High Quality White Noise Gen
in [Design]
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From: George Herold on 3 Jun 2010 12:27 On Jun 3, 10:13 am, John Larkin <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > On Thu, 3 Jun 2010 06:49:53 -0700 (PDT), George Herold > > > > > > <gher...(a)teachspin.com> wrote: > >On Jun 2, 4:36 pm, John Larkin > ><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 2 Jun 2010 12:30:19 -0700 (PDT), George Herold > > >> <gher...(a)teachspin.com> wrote: > >> >On Jun 2, 1:59 pm, whit3rd <whit...(a)gmail.com> wrote: > >> >> 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. > > >> >Ahh, more than one way to skin that cat. > > >> >Say speaking of noise sources, a colleague put together a digital > >> >noise source. A counter steps through a look up table that feeds a > >> >DAC. The lookup table holds a whole comb of sine waves equally spaced > >> >in frequency space up to 32kHz. (I don t recall the frequency spacing > >> >but I could find out.. a few Hz or so.) The phases of all the sine > >> >waves were chosen randomly. The DAC was 12 bit (an AD7541 I > >> >think). The whole thing was clocked several times lower than Nyquist > >> >limit (~128 kHz). Now the problem we observed, (and could never > >> >cure), was intermodulation distortion above the 32 kHz cutoff. The > >> >signals above the cutoff frequency were down by only 50 dB, and my > >> >colleague was expecting something closer to 70 dB down. (Is that > >> >right for 12 bit resolution on the DAC? ) I worked on all the layout > >> >and analog portions of the circuit but could never make it any > >> >better. There was talk about clock jitter on SED recently and I > >> >wondered if this could be the source of the problem? Or maybe you > >> >have some other idea. > > >> The DAC quantization, and any nonlinearity, will add harmonic > >> distortion. Plus the sines may occasionally peak together and clip the > >> dac. Plus it's not trivial to get -70 dB distortion at these > >> frequencies. > > >> We use random number generators and boxcar filters to generate > >> Gaussian noise to feed into dacs. This little box does this, all in a > >> Spartan3 FPGA... > > >>http://www.highlandtechnology.com/DSS/T346DS.html > > >> Rob cleverly, somehow, allowed the user to program the noise bandwidth > >> from mHz to 2 MHz without affecting the RMS amplitude. > > >Very nice, (Are prices listed on your website?) Y'all make stuff > >that's several orders of magnitude above what we're doing. > > >You know what I'd really like to make (and could probabbly sell too.) > >is a random pulse generator. Short little pulses maybe 10nS or less > >coming at an average rate of 1us or so. This would be a psuedo shot > >noise generator. With a pot on the output one could change the > >amplitude of the pulses and see how the noise scaled. ... Hmm it > >would be nicer if the average pulse rate could be changed too. So > >that one could keep the average 'current' the same, but made with > >bigger 'electrons'. Sounds like a digital circuit. (Which I find a > >bit boring) an analog 'something' would be more fun. > > You could start with any noise source, like a zener thing, drive a > comparator to nab the peaks, and use a tach-like feedback to adjust > the comparator trip point to servo the rate. Fire a one-shot, > adjustable width if you like, and a pot to set amplitude. All analog, > pretty simple. You could also do the random rate thing digitally. > > John- Hide quoted text - > > - Show quoted text - Yeah I had something like that in mind. I didn't quite know how to do the random rate part.. By tach-like I assume you mean a frequency to voltage converter. "You could also do the random rate thing digitally." Oh that sounds easier than the tach-servo idea. Divide by N before the one shot. I don't need fine resolution on the rate, just the ability to change it and then adjust the pulse height. Are one shot's still 'kosher'? I once talked with an FPGA guy who had never heard of them. Anything 'better' than the 74AHC123 from NXP? This has a minimum pulse width of 5ns which would be fine. Other one shots looked to be a lot slower. Ahh so many circuits to try and so little time. George H.
