[time-nuts] Opamp datasheet noise specs and their relation to phase noise
Hi All, Opamp noise is usually specified in it's datasheets as input voltage noise, V/sqrt(Hz), and input current noise, A/sqrt(Hz) versus frequency, Hz. Is it possible to estimate the opamp's phase noise at a specific frequency (say a sinusoid at 10MHz or 100MHz) from these curves? I'm assuming it can be seen as amplitude noise that is converted to phase noise. 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? I've seen some earlier posts where opamp phase noise were measured (thanks Bruce, thanks Ulrich) and found to be remarkably good. Another question that could be asked is: What about the phase noise of those Mini-Circuits RF amplifiers that are only specified by noise figure (NF), dB? Can anyone point me to some literature that solves this problem? Many thanks, Stephan. ___ 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.
Re: [time-nuts] AVAR <-> S_Y conversion
On 03/05/2015 07:23 PM, Attila Kinali wrote: > Servus! Servus :) > On Thu, 05 Mar 2015 14:35:51 +0100 > Wolfgang Wallner wrote: > >> For the random walk noise the expected line is off by a factor of >> exactly 2 from the calculated plot, and I don't know how to explain this >> behavior. > > I'm probably the wrong one to answer, as I have never done any noise > simulation or even read up the relevant papers, but... > A factor of 2 sounds like the difference you would get between one sided > and two sided noise PSD's. > I calculate the one-sided PSD of the FFD data as described in [1] (first paragraph), so the code looks like this: xdft = fft(x); xdft = xdft(1:N/2+1); psdx = (1/(Fs*N)) * abs(xdft).^2; psdx(2:end-1) = 2*psdx(2:end-1); Remark: Before calculating the PSD, I split the data into parts of equal size, calculate the PSD for each one, and average over the set of PSDs. This improves the graphical visualization a lot. As the result matches my expectation exactly for 4 different kinds of noise, I would have assumed that this PSD calculation approach is quite reasonable. As I see the unexpected behavior only with random walk noise, and the main difference in the calculation is the term A, I would suspect that it has something to do with it. However, I'm a novice in this field, so any hint is very appreciated. regards, Wolfgang [1] http://de.mathworks.com/help/signal/ug/psd-estimate-using-fft.html ___ 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.
Re: [time-nuts] 50 ohm driver
I got excited when I found the SN74AS1004AD which has the exact same function and pin out but delivers 48 mA per channel. While one output did give me 0-3.0 V into 50 ohm, combining them, as we do for the 74AC, actually produced worse results. Can you post a link to the datasheet for your product, so we have some idea of what you're attempting? The AS devices are bipolar logic (fancy modern schottky TTL) -- they don't even pretend to pull closer than 1.5v to VCC (so, 3.5v with Vcc = 5v), and the spec is only >2v. The datasheet clearly indicates this (if one didn't already know that they were bipolar parts from the AS series designation). When you are depending on the analog performance of logic gates, it is up to you to pay attention to everything on the datasheet, not just one spec. Any ringing on the line (from mistermination) will very likely destroy the AC14s if you run them with no series resistors. At least build a discrete CMOS inverter stage with medium power, low-threshold MOSFETS. 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] How does NTP's local clock estimation work in detail?
Moin, I am a little bit stuck here. I am trying to work out the math behind a synchronization of clocks problem. It shouldn't be too difficult, but every couple of minutes I'm getting stuck at some details and it takes me always a lot of time to get around it. I'm pretty sure that what I am doing is similar to what NTP does locally after it got an estimate on it's time difference. I tried to look up what NTP does but I got lost in the huge amount of papers/presentations. Could someone point me to some papers that explain what NTP does locally? If possible in compact form. Thanks in advance Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ 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.
Re: [time-nuts] AVAR <-> S_Y conversion
Wolfgang, I have checked several sources, and they match up with the IEEE 1139 in this regard. I have also evaluated the equation for Allan variance for the random walk noise, and it matches up with the references and what I put here: https://en.wikipedia.org/wiki/Allan_variance#Power-law_noise So, the A formula you have matches up. You will need to find another source of the mismatch. Cheers, Magnus On 03/06/2015 11:04 AM, Wolfgang Wallner wrote: On 03/05/2015 07:23 PM, Attila Kinali wrote: Servus! Servus :) On Thu, 05 Mar 2015 14:35:51 +0100 Wolfgang Wallner wrote: For the random walk noise the expected line is off by a factor of exactly 2 from the calculated plot, and I don't know how to explain this behavior. I'm probably the wrong one to answer, as I have never done any noise simulation or even read up the relevant papers, but... A factor of 2 sounds like the difference you would get between one sided and two sided noise PSD's. I calculate the one-sided PSD of the FFD data as described in [1] (first paragraph), so the code looks like this: xdft = fft(x); xdft = xdft(1:N/2+1); psdx = (1/(Fs*N)) * abs(xdft).^2; psdx(2:end-1) = 2*psdx(2:end-1); Remark: Before calculating the PSD, I split the data into parts of equal size, calculate the PSD for each one, and average over the set of PSDs. This improves the graphical visualization a lot. As the result matches my expectation exactly for 4 different kinds of noise, I would have assumed that this PSD calculation approach is quite reasonable. As I see the unexpected behavior only with random walk noise, and the main difference in the calculation is the term A, I would suspect that it has something to do with it. However, I'm a novice in this field, so any hint is very appreciated. regards, Wolfgang [1] http://de.mathworks.com/help/signal/ug/psd-estimate-using-fft.html ___ 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.
Re: [time-nuts] How does NTP's local clock estimation work in detail?
Why not look at Poul-Henning Kamp's excellent rework: ntimed, http://phk.freebsd.dk/time/20140926.html He rethinks and reworks the client part of ntp, And makes a very approachable series of explanations at his site. On Fri, Mar 6, 2015 at 11:23 AM, Attila Kinali wrote: > Moin, > > I am a little bit stuck here. I am trying to work out the math behind > a synchronization of clocks problem. It shouldn't be too difficult, > but every couple of minutes I'm getting stuck at some details and it > takes me always a lot of time to get around it. I'm pretty sure that > what I am doing is similar to what NTP does locally after it got an > estimate on it's time difference. I tried to look up what NTP does > but I got lost in the huge amount of papers/presentations. > > Could someone point me to some papers that explain what NTP > does locally? If possible in compact form. > > Thanks in advance > > Attila Kinali > > -- > It is upon moral qualities that a society is ultimately founded. All > the prosperity and technological sophistication in the world is of no > use without that foundation. > -- Miss Matheson, The Diamond Age, Neil Stephenson > ___ > 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.
Re: [time-nuts] Opamp datasheet noise specs and their relation to phase noise
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.
