Thread: modulation of wireless siginals, why isn't this a problem?

Signal modulation can be achieved by varying the frequency of a signal right? According to the formula $\displaystyle f=\frac{v}{\lambda}=\frac{3 * 10^8m/s}{\lambda}$ as the velocity changes so does the frequency. If the wavelength of a wireless signal passes from one medium to another (say air to wall) wouldn't this change the velocity and consequently change the frequency and mess up the modulation?

Originally Posted by superdude

Signal modulation can be achieved by varying the frequency of a signal right? According to the formula $\displaystyle f=\frac{v}{λ}$ as the velocity changes so does the frequency. If the wavelength of a wireless signal passes from one medium to another (say air to wall) wouldn't this change the velocity and consequently change the frequency and mess up the modulation?

Be more specific in future, I presume you are talking about sound. Your formula appears to be that for a Doppler shift and is wrong: $\displaystyle v/c$ is (approximately) the fractional frequency shift, as such the modulation is also shifted but if the modulation bandwidth is small compared to the carrier (and/or the $\displaystyle v \ll c$) this can be negligible. Which you may or may not have to compensate for depending of the bandwidth, type of modulation and what you are trying to do.


CB

I came across the concept of modulation while reading about computer networks. It was my understanding that modulation works by varying a single aspect of a wave (e.g. frequency) such that every variation has a corresponding meaning.
If a wave changes medium then it's frequency changes, correct? If a wave changes from traveling from air to a wall, wouldn't that change it's frequency and therefore corrupt the data it represents? Are you saying that the change in frequency would be insignificant?

Originally Posted by superdude

I came across the concept of modulation while reading about computer networks. It was my understanding that modulation works by varying a single aspect of a wave (e.g. frequency) such that every variation has a corresponding meaning.
If a wave changes medium then it's frequency changes, correct? If a wave changes from traveling from air to a wall, wouldn't that change it's frequency and therefore corrupt the data it represents? Are you saying that the change in frequency would be insignificant?

First you don't usually have a antenna in a wall, it is in free space, and passing from free space through a wall and back into free space results in the original signal being restored.

The other thing is that the change in the signal between mediums and or Doppler is predictable and hence can be compensated for.

Also if the fractional frequency shift is small compared to the carrier the change can be negligible for most purposes.

Nothing is corrupted, the signal undrgoes a transformation process through which the original can be recovered (if it is not ignorable, or self corrected).



CB

Can anyone recommend books or articles on modulation?

@CB where did the number 955 come from? I thought wireless signals are essentially electromagnetic waves so that's why I used the speed of light.

Originally Posted by superdude

Can anyone recommend books or articles on modulation?

@CB where did the number 955 come from? I thought wireless signals are essentially electromagnetic waves so that's why I used the speed of light.

It looks like the speed of sound in some medium and some units, though I don't recognise them, it is slightly too low to be the speed of sound in air in ft/s at room temperature. It might be a combination of the speed of light or sound and the carrier frequency, ... Go back to your source for this formula and see what it says about that constant.

CB

Originally Posted by CaptainBlack

It looks like the speed of sound in some medium and some units, though I don't recognise them, it is slightly too low to be the speed of sound in air in ft/s at room temperature. It might be a combination of the speed of light or sound and the carrier frequency, ... Go back to your source for this formula and see what it says about that constant.

CB

That is not what I had originally wrote, I thought a mod (presumably you) changed it.
$\displaystyle f=\frac{v}{\lambda}=\frac{3 * 10^8m/s}{\lambda}$

When characteristics of a signals wave are given, do they normally refer to the carrier wave? For example if a manual says the wavelength is 10cm would that be referring to the carrier wave or the actual data's wave.

Originally Posted by superdude

When characteristics of a signals wave are given, do they normally refer to the carrier wave? For example if a manual says the wavelength is 10cm would that be referring to the carrier wave or the actual data's wave.

Always the carrier

CB

Originally Posted by superdude

That is not what I had originally wrote, I thought a mod (presumably you) changed it.
$\displaystyle f=\frac{v}{\lambda}=\frac{3 * 10^8m/s}{\lambda}$

I never touched it, and the post history shows no record of any edit before your latest. In that formula relating speed of light frequency and wavelength it is the wave length that changes not the frequency when moving from one medium to another.

CB