Question on Spread Spectrum

[scottieman]

Hi All

If this topic doesn't suit this specify forum, do let me know where which I should post.

My question is, will I observe the modulating signal tone in the spectrum.
For example, I have a 100MHz signal, which is modulated by a 32kHz signal. We know the 100MHz signal is then being modulated to have spread spectrum.
How about the 32kHz modulating signal. Will it also appear in the spectrum?
I am thinking the 32kHz modulating signal may have higher signal level than the fundamental tone of that modulated 100MHz signal. If this is the case, it looks like spread the spectrum does not help as we do introduce another lower frequency tone in the spectrum.

Thanks for reading on such simple question as I am too green on this topic.

 

[Mityan]

Will it also appear in the spectrum? - Sure it will
Your modulating and fundamental tone levels are related to "modulation depth" term.
If you mean spectrum spreading by the phase manipulation, then your excessive modulation depth will distort the signal and optimal filter/correlator for spread spectrum will not work, I think
But why do you need any extra modulation (that 32 kHz) on DSSS?

 

[zorro]

My question is, will I observe the modulating signal tone in the spectrum.
...
How about the 32kHz modulating signal. Will it also appear in the spectrum?

It shouldn't appear. it is not part of the modulated signal.
It can be present at the output because the modulator is not ideal, but it is easily removed by passband filtering, leaving only the part of interest (modulated signal).
Regards

Z

 

[fpgaengineer]

I think you will have to "demodulate" it again, like dejittering is applyid in radar technique, where phase instabilities are measuered and later compensated.

 

[scottieman]

Will it also appear in the spectrum? - Sure it will
Your modulating and fundamental tone levels are related to "modulation depth" term.
If you mean spectrum spreading by the phase manipulation, then your excessive modulation depth will distort the signal and optimal filter/correlator for spread spectrum will not work, I think
But why do you need any extra modulation (that 32 kHz) on DSSS?

Thanks Mityan
Actually my application is not in communication field and I have no idea of what DSSS really means, sorry
In my app, I have a PWM signal and I want to reduce the signal power at the fundamental tone and other harmonics. That's why I come up an idea of spread spectrum.
Then I am curious will the modulating signal (e.g. 32kHz here) appear in the spectrum too.

- - - Updated - - -

Thanks Z
I did think of the same as you do...
analogue is mixing up a signal with DC to bandwidth B by a very high frequency carrier, we should not see the signal at the spectrum from DC to B frequency.
Just I am not good at those signal processing an so seek help here