From: Phil Hobbs on 3 Jun 2010 17:11 On 6/3/2010 9:49 AM, George Herold wrote: > On Jun 2, 4:36 pm, John Larkin > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >> On Wed, 2 Jun 2010 12:30:19 -0700 (PDT), George Herold >> >> >> >> >> >> <gher...(a)teachspin.com> wrote: >>> On Jun 2, 1:59 pm, whit3rd<whit...(a)gmail.com> wrote: >>>> 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. >> >>> Ahh, more than one way to skin that cat. >> >>> Say speaking of noise sources, a colleague put together a digital >>> noise source. A counter steps through a look up table that feeds a >>> DAC. The lookup table holds a whole comb of sine waves equally spaced >>> in frequency space up to 32kHz. (I don t recall the frequency spacing >>> but I could find out.. a few Hz or so.) The phases of all the sine >>> waves were chosen randomly. The DAC was 12 bit (an AD7541 I >>> think). The whole thing was clocked several times lower than Nyquist >>> limit (~128 kHz). Now the problem we observed, (and could never >>> cure), was intermodulation distortion above the 32 kHz cutoff. The >>> signals above the cutoff frequency were down by only 50 dB, and my >>> colleague was expecting something closer to 70 dB down. (Is that >>> right for 12 bit resolution on the DAC? ) I worked on all the layout >>> and analog portions of the circuit but could never make it any >>> better. There was talk about clock jitter on SED recently and I >>> wondered if this could be the source of the problem? Or maybe you >>> have some other idea. >> >> The DAC quantization, and any nonlinearity, will add harmonic >> distortion. Plus the sines may occasionally peak together and clip the >> dac. Plus it's not trivial to get -70 dB distortion at these >> frequencies. >> >> We use random number generators and boxcar filters to generate >> Gaussian noise to feed into dacs. This little box does this, all in a >> Spartan3 FPGA... >> >> http://www.highlandtechnology.com/DSS/T346DS.html >> >> Rob cleverly, somehow, allowed the user to program the noise bandwidth >> from mHz to 2 MHz without affecting the RMS amplitude. > > Very nice, (Are prices listed on your website?) Y'all make stuff > that's several orders of magnitude above what we're doing. > > You know what I'd really like to make (and could probabbly sell too.) > is a random pulse generator. Short little pulses maybe 10nS or less > coming at an average rate of 1us or so. This would be a psuedo shot > noise generator. With a pot on the output one could change the > amplitude of the pulses and see how the noise scaled. ... Hmm it > would be nicer if the average pulse rate could be changed too. So > that one could keep the average 'current' the same, but made with > bigger 'electrons'. Sounds like a digital circuit. (Which I find a > bit boring) an analog 'something' would be more fun. > > George H. >> >> John- Hide quoted text - >> >> - Show quoted text - > How about avalanching a phototransistor? ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
From: George Herold on 3 Jun 2010 19:44 On Jun 3, 5:11 pm, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > On 6/3/2010 9:49 AM, George Herold wrote: > > > > > > > On Jun 2, 4:36 pm, John Larkin > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 2 Jun 2010 12:30:19 -0700 (PDT), George Herold > > >> <gher...(a)teachspin.com> wrote: > >>> On Jun 2, 1:59 pm, whit3rd<whit...(a)gmail.com> wrote: > >>>> 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. > > >>> Ahh, more than one way to skin that cat. > > >>> Say speaking of noise sources, a colleague put together a digital > >>> noise source. A counter steps through a look up table that feeds a > >>> DAC. The lookup table holds a whole comb of sine waves equally spaced > >>> in frequency space up to 32kHz. (I don t recall the frequency spacing > >>> but I could find out.. a few Hz or so.) The phases of all the sine > >>> waves were chosen randomly. The DAC was 12 bit (an AD7541 I > >>> think). The whole thing was clocked several times lower than Nyquist > >>> limit (~128 kHz). Now the problem we observed, (and could never > >>> cure), was intermodulation distortion above the 32 kHz cutoff. The > >>> signals above the cutoff frequency were down by only 50 dB, and my > >>> colleague was expecting something closer to 70 dB down. (Is that > >>> right for 12 bit resolution on the DAC? ) I worked on all the layout > >>> and analog portions of the circuit but could