[SOLVED] Why don't two signals of different frequencies interfere?

why don't two signals of different frequencies interfere(i know that in frequency domain they poses different band in spectrum).but in time domain it seems they interfere

thank u

Re: signal interference

can you tell me what kind of signals are you using and which frequencies?

regards

Re: signal interference

i mean in general, say in communication persons allocated different frequencies there is no intereference

They do.

At any point in space (such as a receiver aerial) the instantaneous strength of the EM field is the sum of the fields of all the impinging EM waves at that point in time. In a way, all the waves 'modulate' the EM field at that point. At each instant in time, the signal induced into the aerial is that sum.

That's OK; all of the frequencies making up the sum are still present in the resulting signal and can be extracted from that signal. The job of a receiver is to select which frequency it wants to use; by effectively filtering out all others with a resonant circuit that responds to that frequency.

Compare with a sound signal, with many notes played at once. A microphone picks up the total amplitude of the vibration which is made by the combination of each note's vibration at each instant in time. Each different note can be extracted individually again from the signal with a filter; they are all still present in the signal even though they 'interfere' at the microphone.

FoxyRick, explained perfectly

And of course they only "don't interfere" if the assumption
that it's a linear system, is actually true. Which it never is
entirely, except in books. Usually you have to respect and
choose components / limits that make it "true enough".

signals having frequency interfere except in theoretical and a very few rare cases that is if the system is linear and all

But most of the time there is a large level of interference when signals of different frequencies travel together

thank you guys. Can i know how a filter is able to extract a particular band of frequencies though there is interference from neighboring(other) signals of different frequencies.


thank u

The term for the ability to 'tune in' to the required signal, and reject others, is 'selectivity'.

In simple terms, an LC (inductor-capacitor) circuit is made. These circuits have a resonant frequency, that is, a frequency at which they oscillate strongly. Just like a tuning fork oscillates at a particular frequency when struck. In the LC circuit's case, it is the current oscillating backwards and forwards through it. The LC circuit is adjusted to oscillate at the wanted frequency; this is done by turning the tuning control on an old-style radio and can adjust either the variable capacitor or, less usually, the variable inductor.

The LC circuit is connected to the antenna, which responds to a wide band of frequencies even though it might itself be 'tuned' to a certain range.

The LC circuit oscillates most strongly at its resonant frequency, and less strongly the further away from this. So, the antenna's signal will have the required frequency amplified, and others diminished, in the output signal. The output of this goes to whatever the next stage is (for instance an RF amplifier; the LC might be built as part of the RF amp). So, we now have a signal that is stronger in the wanted frequency range, and weaker in the others, getting weaker the further away we get from the tuned frequency. Just like with a tuning fork where you cannot even hear anything but the designed frequency.

The earliest receivers, crystal radios, used a single LC circuit, connected through a diode (the 'detector', actually just a rectifier that removed one half of the modulation envelope) to headphones. Note that a wide range of frequencies is still present even after the LC circuit, but the unwanted frequencies (away from the LC's tuned frequency) are very much weaker than the resonant frequency. The headphones themselves (and ever one's ears) act as a low-pass filter so that what we hear is just the audio modulation on the signal.

A single LC circuit is not very selective though, and as stations get closer together, it is not good enough to separate them.

In a very old design of receiver, the Tuned Radio Frequency method would have several of these LC/Amp stages to further refine the signal in the wanted frequency at each stage. However, each stage needs retuning when a new frequency is wanted. That's not convenient, so other methods are used now after a single selectivity stage. Some such methods are Direct Conversion and Super-heterodyne. In any case though, the same principles of resonance are applied in several stages to continually improve selectivity.

so u are able to extract your signal without any problem posed by other signal of different frequency if u have a filter which is tuned to your signal's frequency ?


thank you

- - - Updated - - -

so u are able to extract your signal without any problem posed by other signal of different frequency if u have a filter which is tuned to your signal's frequency ?


thank you

Basically, yes. Provided that the transmissions are not so close together that the receiver cannot separate them because its circuits have too wide a bandwidth.

Transmissions are usually 'channelised' - meaning each transmitted signal is separated from its neighbours by a fixed amount. That amount is calculated according to the bandwidth required for the type of transmissions. For instance, narrow-band FM voice channels might have a 25 KHz separation to allow for the audio range of modulation. The centre frequency of a channel will be modulated either side of that centre by the voice signal that is superimposed onto it, and can deviate by up to 12.5KHz either side without interfering with the adjacent channel.

If the selectivity of the LC/RF-Amp cannot be made narrow enough, then special filters can be obtained such as ceramic or crystal filters. These work on the same principle of resonance, but have a much narrower range of frequency (narrower bandwidth) and can be obtained down to a KHz bandwidth or so. These are used in SSB reception, for example, where narrow and accurate filtering is required to obtain a usable signal free from interference.

Edit to add: Just to be sure you understand - bandwidth refers to how wide a range of frequencies are 'acceptable', outside of which the frequencies are considered to be attenuated. Quantitatively it would be given as dB point, for instance the distance from the centre frequency at which the signal is attenuated to -3dB. All such circuits will have a bandwidth around their centre, operating frequency. That is fortunate, otherwise the modulation of the signal (which causes the frequency to shift above and below the centre) could not pass through the circuit.

Although many good thing have been said about practical aspects of signal selection, I don't agre with the principle answer that's been given on the original question by saying any signals of different frequency will interfer, see e.g. post #4. The answer uses interference in a too broad sense, respectively confuses it with intermodulation and similar non-linear effects.

IMHO the common technical term "interference" is well characterized in the below quoted Wikipedia article:

Interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency.

Click to expand...

https://en.wikipedia.org/wiki/Interference_(wave_propagation)

if i am right,so what does it signifies signals having different frequencies doesn't cause any problem or distortion(interfere) with other one (because u r able to extract required signal using filter as mentioned) so back 2 my question why does this happen?

thank you

superposition principle

Hi, laldee18!
Clear ur mind from the confusions u have and understand what I tell u now.
Mr. FvM, added the quote correctly and also others explained it well, but, I will like to add some explanation to it and try to give u the whole missing picture.
From the famous superposition principle, we can see that depending on the amplitudes and phases of various signals, they can get added or subtracted forming a new different signal. The best example of this is Fourier Series which shows signals can be expressed as weighted sum of sinusoids having different frequencies and phases.
Consider two signals having same frequency. Using superposition they will add/subtract depending on their phases. But, here the thing is since they have the same frequency u will not be able to separate these using a filter as filter recognises signals based on their frequencies. In this way, these two different signals having same frequencies "Interfere" with each making us unable to find which signal is which at the receiver's end.
Now, assume new two signals having different frequencies, the case of ur question. Here, as stated above, a new waveform will be formed again based on superposition principle.
And as Mr. Foxyrick explained we can separate these signals using a filter and tuning to the perfect required frequency. These can be separated since the filter can recognise these as different entities.
In this way signals with different frequencies don't interfere while signals with same frequencies do.
I hope U will understand now.