# modulate a signal onto another

Discussion in 'Electronic Basics' started by Andrew Holme, May 21, 2005.

1. ### Andrew HolmeGuest

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

2. ### PeteSGuest

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

3. ### Bob EldredGuest

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

4. ### JohnGuest

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

5. ### Bob MastaGuest

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