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# What does "dBc/Hz" mean? Help newbie please :(

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#### knack

##### Member level 2
how to calculate dbc/hz

Hi..
It's the first post on this excellent forum!! I hope to get help from you proffessionals!
In PLL VCO specs, noise is represented by the unit: dBc/Hz..
What does it mean?? Where did it come from, i.e. its origin??
Any tutorials about PLL blocks intrensic noise?? My big dream now is to get clear detailed explaination for it!
Any tutorials about noise in general??

I hope that it's the correct place to post.

-- Knack

#### eternal_nan

##### Full Member level 3
what is does dbc mean

It is the power at a given frequency compared (by taking ratio) with the power at the 'carrier' frequency. In PLL applications carrier means the frequency at which you 'want' the VCO to run.

### knack

Points: 2

#### knack

##### Member level 2
origin of -174 dbm/hz

eternal_nan said:
It is the power at a given frequency compared (by taking ratio) with the power at the 'carrier' frequency. In PLL applications carrier means the frequency at which you 'want' the VCO to run.

Carrier=>VCO freq. , then, what's the stated "given frequency"?

Also, if it's a measurement at certain frequency, i couldn't totally understand why should this add in units the ".../Hz" !!

Thnx again,
--Knack

#### engr_najam

##### Full Member level 2
sometimes in filters it is known as the the fractional loss in power compared to some reference power with an increase in frequency

#### eternal_nan

##### Full Member level 3
It is any frequency in the spectrum *except* for the carrier frequency. Usually you will get a gaussian looking waveform centered on the carrier, that means that most of the power of the signal is concentrated around the VCO frequency. You get the dBc/Hz figure by taking the value at adjacent frequencies and dividing by value at VCO frequency.

### knack

Points: 2

#### safwatonline

it is Power Spectral density , i.e. u need to integrate on freq. range to get Power and hence the term /Hz

### knack

Points: 2

#### wonbef

##### Full Member level 3
it is the unit of Power Spectral density.

### knack

Points: 2

#### knack

##### Member level 2
Thank you all for replies!!

to check ma self, is dBc= VCO power(in dBm) - Adjacent freq or 1st harmonic power (in dBm) ??

If so, this's easy to calculate, but how to calculate for it to be dBc/Hz ??

e.g. assume center freq. power=20dBm, and the first adjacent component power=-50dBm, then do I have -70dBc difference between them?? and/or I have -70dBc/Hz ........ (don't know what to say about it?).

Thank u very much,

Cheers

--Knack