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# Why do we use dBc/Hz as measurement unit?

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#### CQCQ

##### Member level 3
Why use dBc/Hz? Thanks in advanced!

#### biff44

##### Advanced Member level 6
dbc hz

Because it is more impressive on a data sheet than dBc/MHz!

##### Full Member level 6
dbc/hz

Because when you are measuring the power of any signal besides a perfect CW tone you have to specify the bandwidth it's measured in. If you were comparing two CW tones it would be dBc. Comparing a tone to noise must be dBc/Hz. Also, it's in dB because it's easier to add than multiply. If you want to know the thermal noise power in a 1 kHz bandwidth is simply -174 dBm/Hz + 30 dBHz = -144 dBm. That's a simple example but when you have to do a link budget with all sorts of parameters you'll see why it's easier to take logarithms and scribble the answer out on paper.

ferdem

Points: 2

### ferdem

Points: 2

#### wilwal

##### Newbie level 6
dbm/hz dbc/hz

The units of dBc/Hz refer to dB below the carrier measured in
a 1-Hz bandwidth.

#### CQCQ

##### Member level 3
dbc add spectrum log db

wilwal said:
The units of dBc/Hz refer to dB below the carrier measured in
a 1-Hz bandwidth.
Why in 1-Hz bandwidth.

#### LvW

##### Advanced Member level 6
what is dbc

CQCQ said:
wilwal said:
Why in 1-Hz bandwidth.

It´s not a natural law - that means you also could specify/measure it, for example, in a 3.5-Hz bandwidth. However, would this make sense ?
If you normalize to a 1-Hz bandwidth you have only to multiply with the actual bandwidth to get the effective noise level referenced to the carrier.

#### Eugen_E

##### Full Member level 6
calculate dbc

The noise is described in frequency domain by its spectral density, expressed in watt/hertz:
https://en.wikipedia.org/wiki/Noise_spectral_density
https://en.wikipedia.org/wiki/Spectral_density

So if you need to get noise power, in a specific bandwidth, you should integrate it over frequency (calculate the spectrum area inside bandwidth).
However, spectral density vs frequency is not constant (except for white noise), so u need to know the power in a small bandwith, the smaller the better, 1Hz is appropriate. Thus u can know precisely the spectrum shape and can calculate power in a specific bandwidth.

Because spectrum is not flat, for oscillators the noise is given as dBc/Hz at x KHz offset from central frequency. An unit as dB/MHz is usefull only if the noise power is quite constant in a 1MHz bandwidth, egg thermal noise, but not for an oscillator.

#### srikanth_cvs

##### Newbie level 2
Re: Why use dBc/Hz

How to measure the noise/spur power in spectrum analyser in dBm/MHz??

#### tony_lth

##### Advanced Member level 5
Re: Why use dBc/Hz

Hi, srikanth_cvs, why you want to use this unit? I am just out of curious.

#### lisamoni

##### Junior Member level 2
Re: Why use dBc/Hz

what kind of frequency analysar i can use?can i download the software from net?please,can u give a link?please

#### tony_lth

##### Advanced Member level 5
Re: Why use dBc/Hz

you can use ADS, it can calculate phase noise which unit is dBc/Hz.

##### Newbie level 6
Re: Why use dBc/Hz

that means power in db on a carrier frequency in a 1 hz bandwidth.

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