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From: John Larkin on 14 Jul 2010 23:27 On Wed, 14 Jul 2010 09:46:09 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >On Jul 14, 12:49�am, "JosephKK"<quiettechb...(a)yahoo.com> wrote: >> On Mon, 12 Jul 2010 23:23:56 -0400, Phil Hobbs >> >> >> >> >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> >JosephKK wrote: >> >> On Mon, 12 Jul 2010 09:37:07 -0400, Phil Hobbs >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> >> >>> JosephKK wrote: >> >>>> On Fri, 9 Jul 2010 10:22:34 -0700 (PDT), j <jdc1...(a)gmail.com> wrote: >> >> >>>>> Resolution of noise vs frequency, (as in bw), is the issue in phase >> >>>>> noise measurements. �The OP never stated the offset from the carrier >> >>>>> nor bandwidth. �Or maybe I just missed it. >> >> >>>>> It�s not clear to me why JosephKK thinks this would be either a time >> >>>>> consuming or difficult measurement to make. �Assuming the appropriate >> >>>>> measurement system is in hand 100 dBc numbers are easily achievable. >> >>>>> Whether it�s 60 Hz or several GHz�s the global issues are the same in >> >>>>> making a phase noise measurement. >> >> >>>>> But having said the above, without the OP responding I guess it really >> >>>>> doesn�t matter. �But I�d like to know more about the application and >> >>>>> derive solutions from there. >> >> >>>> OK. �For a carrier of 60 MHz. �Pick an instrument or test setup of your >> >>>> choice, state the model[s]. �Clearly explain just what is going on in the >> >>>> measurement and the time it takes to accumulate sufficient data to make >> >>>> the measurement. �Explain why it takes that much time to reach a reliable >> >>>> measurement of -100 dBc phase noise at that carrier frequency. >> >> >>>> Now see how well it scales to one million times lower fundamental >> >>>> frequency without a similar scaling in measurement time. >> >>> It's the modulation frequency that's relevant, not the carrier >> >>> frequency. �Measurements get slower when you reduce the bandwidth. >> >> >>> (You can see why this doesn't work if you imagine running it >> >>> backwards--mixing or multiplying up to some very high frequency doesn't >> >>> allow you to make a measurement with 1 Hz bandwidth any faster. >> >> >>> Cheers >> >> >>> Phil Hobbs >> >> >> Now what is the equivalent bandwidth of -100 dBc for a 60 Hz carrier? >> >> Since you said 20 log() basis 60 * 10^-5 is 600 microHz. �That would have >> >> to take some minutes, and if you wanted a proper 10 to 1 measurement >> >> buffer, it takes ten times longer. �Call it 10,000 seconds? �A few hours. >> >> And the reference stability etc., i remarked on is coming into play. >> >> >You're confused, I'm afraid. �-100 dBc phase noise in a given bandwidth >> >(say 1 Hz, but it doesn't matter) is 7 microradians RMS. �Using a 5V >> >swing and a CMOS analogue gate as a phase detector, that's >> >> >dV = 7e-6 rad RMS * 5V/(pi rad) = 11 microvolts RMS, >> >> >which is trivial to measure in a 1 Hz bandwidth in a few seconds--it's >> >80 dB above the noise of a good op amp, so you just have to wait for the >> >filter to settle. >> >> >Cheers >> >> >Phil Hobbs >> >> OK. �I have your "Making it All Work" and AoE 2nd Ed and more. �Where do >> i go to get less confused? �This phase noise measurement is twisted.- Hide quoted text - >> >> - Show quoted text - > >Hi Joseph, I'm trying to get my head around this too. (I like Phil's >intro to section 13.6, "We live in a fallen world, so the signals we >process are never free of noise, distortion, and extraneous >interfering signals.")* > >I think it would help me if I understood how one measures the phase >noise. My simple minded approach would be to trigger my digital >'scope on the carrier zero crossing, and then look 'down stream' 100 >or 1,000 periods later and see how much 'jitter' there was in the >delayed zero crossing. Seems like there must be a better way. That's pretty much what the Allan Variance does, http://en.wikipedia.org/wiki/Allan_variance which is to characterize the jitter as a function of the time, or equivalently the number of skipped edges, between the edges you measure. Cheap crystal oscillators may have a few ps RMS between adjacent clock edges (aka cycle-to-cycle jitter) but may have many nanoseconds of jitter if you measure edges that are a second apart. You've got to be careful about the scope jitter, too. It may well be worse then the oscillator you're trying to measure. The "real" way to measure phase noise in the frequency domain is to