never make it any > >>> better. There was talk about clock jitter on SED recently and I > >>> wondered if this could be the source of the problem? Or maybe you > >>> have some other idea. > > >> The DAC quantization, and any nonlinearity, will add harmonic > >> distortion. Plus the sines may occasionally peak together and clip the > >> dac. Plus it's not trivial to get -70 dB distortion at these > >> frequencies. > > >> We use random number generators and boxcar filters to generate > >> Gaussian noise to feed into dacs. This little box does this, all in a > >> Spartan3 FPGA... > > >>http://www.highlandtechnology.com/DSS/T346DS.html > > >> Rob cleverly, somehow, allowed the user to program the noise bandwidth > >> from mHz to 2 MHz without affecting the RMS amplitude. > > > Very nice, (Are prices listed on your website?) Y'all make stuff > > that's several orders of magnitude above what we're doing. > > > You know what I'd really like to make (and could probabbly sell too.) > > is a random pulse generator. Short little pulses maybe 10nS or less > > coming at an average rate of 1us or so. This would be a psuedo shot > > noise generator. With a pot on the output one could change the > > amplitude of the pulses and see how the noise scaled. ... Hmm it > > would be nicer if the average pulse rate could be changed too. So > > that one could keep the average 'current' the same, but made with > > bigger 'electrons'. Sounds like a digital circuit. (Which I find a > > bit boring) an analog 'something' would be more fun. > > > George H. > > >> John- Hide quoted text - > > >> - Show quoted text - > > How about avalanching a phototransistor? ;) > > Cheers > > Phil Hobbs > > -- > Dr Philip C D Hobbs > Principal > ElectroOptical Innovations > 55 Orchard Rd > Briarcliff Manor NY 10510 > 845-480-2058 > hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted text - > > - Show quoted text - I never heard of it. Google gets a lot of hits though, I'll have to explore tomorrow when I have higher bandwidth at work. Do I need a special phototransistor or will anyone work? I assume I have to reverse bias the BE junction.... But won't current flow out the collector? Thanks for something fun to think about. George H.
From: langwadt on 3 Jun 2010 21:17 On 3 Jun., 23:11, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > On 6/3/2010 9:49 AM, George Herold wrote: > > > > > On Jun 2, 4:36 pm, John Larkin > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > >> On Wed, 2 Jun 2010 12:30:19 -0700 (PDT), George Herold > > >> <gher...(a)teachspin.com> wrote: > >>> On Jun 2, 1:59 pm, whit3rd<whit...(a)gmail.com> wrote: > >>>> 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. > > >>> Ahh, more than one way to skin that cat. > > >>> Say speaking of noise sources, a colleague put together a digital > >>> noise source. A counter steps through a look up table that feeds a > >>> DAC. The lookup table holds a whole comb of sine waves equally spaced > >>> in frequency space up to 32kHz. (I don t recall the frequency spacing > >>> but I could find out.. a few Hz or so.) The phases of all the sine > >>> waves were chosen randomly. The DAC was 12 bit (an AD7541 I > >>> think). The whole thing was clocked several times lower than Nyquist > >>> limit (~128 kHz). Now the problem we observed, (and could never > >>> cure), was intermodulation distortion above the 32 kHz cutoff. The > >>> signals above the cutoff frequency were down by only 50 dB, and my > >>> colleague was expecting something closer to 70 dB down. (Is that > >>> right for 12 bit resolution on the DAC? ) I worked on all the layout > >>> and analog portions of the circuit but could never make it any > >>> better. There was talk about clock jitter on SED recently and I > >>> wondered if this could be the source of the problem? Or maybe you > >>> have some other idea. > > >> The DAC quantization, and any nonlinearity, will add harmonic > >> distortion. Plus the sines may occasionally peak together and clip the > >> dac. Plus it's not trivial to get -70 dB distortion at these > >> frequencies. > > >> We use random number generators and boxcar filters to generate > >> Gaussian noise to feed into dacs. This little box does this, all in a > >> Spartan3 FPGA... > > >>http://www.highlandtechnology.com/DSS/T346DS.html > > >> Rob cleverly, somehow, allowed the user to program the noise bandwidth > >> from mHz to 2 MHz without affecting the RMS