John Larkin and low jitter clocks
in [Design]
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From: Vladimir Vassilevsky on 30 May 2010 09:53 Stephan Goldstein wrote: > On Sat, 29 May 2010 00:37:13 -0500, Vladimir Vassilevsky > <nospam(a)nowhere.com> wrote: > > >> >>Glenn Gundlach wrote: >> >> >>>There is a guy on DIY audio stating that jitter in SPDIF digital audio >>>causes distortions. OK. He also claims to have jitter in the 100 fs >>>area. Is he blowing smoke about the jitter or can he really achieve >>>sub pico second jitter? >> >>Sub-picosecond jitter is non-trivial. >> >>Jitter = Noise/Slew_rate >> >>So, for the reasonable slew rates ~1e9 V/s the noise should be ~1uV. >>This requires narrow band high frequency design. >> >> >>>It seems to me that jitter in that system is >>>similar to Time Base errors in video systems. In analog video 1-2 nS >>>was achieved in both analog and digital Time Base Correction (TBC) >>>systems but I'm having problems getting my head around 100 fS and - is >>>it REALLY audible? >> >> >>S/(THD + N) impairment = Jitter x Frequency x 2Pi/sqrt(3) >> >>For 20kHz and 120dB, jitter should be ~ 10 pS. There is absolutely no >>point in pushing into femtosecond area. >> >> > > A coworker has been concerned about jitter-induced degradation in > a data acquisition system and has been trying to find a simple > equation like this one. Can you give me a reference I can pass > along to him? He's an analog-circuits guy like myself, and not > terribly familiar with the digital literature. I can't recall any book from the top of my head; however it is not difficult to do the numbers. Jitter is a small angle phase modulation of the signal; therefore the effect of jitter is essentially the same as the effect of the AM. That creates noise sidebands on both sides. The rms amplitude of the sidebands is proportional to the signal frequency. Now integrate the noise through the bandwidth of interest and you have the answer. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Stephan Goldstein on 30 May 2010 10:01 On Sun, 30 May 2010 08:53:10 -0500, Vladimir Vassilevsky <nospam(a)nowhere.com> wrote: > > >Stephan Goldstein wrote: > >> On Sat, 29 May 2010 00:37:13 -0500, Vladimir Vassilevsky >> <nospam(a)nowhere.com> wrote: >> >> >>> >>>Glenn Gundlach wrote: >>> >>> >>>>There is a guy on DIY audio stating that jitter in SPDIF digital audio >>>>causes distortions. OK. He also claims to have jitter in the 100 fs >>>>area. Is he blowing smoke about the jitter or can he really achieve >>>>sub pico second jitter? >>> >>>Sub-picosecond jitter is non-trivial. >>> >>>Jitter = Noise/Slew_rate >>> >>>So, for the reasonable slew rates ~1e9 V/s the noise should be ~1uV. >>>This requires narrow band high frequency design. >>> >>> >>>>It seems to me that jitter in that system is >>>>similar to Time Base errors in video systems. In analog video 1-2 nS >>>>was achieved in both analog and digital Time Base Correction (TBC) >>>>systems but I'm having problems getting my head around 100 fS and - is >>>>it REALLY audible? >>> >>> >>>S/(THD + N) impairment = Jitter x Frequency x 2Pi/sqrt(3) >>> >>>For 20kHz and 120dB, jitter should be ~ 10 pS. There is absolutely no >>>point in pushing into femtosecond area. >>> >>> >> >> A coworker has been concerned about jitter-induced degradation in >> a data acquisition system and has been trying to find a simple >> equation like this one. Can you give me a reference I can pass >> along to him? He's an analog-circuits guy like myself, and not >> terribly familiar with the digital literature. > >I can't recall any book from the top of my head; however it is not >difficult to do the numbers. Jitter is a small angle phase modulation >of the signal; therefore the effect of jitter is essentially the same as >the effect of the AM. That creates noise sidebands on both sides. The >rms amplitude of the sidebands is proportional to the signal frequency. >Now integrate the noise through the bandwidth of interest and you have >the answer. > > >Vladimir Vassilevsky >DSP and Mixed Signal Design Consultant >http://www.abvolt.com That's exactly the approach he took, but numerically using the simulators. He didn't try simple calculus... Thank you. Steve
