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The answer seems obvious (frequencies restricted by chips) but I'm looking for a clear and accurate explanation. Several examples of SDR projects connecting an SDR dongle to a smartphone are available on the web, such as here or here etc. Thing is, the SDR dongle is not required to demodulate, otherwise I could not listen to my favorite FM station using nothing more than my headphones as an antenna (without mentioning the actual GSM/4G... bands), as I've done myself. Another webpage explores the possibility of converting smartphones for radio communications, while suggesting the smartphone limitations:

"Despite the progress, the smartphones still could not communicate with handheld devices running the frequencies because the phones support different bands"

A short SO post indeed suggests the frequencies supported by a smartphone are very limited by the chips within it:

" a device manufacturer has those, and none of those will have anything to do with changing the frequency, as those are fixed by the chipset manufacture"

Therefore I ask the question: what exactly limits the possibilities of using my smartphone as an SDR, for example with my headphones as an antenna, to listen to other frequencies than FM radio stations (typically CB, VHF 144-146 MHz, PMR 446 MHz) ? Would the setup theoretically work but not well enough ? What would limit the demodulation capabilities of my smartphone, when not helped by an SDR dongle ? What are the limits exactly ? There is a lot of information available on SDR online but I'm still a little confused, hence my question here (not sure if this belongs here or should rather be posted on SO/signal processing stack exchange Q&A).

Blupon
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    A lot will depend on the specific phone/chipset. SDR is generally something that really's on chip specific setups, and that requires someone to have done the work to work out what is possible for a specific chipset in a specific configuration, and then get the data to the SDR application. Most dedicated low cost SDR dongles are built around a very small number of chipset/config setups where this work that has already been done. A phone may use a different chipset, or may use it in a different config. – user1937198 May 03 '21 at 01:40
  • https://ham.stackexchange.com – Rodrigo de Azevedo May 03 '21 at 01:41
  • To some up: A phone might or might not be able to use built in FM capabilities for SDR, but the biggest limiter is having the knowledge of what its capable of, and how to use it. – user1937198 May 03 '21 at 01:44
  • I agree with @user1937198 For Nokia Lumia series smart phone with windows OS, even FM was not available initially due to lack of support from OS. Later, the support from OS was added during an OS update. So it might just be a matter of the manufacturer not finding it worth the effort to test, certify and mantain the software and hardware combination. – AJN May 03 '21 at 01:45
  • The dongle is the digital radio that extends the bandwidth and interface on USB , what the smart phone cannot do without. – Tony Stewart EE75 May 03 '21 at 02:08
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    Ignoring the bandwidth and frequencies cellphones support, their radios are also designed to very, very efficiently send and receive a handful of protocols using highly specialized hardware. There would be no reason to include SDR functionality on the chip, so your options would probably be very limited even if you had complete documentation and could arbitrarily change mixer settings, etc. – user1850479 May 03 '21 at 04:33
  • @user1850479 The Wifi/Cellular modem on Qualcomm SoCs is implemented using proprietary SDR on a DSP. So the fuctionality is there on a different protocol on the same chipsets, so if they used the same approach for FM radio, then the capability would be there. Getting access to that information outside Qualcomm or a major phone manufacturer is unlikely. – user1937198 May 03 '21 at 10:39
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    Apart from the radio chip itself, the phone will use an on-board antenna and filters picked for certain frequencies. – Lundin May 03 '21 at 11:21

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