Re: [Discuss-gnuradio] LPF in FM receiver

[Marcus Müller]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hi Aditya, Hi Pablo,

the multiples-of-carrier-frequency problem is being solved in hardware
(you can't do this in software, since mixing is still analog domain).

Anyway, as Aditya explained, since you are processing baseband
signals, a lowpass is actually a bandpass (you might at least think of
it as such) from -f_c to +f_c.
You need to do this, because the FM receiver should only deal with the
signal in the interesting band.

Greetings,
Marcus


On 11.02.2014 17:35, Aditya Dhananjay wrote:
> To continue: It is always easier to process signals in baseband
> (centered around 0) than it is to process RF signals (due to the
> high sampling rates required).
> 
> Given that the bandwidth you are interested in is in the order of
> 100kHz (typical of FM radio signals), it is easy to see why radio
> receivers are designed the way they are.
> 
> 
> On Tue, Feb 11, 2014 at 11:31 AM, Aditya Dhananjay
> <address@hidden>wrote:
> 
>> 
>> 
>>> In the FM receiver: Why a LPF is used instead of a BPF in order
>>> to select the Radio channel that we want to hear? That is the
>>> only part I don't understand.
>>> 
>>> 
>> Hello Pablo,
>> 
>> Let's say you are trying to receive an FM signal centered around
>> 100MHz. In other words, the signal of interest is from
>> (100-delta)MHz through (100+delta)MHz.
>> 
>> My understanding is that at the receiver, you are first
>> demodulating the signal by multiplying the incoming signal with
>> the carrier frequency of 100MHz. This leads to two bands:
>> 
>> a) Centered around 0: (-delta through +delta)
>> 
>> b) Centered around 200MHz: (200-delta) Mhz through (200+delta)
>> MHz.
>> 
>> Signal (a) is what you are interested in, while signal (b) needs
>> to be eliminated. This is where the LPF comes in.
>> 
>> Best, Aditya
>> 
>> 
> 
> 
> 
> _______________________________________________ Discuss-gnuradio
> mailing list address@hidden 
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1
Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/

iQEcBAEBAgAGBQJS+lWUAAoJEAFxB7BbsDrLr8oH/iTpkA8ohIKeifYY75LuOPPs
B8dZn4vo2c/6lSUL7ucM9l/pD215nrGytBClVPOq2QJHvHwQ5O+BWNVXH/Fm7sRn
iB0rb1eYsbnqZnABZyG3K7TE34KoAmmrTm6IIW7fH6Gk7qGODYH8mtaqbKcLna7G
bXbWh7J3woNJpYClq/VfEJubh8trb0DKfKzFzgIqwJBy42QOqO1Cca9szB/psl7t
cb/fqUKUe6UrfQnmQcCCkN4DCXeixd7Olx4Zju/o9PvsBmiup63AfKeEh5aHbCbG
h7jOMhG95g5g2VjchCF4xi5TNPxYrj+uXvFrb8LwH5eZWmceipNuUJVASGCJXKk=
=Y11I
-----END PGP SIGNATURE-----