High Quality White Noise Gen
in [Design]
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From: MooseFET on 10 Jun 2010 04:31 On Jun 9, 11:15 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Wed, 9 Jun 2010 07:32:09 -0700 (PDT), MooseFET <kensm...(a)rahul.net> > wrote: > > > > >On Jun 9, 10:03 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 09 Jun 2010 04:46:44 -0700, > > >> "JosephKK"<quiettechb...(a)yahoo.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. > > >> >Actually differencing them in twos, then summing seems more likely to > >> >reduce the asymmetry. > > >> When did addition stop being associative? > > >Perhaps when you do it with an op-amp. > > >(10+10)-(10+10) clips > > >(10-10)+(10-10) doesn't > > >That would be a whole lot of noise spiking :) > > The signals (zener noise) are mildly asymmetric noise, not DC values. > When you add noise, it doesn't matter what order you add or subtract > them in. When you add asymmetric noise, the clipping rate will be greater than when you subtract them. The effect is small but not zero.
From: John Larkin on 10 Jun 2010 09:53 On Thu, 10 Jun 2010 01:31:12 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On Jun 9, 11:15 pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 9 Jun 2010 07:32:09 -0700 (PDT), MooseFET <kensm...(a)rahul.net> >> wrote: >> >> >> >> >On Jun 9, 10:03 pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Wed, 09 Jun 2010 04:46:44 -0700, >> >> >> "JosephKK"<quiettechb...(a)yahoo.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. >> >> >> >Actually differencing them in twos, then summing seems more likely to >> >> >reduce the asymmetry. >> >> >> When did addition stop being associative? >> >> >Perhaps when you do it with an op-amp. >> >> >(10+10)-(10+10) clips >> >> >(10-10)+(10-10) doesn't >> >> >That would be a whole lot of noise spiking :) >> >> The signals (zener noise) are mildly asymmetric noise, not DC values. >> When you add noise, it doesn't matter what order you add or subtract >> them in. > >When you add asymmetric noise, the clipping rate will be greater than >when you subtract them. The effect is small but not zero. If you are determined to be a PITA, yes. John
From: MooseFET on 10 Jun 2010 10:01 On Jun 10, 9:53 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 10 Jun 2010 01:31:12 -0700 (PDT), MooseFET > > > > <kensm...(a)rahul.net> wrote: > >On Jun 9, 11:15 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 9 Jun 2010 07:32:09 -0700 (PDT), MooseFET <kensm...(a)rahul.net> > >> wrote: > > >> >On Jun 9, 10:03 pm, John Larkin > >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> On Wed, 09 Jun 2010 04:46:44 -0700, > > >> >> "JosephKK"<quiettechb...(a)yahoo.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. > > >> >> >Actually differencing them in twos, then summing seems more likely to > >> >> >reduce the asymmetry. > > >> >> When did addition stop being associative? > > >> >Perhaps when you do it with an op-amp. > > >> >(10+10)-(10+10) clips > > >> >(10-10)+(10-10) doesn't > > >> >That would be a whole lot of noise spiking :) > > >> The signals (zener noise) are mildly asymmetric noise, not DC values. > >> When you add noise, it doesn't matter what order you add or subtract > >> them in. > > >When you add asymmetric noise, the clipping rate will be greater than > >when you subtract them. The effect is small but not zero. > > If you are determined to be a PITA, yes. > Sorry about the tone. I didn't sleep well last night. .... or perhaps yes, I am determined to be a PITA. maybe with more coffee I will know which it is. > John
From: John Larkin on 10 Jun 2010 11:49 On Thu, 10 Jun 2010 07:01:03 -0700 (PDT), MooseFET <kensmith(a)rahul.net> wrote: >On Jun 10, 9:53 pm, John Larkin ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Thu, 10 Jun 2010 01:31:12 -0700 (PDT), MooseFET >> >> >> >> <kensm...(a)rahul.net> wrote: >> >On Jun 9, 11:15 pm, John Larkin >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> On Wed, 9 Jun 2010 07:32:09 -0700 (PDT), MooseFET <kensm...(a)rahul.net> >> >> wrote: >> >> >> >On Jun 9, 10:03 pm, John Larkin >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> >> On Wed, 09 Jun 2010 04:46:44 -0700, >> >> >> >> "JosephKK"<quiettechb...(a)yahoo.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. >> >> >> >> >Actually differencing them in twos, then summing seems more likely to >> >> >> >reduce the asymmetry. >> >> >> >> When did addition stop being associative? >> >> >> >Perhaps when you do it with an op-amp. >> >> >> >(10+10)-(10+10) clips >> >> >> >(10-10)+(10-10) doesn't >> >> >> >That would be a whole lot of noise spiking :) >> >> >> The signals (zener noise) are mildly asymmetric noise, not DC values. >> >> When you add noise, it doesn't matter what order you add or subtract >> >> them in. >> >> >When you add asymmetric noise, the clipping rate will be greater than >> >when you subtract them. The effect is small but not zero. >> >> If you are determined to be a PITA, yes. >> >Sorry about the tone. I didn't sleep well last night. >... or perhaps yes, I am determined to be a PITA. >maybe with more coffee I will know which it is. > >> John I've had my coffee, two cups of Peets. Snarl. Snap. Noise amplifiers need, well, infinite headroom, so we can never get it right. John
From: MooseFET on 10 Jun 2010 20:50
On Jun 10, 11:49 pm, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 10 Jun 2010 07:01:03 -0700 (PDT), MooseFET > > > > <kensm...(a)rahul.net> wrote: > >On Jun 10, 9:53 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Thu, 10 Jun 2010 01:31:12 -0700 (PDT), MooseFET > > >> <kensm...(a)rahul.net> wrote: > >> >On Jun 9, 11:15 pm, John Larkin > >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> On Wed, 9 Jun 2010 07:32:09 -0700 (PDT), MooseFET <kensm...(a)rahul.net> > >> >> wrote: > > >> >> >On Jun 9, 10:03 pm, John Larkin > >> >> ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> >> >> On Wed, 09 Jun 2010 04:46:44 -0700, > > >> >> >> "JosephKK"<quiettechb...(a)yahoo.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. > > >> >> >> >Actually differencing them in twos, then summing seems more likely to > >> >> >> >reduce the asymmetry. > > >> >> >> When did addition stop being associative? > > >> >> >Perhaps when you do it with an op-amp. > > >> >> >(10+10)-(10+10) clips > > >> >> >(10-10)+(10-10) doesn't > > >> >> >That would be a whole lot of noise spiking :) > > >> >> The signals (zener noise) are mildly asymmetric noise, not DC values. > >> >> When you add noise, it doesn't matter what order you add or subtract > >> >> them in. > > >> >When you add asymmetric noise, the clipping rate will be greater than > >> >when you subtract them. The effect is small but not zero. > > >> If you are determined to be a PITA, yes. > > >Sorry about the tone. I didn't sleep well last night. > >... or perhaps yes, I am determined to be a PITA. > >maybe with more coffee I will know which it is. > > >> John > > I've had my coffee, two cups of Peets. Snarl. Snap. Peet's smells rotten to me. I think they ferment it. > Noise amplifiers need, well, infinite headroom, > so we can never get it > right. If the noise is to be white and not contrived, yes, there is no way to really do it. I have had a few cases where I needed noise that was more like a a uniform probability over a band of values and also a flat spectrum over a band. It can be done. The crappy version is just the usual white noise thing feeding into a pair of diodes. The log like curve of the diodes does not that bad of a job. |