get two of the things you want to test, set them to slightly different frequencies, and mix their outputs, then analyse the mixer output. That's about the only way to characterize really quiet sources. Of course you have to make sure they aren't injection locking or showing the same line-hum jitter or any other sneaky correlation. We have a couple of 10 MHz atomic clocks around here, one rubidium and one cesium. If you trigger a scope from one and look at the rising edge of the other, you could swear that the scope is triggered internally. At, say, 5 ns/div, it looks rock steady. Come back a half hour later and the trace has drifted a little left or right. That test is the time-domain equivalent of the mixer thing. That's how we test the timing system stuff we did for NIF: trigger a scope from one unit, observe the rising edge of another. John
From: Bob Masta on 15 Jul 2010 07:33 On Wed, 14 Jul 2010 09:46:09 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >*Does this mean there is no noise in heaven? (all R's have zero >temperature) All the Real terms are zero... heaven is purely Imaginary! <g> Best regards, Bob Masta DAQARTA v5.10 Data AcQuisition And Real-Time Analysis www.daqarta.com Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, FREE Signal Generator Pitch Track, Pitch-to-MIDI DaqMusic - FREE MUSIC, Forever! (Some assembly required) Science (and fun!) with your sound card!
From: George Herold on 15 Jul 2010 10:03 On Jul 14, 11:27 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 14 Jul 2010 09:46:09 -0700 (PDT), George Herold > > > > > > <gher...(a)teachspin.com> wrote: > >On Jul 14, 12:49 am, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > >> On Mon, 12 Jul 2010 23:23:56 -0400, Phil Hobbs > > >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > >> >JosephKK wrote: > >> >> On Mon, 12 Jul 2010 09:37:07 -0400, Phil Hobbs > >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > > >> >>> JosephKK wrote: > >> >>>> On Fri, 9 Jul 2010 10:22:34 -0700 (PDT), j <jdc1...(a)gmail.com> wrote: > > >> >>>>> Resolution of noise vs frequency, (as in bw), is the issue in phase > >> >>>>> noise measurements. The OP never stated the offset from the carrier > >> >>>>> nor bandwidth. Or maybe I just missed it. > > >> >>>>> Its not clear to me why JosephKK thinks this would be either a time > >> >>>>> consuming or difficult measurement to make. Assuming the appropriate > >> >>>>> measurement system is in hand 100 dBc numbers are easily achievable. > >> >>>>> Whether its 60 Hz or several GHzs the global issues are the same in > >> >>>>> making a phase noise measurement. > > >> >>>>> But having said the above, without the OP responding I guess it really > >> >>>>> doesnt matter. But Id like to know more about the application and > >> >>>>> derive solutions from there. > > >> >>>> OK. For a carrier of 60 MHz. Pick an instrument or test setup of your > >> >>>> choice, state the model[s]. Clearly explain just what is going on in the > >> >>>> measurement and the time it takes to accumulate sufficient data to make > >> >>>> the measurement. Explain why it takes that much time to reach a reliable > >> >>>> measurement of -100 dBc phase noise at that carrier frequency. > > >> >>>> Now see how well it scales to one million times lower fundamental > >> >>>> frequency without a similar scaling in measurement time. > >> >>> It's the modulation frequency that's relevant, not the carrier > >> >>> frequency. Measurements get slower when you reduce the bandwidth. > > >> >>> (You can see why this doesn't work if you imagine running it > >> >>> backwards--mixing or multiplying up to some very high frequency doesn't > >> >>> allow you to make a measurement with 1 Hz bandwidth any faster. > > >> >>> Cheers > > >> >>> Phil Hobbs > > >> >> Now what is the equivalent bandwidth of -100 dBc for a 60 Hz carrier? > >> >> Since you said 20 log() basis 60 * 10^-5 is 600 microHz. That would have > >> >> to take some minutes, and if you wanted a proper 10 to 1 measurement > >> >> buffer, it takes ten times longer. Call it 10,000 seconds? A few hours. > >> >> And the reference stability etc., i remarked on is coming into play.. > > >> >You're confused, I'm afraid. -100 dBc phase noise in a given bandwidth > >> >(say 1 Hz, but it doesn't matter) is 7 microradians RMS. Using a 5V > >> >swing and a CMOS analogue gate as a phase detector, that's > > >> >dV = 7e-6 rad RMS * 5V/(pi rad) = 11 microvolts RMS, > > >> >which is trivial to measure in a 1 Hz bandwidth in a few seconds--it's > >> >80 dB above the noise of a good op amp, so you just have to wait for the > >> >filter to settle. > > >> >Cheers > > >> >Phil Hobbs > > >> OK. I have your "Making it All Work" and AoE 2nd Ed and more. Where do > >> i go to get less confused? This phase noise measurement is twisted.