Thank you Charles for the detailed answer. Any good texts on the subject that you can recommend?
At the moment I have the books written by Motchenbacher and Henry Ott. NIST papers are another obvious source, but sometimes it is a little hard to fill in, or understand, the details without adequate pre-knowledge/experience. Regards, Stephan. On 6 March 2015 at 23:01, Charles Steinmetz <csteinm...@yandex.com> wrote: > Stephan wrote: > > Opamp noise is usually specified in its datasheets as input voltage noise, >> V/sqrt(Hz), and input current noise, A/sqrt(Hz) versus frequency, Hz. >> > > Actually, those are the voltage and current noise *densities*. I'm not > trying to be gratuitously picky, it's just that casual designers' > understanding of noise, and their noise calculations, often come to grief > because of just this kind of confusion. > > Is it possible to estimate the opamp's phase noise at a specific frequency >> (say a sinusoid at 10MHz or 100MHz) from these curves? >> > > No, not really, because: > > I'm assuming it can be seen as amplitude noise that is converted to phase >> noise. >> > > Correct. Some of the output noise of the amplifier is converted to phase > noise "directly" -- meaning, any instantaneous measurement of the sine wave > has an uncertainty in both amplitude and time due to the added noise. BUT > this is not the main AM to PM conversion problem. > > The input noise (typically, but not exclusively at baseband rather than at > the RF frequency) also modulates the operating parameters of the amplifier > itself (typically, but not exclusively by modulating internal device > capacitances), causing the amplifier's frequency response at RF to be > modulated in synchronism with the noise -- which modulates the phase of the > RF being amplified or processed by the amplifier. This process is > different for every amplifier topology, so there is no way to calculate > phase noise from the amplifier noise specifications without much, much more > information (specifically, a very detailed model of the amplifier based on > very detailed models of its component parts). It needs to be measured. > [There may be nonlinear modeling software capable of making a first > approximation, but I'm not aware of any.] > > Since the AM to PM conversion is most troublesome at baseband, PN is > reduced by using devices with low noise at low frequencies (especially > flicker or 1/f noise) and by making the internal amplifier gain low at low > frequencies, for example by shunting low frequencies to ground with > inductors where possible. > > Which in turn make it seem to me that it is dependent on the slope >> of the zero-crossing. Meaning it is dependent on signal amplitude and >> frequency? >> > > Not relevant. > > Best regards, > > Charles > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/ > mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.