kinds of filters to prevent out of band emissions from being generated. The analog parts like the amplifiers are also precisely tuned to only use certain bands.

This particular hardware may be limited to those frequencies but I believe the Mediatek combo chips that do WiFi+BT+GPS+GLONASS+Beidou+FM as well as GSM are capable of a much wider range. WiFi and BT are at 2.4GHz but GSM/UMTS/LTE reach up to 2.6GHz and down to 800-900MHz, and the various navigation systems use frequencies in the 1.x GHz range. From what I've read, only the FM hardware is different due to its vastly different frequency (76-108MHz, although the hardware could probably go beyond those ranges slightly) but everything else is based on the same SDR.

Which chips are those? Most of those solutions I had seen were not actually a single output stage but rather each band had dedicated pins that went out to dedicated PA's in many cases. So wifi/BT has its own antenna and front end etc.. same for the other bands..

Start on page 34 of this:

http://www.datasheetspdf.com/datasheet/download.php?id=73999...

The combo chip itself probably has multiple SDRs internally so that e.g. you can use GPS and WiFi/BT simultaneously, but they can be connected externally to the same antenna through a diplexer.

Is this a limitation of the antenna, or the hardware itself?

Probably both. The antenna's designed for a specific band, and on top of it the actual frequency synthesis hardware is likely only designed to be capable of generating signals in certain bands, as are all the amplifiers etc.

Yes, Wi-Fi bands only.

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