- Hide quoted text - > > >> - Show quoted text - > > >Hi Joseph, I'm trying to get my head around this too. (I like Phil's > >intro to section 13.6, "We live in a fallen world, so the signals we > >process are never free of noise, distortion, and extraneous > >interfering signals.")* > > >I think it would help me if I understood how one measures the phase > >noise. My simple minded approach would be to trigger my digital > >'scope on the carrier zero crossing, and then look 'down stream' 100 > >or 1,000 periods later and see how much 'jitter' there was in the > >delayed zero crossing. Seems like there must be a better way. > > That's pretty much what the Allan Variance does, > > http://en.wikipedia.org/wiki/Allan_variance > > which is to characterize the jitter as a function of the time, or > equivalently the number of skipped edges, between the edges you > measure. Cheap crystal oscillators may have a few ps RMS between > adjacent clock edges (aka cycle-to-cycle jitter) but may have many > nanoseconds of jitter if you measure edges that are a second apart. > > You've got to be careful about the scope jitter, too. It may well be > worse then the oscillator you're trying to measure. > > The "real" way to measure phase noise in the frequency domain is to > get two of the things you want to test, set them to slightly different > frequencies, and mix their outputs, then analyse the mixer output. > That's about the only way to characterize really quiet sources. Of > course you have to make sure they aren't injection locking or showing > the same line-hum jitter or any other sneaky correlation. OK that sounds easy enough... you then have to add a third oscillator and measure each againts the other if you want to really know any individual bandwidth. > > We have a couple of 10 MHz atomic clocks around here, one rubidium and > one cesium. If you trigger a scope from one and look at the rising > edge of the other, you could swear that the scope is triggered > internally. At, say, 5 ns/div, it looks rock steady. Come back a half > hour later and the trace has drifted a little left or right. Fun! I did the same trick when comparing digital function generators. This cheap protek one had a 'hick up' every second or so and the transistion would slowly march across the screen. > > That test is the time-domain equivalent of the mixer thing. That's how > we test the timing system stuff we did for NIF: trigger a scope from > one unit, observe the rising edge of another. > > John- Hide quoted text - > > - Show quoted text - Thanks again John and Phil. George H.
From: John Larkin on 15 Jul 2010 10:17
On Thu, 15 Jul 2010 07:03:01 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >On Jul 14, 11:27�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 14 Jul 2010 09:46:09 -0700 (PDT), George Herold >> >> >> >> >> >> <gher...(a)teachspin.com> wrote: >> >On Jul 14, 12:49�am, "JosephKK"<quiettechb...(a)yahoo.com> wrote: >> >> On Mon, 12 Jul 2010 23:23:56 -0400, Phil Hobbs >> >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> >> >JosephKK wrote: >> >> >> On Mon, 12 Jul 2010 09:37:07 -0400, Phil Hobbs >> >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> >> >> >>> JosephKK wrote: >> >> >>>> On Fri, 9 Jul 2010 10:22:34 -0700 (PDT), j <jdc1...(a)gmail.com> wrote: >> >> >> >>>>> Resolution of noise vs frequency, (as in bw), is the issue in phase >> >> >>>>> noise measurements. �The OP never stated the offset from the carrier >> >> >>>>> nor bandwidth. �Or maybe I just missed it. >> >> >> >>>>> It�s not clear to me why JosephKK thinks this would be either a time >> >> >>>>> consuming or difficult measurement to make. �Assuming the appropriate >> >> >>>>> measurement system is in hand 100 dBc numbers are easily achievable. >> >> >>>>> Whether it�s 60 Hz or several GHz�s the global issues are the same in >> >> >>>>> making a phase noise measurement. >> >> >> >>>>> But having said the above, without the OP responding I guess it really >> >> >>>>> doesn�t matter. �But I�d like to know more about the application and >> >> >>>>> derive solutions from there. >> >> >> >>>> OK. �For a carrier of 60 MHz. �Pick an instrument or test setup of your >> >> >>>> choice, state the model[s]. �Clearly explain just what is