High Quality White Noise Gen
in [Design]
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From: John Larkin on 2 Jun 2010 10:55 On Wed, 2 Jun 2010 07:32:55 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >On Jun 1, 5:51�pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Tue, 1 Jun 2010 11:35:59 -0700 (PDT), George Herold >> >> >> >> >> >> <gher...(a)teachspin.com> wrote: >> >On May 31, 12:56�pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Mon, 31 May 2010 12:23:10 GMT, jimsl...(a)esterlux.com (Jim Slone) >> >> wrote: >> >> >> >What are the best options for high quality audio white noise >> >> >generation? >> >> >> >I have been using generic diodes and reversed biased transistors. Then >> >> >someone mentioned there are special parts available with better >> >> >characteristics. >> >> >> >Can anyone please give me a pointer? >> >> >> >Jim Slone >> >> >> You can buy noise diodes from lots of people... just google <noise >> >> diode> >> >> >Does anyone know what makes a high price "noise diode" any better than >> >your garden variety Zener? >> >> Probably a very small junction area (for low capacitance, high current >> density) and maybe some doping profile. Not a power device! >> >> Regular zeners get spikey and asymmetric and sort of oscillate at low >> current. You can get noise diodes that behave at low currents. >> >> >> >> >> If you want really flat, really gaussian noise, a mathematical random >> >> stream (single-bit) or random word (dac) generator is probably best. >> >> See AoE for details. >> >> >> For audio, it doesn't matter much. A 10-volt zener biased at a few mA >> >> is fine. >> >> >Yup, and if the voltage asymmetry is a problem you can add the signal >> >from two diodes, one biased from the positve supply and the other from >> >the negative. �(Though I've never tried this trick.) >> >> Or sum the signals from a bunch of them. Central limit theorem. > >Well that is not going to get rid of the voltage asymmetery. > >If you need real Gaussian noise you can look at the shot noise from a >photodiode illuminated by an LED. Gives you noise ~100 times bigger >than the johnson noise of the sense resistor. (Assuming a 5 Volt DC >drop across R). But this has one big drawback. It's very sensitve to >vibrations. Shot noise is the ultimate asymmetric waveform. It's made of single-photon unidirectional spikes. If it manages to be Gaussian, it's because a lot of asymmetric signals are being summed. Central limit theorem. http://en.wikipedia.org/wiki/Illustration_of_the_central_limit_theorem John
From: George Herold on 2 Jun 2010 13:37 On Jun 2, 10:55 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 2 Jun 2010 07:32:55 -0700 (PDT), George Herold > > > > > > <gher...(a)teachspin.com> wrote: > >On Jun 1, 5:51 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Tue, 1 Jun 2010 11:35:59 -0700 (PDT), George Herold > > >> <gher...(a)teachspin.com> wrote: > >> >On May 31, 12:56 pm, John Larkin > >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> On Mon, 31 May 2010 12:23:10 GMT, jimsl...(a)esterlux.com (Jim Slone) > >> >> wrote: > > >> >> >What are the best options for high quality audio white noise > >> >> >generation? > > >> >> >I have been using generic diodes and reversed biased transistors. Then > >> >> >someone mentioned there are special parts available with better > >> >> >characteristics. > > >> >> >Can anyone please give me a pointer? > > >> >> >Jim Slone > > >> >> You can buy noise diodes from lots of people... just google <noise > >> >> diode> > > >> >Does anyone know what makes a high price "noise diode" any better than > >> >your garden variety Zener? > > >> Probably a very small junction area (for low capacitance, high current > >> density) and maybe some doping profile. Not a power device! > > >> Regular zeners get spikey and asymmetric and sort of oscillate at low > >> current. You can get noise diodes that behave at low currents. > > >> >> If you want really flat, really gaussian noise, a mathematical random > >> >> stream (single-bit) or random word (dac) generator is probably best.. > >> >> See AoE for details. > > >> >> For audio, it doesn't matter much. A 10-volt zener biased at a few mA > >> >> is fine. > > >> >Yup, and if the voltage asymmetry is a problem you can add the signal > >> >from two diodes, one biased from the positve supply and the other from > >> >the negative. (Though I've never tried this trick.) > > >> Or sum the signals from a bunch of them. Central limit theorem. > > >Well that is not going to get rid of the voltage asymmetery. > > >If you need real Gaussian noise you can look at the shot noise from a > >photodiode illuminated by an LED. Gives you noise ~100 times bigger > >than the johnson noise of the sense resistor. (Assuming a 5 Volt DC > >drop across R). But this has one big drawback. It's very sensitve to > >vibrations. > > Shot noise is the ultimate asymmetric waveform. It's made of > single-photon unidirectional spikes. If it manages to be Gaussian, > it's because a lot of asymmetric signals are being summed. Central > limit theorem. > > http://en.wikipedia.org/wiki/Illustration_of_the_central_limit_theorem > > John- Hide quoted text - > > - Show quoted text - Hmmm, you are right.... I still don't think that summing the voltage noise from a bunch of unipolarized zeners is going to get rid of the voltage assymetry. But I'd be happy to be wrong too. Have you ever tried this? It would be simple enough to put 5 or 6 together and see what the output looks like. (As long as you don't mind my summing with an opamp)... Maybe I can find some 'fun' time on Friday. George H.
