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modulate a signal onto another

Well, there are a number of methods of modulation.

In the Analog sense, there are phase, frequency and amplitude (that is, the carrier is changed by the modulating signal in one of those aspects). For the analog model, the carrier (to be effective) must be a higher frequency than the greatest offset caused by the modulating signal. In AM, this is the band of frequencies of the modulating signal. For frequency modulation, it depends on the characteristics of the modulator. For phase, it can be very low, but should be at least the bandwidth of the modulating signal (as phase modulation has some AM characteristics).

In the digital world, other techniques apply. The addition to a PRBS sequence (noted above) is actually used commonly to maintain a (statistically) constant bandwidth on the channel regardless of whether there is a signal present or not. It might be argued by some that is not really modulation, but that's another story.

There are other forms of digital modulation ( 8bit - 10 bit conversion is common) if we define modulation as making the signal suitable for the transmission medium ( normally it may be defined as imposing a variation onto one signal based on another - the modulating signal).

In the digital arena, the final output rate must be at least equal to the symbol rate of the modulating signal plus any overhead rate. Note I use the term symbol rate, not bit rate. The symbol rate is sometimes known (accurately) as baud rate, whereas bit rate and buad rate are not synonymous except for bits per symbol = 1. To avoid the common confusion over the term, symbol rate is more commonly used.

So there's a whole world of things to learn here - I suggest googling for some definitions.

Cheers

PeteS

Reply to
PeteS
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If I understand your question, you are confucing modulation with simple mixing. In simple mixing the two signals are ADDED together and they can be easily separated by filtering. The signals do not interact or control each another.

In modulation, the two signals are MULTIPLIED together, not added. In this way the amplitude of one signal is controlled by the other signal. Modualtion causes the amplitude or envelope of the higher frequency signal to change with the level of the lower frequency signal. Modualted signals cannot be separated by filtering. This is AM modulation. Other parameters besides amplitude may be used to control a modulated signal such as frequency and phase, FM and PM.

There are no restrictions on what type of signals can modulate each other, sine waves, noise, audio, etc. However, there are practical restrictions on what makes good carriers. To avoid spurious signals, carries need to be clean sinewaves devoid of harmonics. Carriers are usually many times higher in frequency than the modualtion signals. Modulation always causes sum and difference frequencies that must be considered in any carrier system. Bob

Reply to
Bob Eldred

Hi all,

I have a question about modulating one signal onto another. Take for example a sine wave and an audio signal. They are easy to combine and separate. But is this also possible with a random noise signal and an audio wave. Or are there restrictions to what signals can and can not be used as a carrier wave for other signals. And if so how are they defined.

Thanks for any help on this.

Best regards, John

Reply to
John

In direct spread spectrum (DSS) a digitally encoded message is mixed with a pseudo-random sequence (noise) using an XOR gate. The XOR operation is reversible, and the intelligence is extracted at the receiving end by re-generating (and synchronising) an identical pseudo-random sequence, and XORing it with the signal. Without the correct correlated sequence to de-spread it, the transmission is just broadband noise.

Reply to
Andrew Holme

As mentioned by another respondent, there are no limits for simple linear mixing. Assuming you are talking about amplitude modulation, there are also no limits. For frequency modulation, of course, the carrier needs a frequency to be modulated. That pretty much rules out broadband noise as a carrier for FM, at least in the conventional modulation sense. You could have a pseudorandom noise generator whose rate was modulated, however. For narrowband noise, you can modulate the center frequency.

You might want to have a llook at my freeware DaqGen audio signal generator, which allows you to play around with various modulations and hear the results in real-time, as well as viewing the real-time waveform, spectra, and spectrograms. Besides standard waveforms, plus several types of noise sources, you can even load files of arbitrary waves and use them as modulation or carrier.

Let me know if you have any questions.

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

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Home of DaqGen, the FREEWARE signal generator

Reply to
Bob Masta

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