communications questions?

[psi]

I have a few questions regarding communication systems...

What is the difference between narrowband and baseband signals?
Any URLS that can explain the difference between the two?


If I have a x(t) = 5 in an oscilloscope I will probably see a straight line on the aplitude 5. How would this look like in a spectrum analyzer... I am trying to understand the concept of DC and how it would like in oscilloscope and in spectrum analyzer.

Thank you!!

 

[flatulent]

An ideal spectrum analyzer would show a vertical line at zero frequency for a constant voltage. Real world ones cannot display down to zero frequency.

In general, base band signals extend from DC up. The output of a logic gate would produce such signals. USB and RS232 are such signals.

Narrow band signals result from modulating a carrier with a base band signal.

 

[Moss]

Narrow band is signal wich ratio bettwen his highest and lowest frequency is small( bellow 1000). For example human voice have lowest spectral components about 100Hz and highest about 5-6Khz, and baseband composite video signal has lowest harmonics on 25Hz and highest above 5Mhz. Voice is narowband ( ratio is bellow 100) and TV signal is broadband ( ratio is 200 000) But if the TV signal is modilated on carrier 500MHz it is become narowband ( ratio 505/500).
Baseband in general means that the lowest freqency is shifted to the zero.

 

[psi]

hmmm.. seems like there are too many definitions.

I read somewhere that narrowband signals are not modulated.. and baseband signals are modulated signals..

 

[Moss]

It depends on modulation used. But baseband is not modulated signal. You have to read some book about radiocommunications or probably (and harder;-)) somthing like Signals and system" from Allan Oppenheim. Search in ebook section. You can also look at t stanford EE website- www-ee.stanford.edu/class_directory.html There is a lot of information.

 

[Element7k]

I will like to add:

Baseband in communications systems in general is the band of frequencies used to represent the original signal to deliver a source of information. In digital communications, it can take the form of discrete pulse as described by flatulent. For example see:

**broken link removed**

Narrowband in general is when the bandwidth of the modulated signal is small compared to its midband frequency. Many research papers has been written to argue what is actually considered to be narrowband. But I am in agreement with moss.

Element7k

 

[flatulent]

more thoughts

After thinking about this, I want to add to my previous information. Baseband is contrasted with bandpass. Bandpass is the modulated signal.

Narrowband is a relative term. Narrowband interference is narrower than the signal you want to get. Narrowband FM is a modulation index that produces just one set of sidebands.

 

[Element7k]

I remember a good, unambiguous definition and illustration to these questions in the manual of System View. Not used it for years so don't know where its collecting dust. Maybe you can find it in other forums.

Element7k