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CQCQ
Joined: 26 Nov 2006
Posts: 49
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 18 Aug 2008 14:38 Why use dBc/Hz |
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| Why use dBc/Hz? Thanks in advanced!
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biff44
Joined: 24 Dec 2004
Posts: 1295
Helped: 169
Location: New England, USA
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| 18 Aug 2008 16:42 Re: Why use dBc/Hz |
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| Because it is more impressive on a data sheet than dBc/MHz!
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madengr
Joined: 06 Apr 2005
Posts: 388
Helped: 81
Location: Kansas City
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| 18 Aug 2008 18:25 Why use dBc/Hz |
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| 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.
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wilwal
Joined: 19 Mar 2008
Posts: 11
Helped: 1
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| 19 Aug 2008 14:03 Re: Why use dBc/Hz |
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The units of dBc/Hz refer to dB below the carrier measured in
a 1-Hz bandwidth.
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CQCQ
Joined: 26 Nov 2006
Posts: 49
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| 20 Aug 2008 15:28 Re: Why use dBc/Hz |
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| wilwal wrote: |
The units of dBc/Hz refer to dB below the carrier measured in
a 1-Hz bandwidth. |
Why in 1-Hz bandwidth.
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LvW
Joined: 07 May 2008
Posts: 774
Helped: 148
Location: Germany
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| 20 Aug 2008 15:52 Re: Why use dBc/Hz |
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[quote="CQCQ"]
| wilwal wrote: |
Why in 1-Hz bandwidth. |
Itīs 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.
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Eugen_E
Joined: 29 Nov 2004
Posts: 326
Helped: 30
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| 20 Aug 2008 15:59 Re: Why use dBc/Hz |
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The noise is described in frequency domain by its spectral density, expressed in watt/hertz:
http://en.wikipedia.org/wiki/Noise_spectral_density
http://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.
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