amplitude. > > > Very nice, (Are prices listed on your website?) Y'all make stuff > > that's several orders of magnitude above what we're doing. > > > You know what I'd really like to make (and could probabbly sell too.) > > is a random pulse generator. Short little pulses maybe 10nS or less > > coming at an average rate of 1us or so. This would be a psuedo shot > > noise generator. With a pot on the output one could change the > > amplitude of the pulses and see how the noise scaled. ... Hmm it > > would be nicer if the average pulse rate could be changed too. So > > that one could keep the average 'current' the same, but made with > > bigger 'electrons'. Sounds like a digital circuit. (Which I find a > > bit boring) an analog 'something' would be more fun. > > > George H. > > >> John- Hide quoted text - > > >> - Show quoted text - > > How about avalanching a phototransistor? ;) > point it at an Americium source from a smokedetector :) real random noise -Lasse
From: George Herold on 3 Jun 2010 23:12
langwadt(a)fonz.dk wrote: > On 3 Jun., 23:11, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> > wrote: > > On 6/3/2010 9:49 AM, George Herold wrote: > > > > > > > > > On Jun 2, 4:36 pm, John Larkin > > > <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: > > >> On Wed, 2 Jun 2010 12:30:19 -0700 (PDT), George Herold > > > > >> <gher...(a)teachspin.com> wrote: > > >>> On Jun 2, 1:59 pm, whit3rd<whit...(a)gmail.com> wrote: > > >>>> 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. > > > > >>> Ahh, more than one way to skin that cat. > > > > >>> Say speaking of noise sources, a colleague put together a digital > > >>> noise source. A counter steps through a look up table that feeds a > > >>> DAC. The lookup table holds a whole comb of sine waves equally spaced > > >>> in frequency space up to 32kHz. (I don t recall the frequency spacing > > >>> but I could find out.. a few Hz or so.) The phases of all the sine > > >>> waves were chosen randomly. The DAC was 12 bit (an AD7541 I > > >>> think). The whole thing was clocked several times lower than Nyquist > > >>> limit (~128 kHz). Now the problem we observed, (and could never > > >>> cure), was intermodulation distortion above the 32 kHz cutoff. The > > >>> signals above the cutoff frequency were down by only 50 dB, and my > > >>> colleague was expecting something closer to 70 dB down. (Is that > > >>> right for 12 bit resolution on the DAC? ) I worked on all the layout > > >>> and analog portions of the circuit but could never make it any > > >>> better. There was talk about clock jitter on SED recently and I > > >>> wondered if this could be the source of the problem? Or maybe you > > >>> have some other idea. > > > > >> The DAC quantization, and any nonlinearity, will add harmonic > > >> distortion. Plus the sines may occasionally peak together and clip the > > >> dac. Plus it's not trivial to get -70 dB distortion at these > > >> frequencies. > > > > >> We use random number generators and boxcar filters to generate > > >> Gaussian noise to feed into dacs. This little box does this, all in a > > >> Spartan3 FPGA... > > > > >>http://www.highlandtechnology.com/DSS/T346DS.html > > > > >> Rob cleverly, somehow, allowed the user to program the noise bandwidth > > >> from mHz to 2 MHz without affecting the RMS amplitude. > > > > > Very nice, (Are prices listed on your website?) Y'all make stuff > > > that's several orders of magnitude above what we're doing. > > > > > You know what I'd really like to make (and could probabbly sell too.) > > > is a random pulse generator. Short little pulses maybe 10nS or less > > > coming at an average rate of 1us or so. This would be a psuedo shot > > > noise generator. With a pot on the output one could change the > > > amplitude of the pulses and see how the noise scaled. ... Hmm it > > > would be nicer if the average pulse rate could be changed too. So > > > that one could keep the average 'current' the same, but made with > > > bigger 'electrons'. Sounds like a digital circuit. (Which I find a > > > bit boring) an analog 'something' would be more fun. > > > > > George H. > > > > >> John- Hide quoted text - > > > > >> - Show quoted text - > > > > How about avalanching a phototransistor? ;) > > > > point it at an Americium source from a smokedetector :) > real random noise Something optical is nice because you can change light intensity with a knob. George H. > > -Lasse |