From: John Larkin on 2 Jun 2010 13:46 On Wed, 2 Jun 2010 10:37:12 -0700 (PDT), George Herold <gherold(a)teachspin.com> wrote: >On Jun 2, 10:55�am, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 2 Jun 2010 07:32:55 -0700 (PDT), George Herold >> >> >> >> >> >> <gher...(a)teachspin.com> wrote: >> >On Jun 1, 5:51�pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Tue, 1 Jun 2010 11:35:59 -0700 (PDT), George Herold >> >> >> <gher...(a)teachspin.com> wrote: >> >> >On May 31, 12:56�pm, John Larkin >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> >> On Mon, 31 May 2010 12:23:10 GMT, jimsl...(a)esterlux.com (Jim Slone) >> >> >> wrote: >> >> >> >> >What are the best options for high quality audio white noise >> >> >> >generation? >> >> >> >> >I have been using generic diodes and reversed biased transistors. Then >> >> >> >someone mentioned there are special parts available with better >> >> >> >characteristics. >> >> >> >> >Can anyone please give me a pointer? >> >> >> >> >Jim Slone >> >> >> >> You can buy noise diodes from lots of people... just google <noise >> >> >> diode> >> >> >> >Does anyone know what makes a high price "noise diode" any better than >> >> >your garden variety Zener? >> >> >> Probably a very small junction area (for low capacitance, high current >> >> density) and maybe some doping profile. Not a power device! >> >> >> Regular zeners get spikey and asymmetric and sort of oscillate at low >> >> current. You can get noise diodes that behave at low currents. >> >> >> >> If you want really flat, really gaussian noise, a mathematical random >> >> >> stream (single-bit) or random word (dac) generator is probably best. >> >> >> See AoE for details. >> >> >> >> For audio, it doesn't matter much. A 10-volt zener biased at a few mA >> >> >> is fine. >> >> >> >Yup, and if the voltage asymmetry is a problem you can add the signal >> >> >from two diodes, one biased from the positve supply and the other from >> >> >the negative. �(Though I've never tried this trick.) >> >> >> Or sum the signals from a bunch of them. Central limit theorem. >> >> >Well that is not going to get rid of the voltage asymmetery. >> >> >If you need real Gaussian noise you can look at the shot noise from a >> >photodiode illuminated by an LED. �Gives you noise ~100 times bigger >> >than the johnson noise of the sense resistor. �(Assuming a 5 Volt DC >> >drop across R). �But this has one big drawback. �It's very sensitve to >> >vibrations. >> >> Shot noise is the ultimate asymmetric waveform. It's made of >> single-photon unidirectional spikes. If it manages to be Gaussian, >> it's because a lot of asymmetric signals are being summed. Central >> limit theorem. >> >> http://en.wikipedia.org/wiki/Illustration_of_the_central_limit_theorem >> >> John- Hide quoted text - >> >> - Show quoted text - > >Hmmm, you are right.... I still don't think that summing the voltage >noise from a bunch of unipolarized zeners is going to get rid of the >voltage assymetry. But I'd be happy to be wrong too. Have you ever >tried this? It would be simple enough to put 5 or 6 together and see >what the output looks like. (As long as you don't mind my summing >with an opamp)... Maybe I can find some 'fun' time on Friday. > >George H. The math says it must be so. Still, the sum would converge to Gaussian faster if half of the lopsided signals were inverted. Zener noise gets more symmetric at higher currents. 10 mA is usually OK for a small 10-volt zener. I sometimes generate Gaussian-distributed numbers by summing a bunch of RAN() calls, which are uniform on [0,1]. Six to ten works well, and the crest factor is finite and known. John
From: whit3rd on 2 Jun 2010 13:59 On Jun 2, 10:37 am, George Herold <gher...(a)teachspin.com> wrote: > > >> >Yup, and if the voltage asymmetry is a problem you can add the signal > > >> >from two diodes, one biased from the positve supply and the other from > > >> >the negative. > Hmmm, you are right.... I still don't think that summing the voltage > noise from a bunch of unipolarized zeners is going to get rid of the > voltage asymmetry. You can use two zeners on a single power supply, in bridge configuration; couple the output through a transformer to get the difference. Symmetry is guaranteed if you balance the bridge correctly. I've seen zeners (on a curve tracer) that were so noisy that the breakover region was a blur.
