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

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 <csteinmetz@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. >