vborich
Junior Member
Offline
Posts: 23
|
There is widespread confusion regarding this topic.
Technically, phase noise is short for Spectral density of phase noise. You'll commonly see this denoted as S_phi(f) in technical literature. On the other hand, designers usually refer to the noise-to-carrier ratio Lambda(f) simply as "phase noise", even though this quantity measures the total noise power at offset f and therefore includes the contribution, however small, of amplitude noise as well.
In the small-angle approximation, and assuming negligible amplitude noise, S_phi(f) is twice the noise-to-carrier ratio at small offsets or, equivalently, 3 dB HIGHER.
Because the noise power in the noise-to-carrier ratio definition is measured in a single sideband, and because S_phi(f) approximatelly equals twice the single sideband noise-to-carrier ratio, and because the noise-to-carrier ratio is usually referred to as "phase noise", S_phi(f) has come to be known as "double-sideband phase noise".
So Eldo-RF is indeed correct.
Your simulator may in fact compute just one quantity and simply scale the other by 3 dB. Or it could take a more rigorous approach and compute both of them directly. It's easy to verify: Sweep your analysis over a broad range until you hit the noise floor, and then plot them on the same graph. You'll see ~3dB difference near carrier and then the plots will slowly diverge as you approach the knee.
Vuk
|