going on in the >> >> >>>> measurement and the time it takes to accumulate sufficient data to make >> >> >>>> the measurement. �Explain why it takes that much time to reach a reliable >> >> >>>> measurement of -100 dBc phase noise at that carrier frequency. >> >> >> >>>> Now see how well it scales to one million times lower fundamental >> >> >>>> frequency without a similar scaling in measurement time. >> >> >>> It's the modulation frequency that's relevant, not the carrier >> >> >>> frequency. �Measurements get slower when you reduce the bandwidth. >> >> >> >>> (You can see why this doesn't work if you imagine running it >> >> >>> backwards--mixing or multiplying up to some very high frequency doesn't >> >> >>> allow you to make a measurement with 1 Hz bandwidth any faster. >> >> >> >>> Cheers >> >> >> >>> Phil Hobbs >> >> >> >> Now what is the equivalent bandwidth of -100 dBc for a 60 Hz carrier? >> >> >> Since you said 20 log() basis 60 * 10^-5 is 600 microHz. �That would have >> >> >> to take some minutes, and if you wanted a proper 10 to 1 measurement >> >> >> buffer, it takes ten times longer. �Call it 10,000 seconds? �A few hours. >> >> >> And the reference stability etc., i remarked on is coming into play. >> >> >> >You're confused, I'm afraid. �-100 dBc phase noise in a given bandwidth >> >> >(say 1 Hz, but it doesn't matter) is 7 microradians RMS. �Using a 5V >> >> >swing and a CMOS analogue gate as a phase detector, that's >> >> >> >dV = 7e-6 rad RMS * 5V/(pi rad) = 11 microvolts RMS, >> >> >> >which is trivial to measure in a 1 Hz bandwidth in a few seconds--it's >> >> >80 dB above the noise of a good op amp, so you just have to wait for the >> >> >filter to settle. >> >> >> >Cheers >> >> >> >Phil Hobbs >> >> >> OK. �I have your "Making it All Work" and AoE 2nd Ed and more. �Where do >> >> i go to get less confused? �This phase noise measurement is twisted.- Hide quoted text - >> >> >> - Show quoted text - >> >> >Hi Joseph, �I'm trying to get my head around this too. �(I like Phil's >> >intro to section 13.6, "We live in a fallen world, so the signals we >> >process are never free of noise, distortion, and extraneous >> >interfering signals.")* >> >> >I think it would help me if I understood how one measures the phase >> >noise. �My simple minded approach would be to trigger my digital >> >'scope on the carrier zero crossing, and then look 'down stream' 100 >> >or 1,000 periods later and see how much 'jitter' there was in the >> >delayed zero crossing. �Seems like there must be a better way. >> >> That's pretty much what the Allan Variance does, >> >> http://en.wikipedia.org/wiki/Allan_variance >> >> which is to characterize the jitter as a function of the time, or >> equivalently the number of skipped edges, between the edges you >> measure. Cheap crystal oscillators may have a few ps RMS between >> adjacent clock edges (aka cycle-to-cycle jitter) but may have many >> nanoseconds of jitter if you measure edges that are a second apart. >> >> You've got to be careful about the scope jitter, too. It may well be >> worse then the oscillator you're trying to measure. >> >> The "real" way to measure phase noise in the frequency domain is to >> get two of the things you want to test, set them to slightly different >> frequencies, and mix their outputs, then analyse the mixer output. >> That's about the only way to characterize really quiet sources. Of >> course you have to make sure they aren't injection locking or showing >> the same line-hum jitter or any other sneaky correlation. > >OK that sounds easy enough... you then have to add a third oscillator >and measure each againts the other if you want to really know any >individual bandwidth. Usually just dumping the mixer output into a spectrum analyzer is good enough. One assumes the units are identical. You can use three or more DUTs, in pairs, to demonstrate that. >> >> We have a couple of 10 MHz atomic clocks around here, one rubidium and >> one cesium. If you trigger a scope from one and look at the rising >> edge of the other, you could swear that the scope is triggered >> internally. At, say, 5 ns/div, it looks rock steady. Come back a half >> hour later and the trace has drifted a little left or right. > >Fun! I did the same trick when comparing digital function >generators. This cheap protek one had a 'hick up' every second or so >and the transistion would slowly march across the screen. One of my Tek scopes has a few ps of time shift that correlates to a blinking led on the sampling head. John |