1
$\begingroup$

As I understand, the RF frequency-modulated signal centered in the range of $88-108\textrm{ MHz}$, is picked by the earphone antenna and is directly sampled by the sound card adc at $48\textrm{ kHz}$ and then digitally processed. Will this process not cause aliasing? Could anyone please correct my understanding?

$\endgroup$
  • 1
    $\begingroup$ Your question is based on a misunderstanding. The RF signal is not directly sampled by the sound card. $\endgroup$ – Marcus Müller Jun 29 '17 at 8:36
  • $\begingroup$ by the way, capitalization makes a difference. m = milli = $10^{-3}$; M = Mega = $10^6$. Also, Hertz is abbreviated Hz, not hz. $\endgroup$ – Marcus Müller Jun 29 '17 at 8:38
  • 1
    $\begingroup$ (the earphone cable just doubles as an antenna for a dedicated FM receiver; the soundcard hasn't got anything to do with FM reception. In fact, it doesn't even "get" radio frequencies – they're filtered out before and fed to the FM receive chain) $\endgroup$ – Marcus Müller Jun 29 '17 at 8:43
  • $\begingroup$ @MarcusMüller This is a fine answer, why not post below so this one is not left open? I fixed the title. $\endgroup$ – Dan Boschen Jun 29 '17 at 14:54
  • $\begingroup$ @DanBoschen If you say so :-) $\endgroup$ – Marcus Müller Jun 29 '17 at 16:46
2
$\begingroup$

Your question is based on a misunderstanding. The RF signal is not directly sampled by the sound card.

Instead, the earphone cable just doubles as an antenna for a dedicated FM receiver; the soundcard hasn't got anything to do with FM reception.
In fact, it doesn't even "get" radio frequencies – they're filtered out before and fed to the FM receive chain.

$\endgroup$
  • $\begingroup$ @DanBoschen Antenna design was never that easy :-) Just grasp a pair of earphone cables, lay it over somewhere and connect to your FM receiver front end. Now I really wonder how can that shapeless thing can even help in FM reception? It seems there's a high probability of achieving a flat piece of 15 cms along the cable, or it seems there is not even such a requirement on the antenna design, which returns to my inital statement therefore $\endgroup$ – Fat32 Jun 29 '17 at 16:59
  • $\begingroup$ well, they're certainly crappy antennas. But: compared to the demands of cellular data, the requirements of FM radio are very benign, so I guess even with an extremely bad antenna efficiency, when you've got the technology to receive 3G / 4G, you'll do well enough. I also guess that for ~100 MHz signals (which isn't all that far from the baseband bandwidth of 4G), on-chip adjustable matching networks $\endgroup$ – Marcus Müller Jun 29 '17 at 17:15
  • $\begingroup$ well in my mind was the picture of an old sony walkman indeed, but probably the same would apply for 4G reception as well. $\endgroup$ – Fat32 Jun 29 '17 at 17:31
  • $\begingroup$ @Fat32 : AM antennas in pocket radios are even more tiny and shapeless, and have no problem gathering enough EM. As long as the antenna gathers less noise than broadcast RF, a reasonable S/N signal can be amplified with an LNA. $\endgroup$ – hotpaw2 Jun 29 '17 at 20:33
  • 1
    $\begingroup$ @hotpaw2 well, most small antennas are magnetic antennas, essentially coils wound around a ferrite core; they're pretty high-resonance (high-Q). $\endgroup$ – Marcus Müller Jun 29 '17 at 21:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.