Google Groups no longer supports new Usenet posts or subscriptions. Historical content remains viewable.
Dismiss

Frequency Modulation

0 views
Skip to first unread message

Google_User

unread,
Jan 20, 2006, 9:19:08 AM1/20/06
to
Regarding FM, this is explained simply as:

"Thus the RF carrier wave and the input signal can't do much by
themselves, they must be modulated. That is the basis of our
transmitter. An example is useful to illustrate what is actually going
on. If we were to broadcast a 100MHz signal and tune a radio into that
frequency, we would hear nothing. That 100MHz signal has locked or
captured that spot and simply produces a DC value. Now if we were to
move the incoming signal +/-100KHz in either direction at a frequency
of 1000Hz, then we would hear a 1000Hz signal on the radio. If we only
moved +/-10Khz then the sound from the radio would be 1/10th the
original in loudness. Thus the rate or frequency at which we change the
RF carrier produces the audible frequency that we hear, and the further
from the main RF carrier we move, the louder the output will be. This
is the basis of all FM transmitters."

Is this a valid explanation? I thought the carrier frequency was only
allowed to swing up to +/- 15kHz?

Blue Cat

unread,
Jan 20, 2006, 9:36:23 AM1/20/06
to

"Google_User" <mr.wh...@gmail.com> wrote in message
news:1137766748.0...@g14g2000cwa.googlegroups.com...
There are two types of frequency modulation: Narrow band and wide band

Narrow band has limits on the swing of the carrier frequency so that more
channels can occupy a band. This is sufficient for voice communication. Wide
band takes advantage of FM's immunity to static and the ability to transmit
in high fidelity. If you use a wide band FM receiver to tune in a narrow
band signal, the audio will be very faint. Conversely, if you tune in a wide
band signal with a narrow band FM receiver, the audio will be severely
distorted.

What is being descibed in the last message is wide band FM.


Richard Fry

unread,
Jan 20, 2006, 12:07:04 PM1/20/06
to
"Google_User"

> Thus the rate or frequency at which we change the
> RF carrier produces the audible frequency that we hear, and the further
> from the main RF carrier we move, the louder the output will be. This
> is the basis of all FM transmitters."
>
> Is this a valid explanation? I thought the carrier frequency was only
> allowed to swing up to +/- 15kHz?
__________________

The explanation is correct.

Broadcast FM in the US and most/all other locations uses +/-75 kHz deviation
of the carrier frequency as "100%" modulation.

RF

0 new messages