From: George Herold on 2 Jun 2010 14:57
On Jun 2, 1:46 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 2 Jun 2010 10:37:12 -0700 (PDT), George Herold > > > > > > <gher...(a)teachspin.com> wrote: > >On Jun 2, 10:55 am, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 2 Jun 2010 07:32:55 -0700 (PDT), George Herold > > >> <gher...(a)teachspin.com> wrote: > >> >On Jun 1, 5:51 pm, John Larkin > >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> On Tue, 1 Jun 2010 11:35:59 -0700 (PDT), George Herold > > >> >> <gher...(a)teachspin.com> wrote: > >> >> >On May 31, 12:56 pm, John Larkin > >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> >> On Mon, 31 May 2010 12:23:10 GMT, jimsl...(a)esterlux.com (Jim Slone) > >> >> >> wrote: > > >> >> >> >What are the best options for high quality audio white noise > >> >> >> >generation? > > >> >> >> >I have been using generic diodes and reversed biased transistors. Then > >> >> >> >someone mentioned there are special parts available with better > >> >> >> >characteristics. > > >> >> >> >Can anyone please give me a pointer? > > >> >> >> >Jim Slone > > >> >> >> You can buy noise diodes from lots of people... just google <noise > >> >> >> diode> > > >> >> >Does anyone know what makes a high price "noise diode" any better than > >> >> >your garden variety Zener? > > >> >> Probably a very small junction area (for low capacitance, high current > >> >> density) and maybe some doping profile. Not a power device! > > >> >> Regular zeners get spikey and asymmetric and sort of oscillate at low > >> >> current. You can get noise diodes that behave at low currents. > > >> >> >> If you want really flat, really gaussian noise, a mathematical random > >> >> >> stream (single-bit) or random word (dac) generator is probably best. > >> >> >> See AoE for details. > > >> >> >> For audio, it doesn't matter much. A 10-volt zener biased at a few mA > >> >> >> is fine. > > >> >> >Yup, and if the voltage asymmetry is a problem you can add the signal > >> >> >from two diodes, one biased from the positve supply and the other from > >> >> >the negative. (Though I've never tried this trick.) > > >> >> Or sum the signals from a bunch of them. Central limit theorem. > > >> >Well that is not going to get rid of the voltage asymmetery. > > >> >If you need real Gaussian noise you can look at the shot noise from a > >> >photodiode illuminated by an LED. Gives you noise ~100 times bigger > >> >than the johnson noise of the sense resistor. (Assuming a 5 Volt DC > >> >drop across R). But this has one big drawback. It's very sensitve to > >> >vibrations. > > >> Shot noise is the ultimate asymmetric waveform. It's made of > >> single-photon unidirectional spikes. If it manages to be Gaussian, > >> it's because a lot of asymmetric signals are being summed. Central > >> limit theorem. > > >>http://en.wikipedia.org/wiki/Illustration_of_the_central_limit_theorem > > >> John- Hide quoted text - > > >> - Show quoted text - > > >Hmmm, you are right.... I still don't think that summing the voltage > >noise from a bunch of unipolarized zeners is going to get rid of the > >voltage assymetry. But I'd be happy to be wrong too. Have you ever > >tried this? It would be simple enough to put 5 or 6 together and see > >what the output looks like. (As long as you don't mind my summing > >with an opamp)... Maybe I can find some 'fun' time on Friday. > > >George H. > > The math says it must be so. Still, the sum would converge to Gaussian > faster if half of the lopsided signals were inverted. OK I'll just have to try it. I find the math more convincing if I can see it in some experimental result. (Shot noise is a good example, but its hard to get the current low enough so that you could see a non-Gaussian distribution.) > > Zener noise gets more symmetric at higher currents. 10 mA is usually > OK for a small 10-volt zener. Yeah I think that is just the result of the I-V curvature. I'll try running them down at low currents where the asymmetry is larger. This is an experiment to show the central limit theorem and not make a good Gaussian noise source. George H. > > I sometimes generate Gaussian-distributed numbers by summing a bunch > of RAN() calls, which are uniform on [0,1]. Six to ten works well, and > the crest factor is finite and known. > > John- Hide quoted text - > > - Show quoted text - |