From: Jeroen Belleman on 30 May 2010 10:39 On 05/30/2010 03:53 PM, Vladimir Vassilevsky wrote: > > > I can't recall any book from the top of my head; however it is not > difficult to do the numbers. Jitter is a small angle phase modulation of > the signal; therefore the effect of jitter is essentially the same as > the effect of the AM. That creates noise sidebands on both sides. The > rms amplitude of the sidebands is proportional to the signal frequency. > Now integrate the noise through the bandwidth of interest and you have > the answer. I found a nice treatment for this in Microwaves & RF, no less. Here is the reference: B. Drakhlis, Calculate oscillator jitter by using phase noise analysis � Part 1, Microwaves & RF, Vol. 50, No. 1, January 2001, pp. 82�90, 157. Presumably the article should be floating on the web somewhere. Failing that, I summarily reproduced it in a course on AD conversion I gave some time ago. The course text is at: <http://cdsweb.cern.ch/record/1213280/files/p281.pdf> Jeroen Belleman
From: John O'Flaherty on 30 May 2010 12:36 On Sat, 29 May 2010 20:18:30 -0400, Stephan Goldstein <sgoldHAM(a)alum.mit.edu> wrote: >On Sat, 29 May 2010 00:37:13 -0500, Vladimir Vassilevsky ><nospam(a)nowhere.com> wrote: > >> >> >>Glenn Gundlach wrote: >> >>> There is a guy on DIY audio stating that jitter in SPDIF digital audio >>> causes distortions. OK. He also claims to have jitter in the 100 fs >>> area. Is he blowing smoke about the jitter or can he really achieve >>> sub pico second jitter? >> >>Sub-picosecond jitter is non-trivial. >> >>Jitter = Noise/Slew_rate >> >>So, for the reasonable slew rates ~1e9 V/s the noise should be ~1uV. >>This requires narrow band high frequency design. >> >>> It seems to me that jitter in that system is >>> similar to Time Base errors in video systems. In analog video 1-2 nS >>> was achieved in both analog and digital Time Base Correction (TBC) >>> systems but I'm having problems getting my head around 100 fS and - is >>> it REALLY audible? >> >> >>S/(THD + N) impairment = Jitter x Frequency x 2Pi/sqrt(3) >> >>For 20kHz and 120dB, jitter should be ~ 10 pS. There is absolutely no >>point in pushing into femtosecond area. >> >> >>Vladimir Vassilevsky >>DSP and Mixed Signal Design Consultant >>http://www.abvolt.com > >A coworker has been concerned about jitter-induced degradation in >a data acquisition system and has been trying to find a simple >equation like this one. Can you give me a reference I can pass >along to him? He's an analog-circuits guy like myself, and not >terribly familiar with the digital literature. This may be apropos: http://cds.linear.com/docs/Design%20Note/dn1013f.pdf -- John
From: David L. Jones on 31 May 2010 03:43 Glenn Gundlach wrote: > There is a guy on DIY audio stating that jitter in SPDIF digital audio > causes distortions. OK. He also claims to have jitter in the 100 fs > area. Is he blowing smoke about the jitter or can he really achieve > sub pico second jitter? Yes, good crystal oscillators can have sub 1ps RMS jitter. e.g. http://www.crystekcrystals.com/crystal/spec-sheets/clock/C18xx.pdf > It seems to me that jitter in that system is > similar to Time Base errors in video systems. In analog video 1-2 nS > was achieved in both analog and digital Time Base Correction (TBC) > systems but I'm having problems getting my head around 100 fS and - is > it REALLY audible? Nope. Anyone that thinks so is delusional. I had some guy who makes and sells low jitter SPDIF audiophool RCA cables take me to task over my Audiophoolery Blog, claiming he knew it all, and has measurements to prove it etc. And he tried to impress me by saying he's good buddies with Bob Pease, so I'd better shut my mouth, yarda yarda... Hilarious. I'll have to dig up the link... Dave. -- ================================================ Check out my Electronics Engineering Video Blog & Podcast: http://www.eevblog.com
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