Why do we use dBc/Hz as measurement unit?

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

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.

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

Why in 1-Hz bandwidth.

[quote="CQCQ"]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.

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.

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

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

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

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

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