High Quality White Noise Gen
in [Design]
|
From: Phil Hobbs on 4 Jun 2010 10:07 On 6/3/2010 11:12 PM, George Herold wrote: > > > 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 I was just janking your chain--I had no idea that anybody had actually tried that. Phototransistors are so horrible on so many levels that it's pretty amusing to see someone trying to make fast pulses out of them. 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 4 Jun 2010 10:41 On Jun 4, 10:07 am, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > On 6/3/2010 11:12 PM, George Herold wrote: > > > > > > > > > langw...(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 > > I was just janking your chain--I had no idea that anybody had actually > tried that. Phototransistors are so horrible on so many levels that > it's pretty amusing to see someone trying to make fast pulses out of them.. > > 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 - Oh...(Silly me) Well so how 'bout a photo-transistor with three terminals (one where the base has a lead connected to it.) I'll short the base and collector together and then reverse bias the EB junction. With the bias voltage set just below where the thing wants to zener. Then a photon could set the whole thing off? I thought I had a photo transistor around here someplace, but I couldn't find it. (The original prototype of optical puming used a phototransistor, until I convinced every one that a photodiode was, more sensitive (larger area), less noisy and faster.) Ahh the other silly thought that this generated was using an avalanche zener diode as a photo detector. I know the Zener's I use are sensitive to light, so again if I was to park them just below the knee voltage and then hit them with light could I get some gain out of them? Kinda a poor man's APD. George H.
From: Phil Hobbs on 4 Jun 2010 11:32 On 6/4/2010 10:41 AM, George Herold wrote: > On Jun 4, 10:07 am, Phil Hobbs > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> On 6/3/2010 11:12 PM, George Herold wrote: >> >> >> >> >> >> >> >>> langw...(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 >> >> I was just janking your chain--I had no idea that anybody had actually >> tried that. Phototransistors are so horrible on so many levels that >> it's pretty amusing to see someone trying to make fast pulses out of them.. >> >> 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 - > > Oh...(Silly me) Well so how 'bout a photo-transistor with three > terminals (one where the base has a lead connected to it.) I'll short > the base and collector together and then reverse bias the EB > junction. With the bias voltage set just below where the thing wants > to zener. Then a photon could set the whole thing off? > > I thought I had a photo transistor around here someplace, but I > couldn't find it. (The original prototype of optical puming used a > phototransistor, until I convinced every one that a photodiode was, > more sensitive (larger area), less noisy and faster.) > > Ahh the other silly thought that this generated was using an avalanche > zener diode as a photo detector. I know the Zener's I use are > sensitive to light, so again if I was to park them just below the knee > voltage and then hit them with light could I get some gain out of > them? Kinda a poor man's APD. > > George H. Shot noise is really, really Gaussian--I've measured it out to 7.1 sigma, where (false alarm rate)/(bandwidth) = 10**-11 (about 1 count per day in a 1 MHz bandwidth). A decent comparator driving a one-shot can make nice Poissonian pulses (as someone already suggested). You can make okay phototransistors by connecting a real photodiode between the base and collector of a real transistor. Still slow, but much more sensitive. The next big advance is to replace the transistor with an op amp. ;) 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: Bill Sloman on 4 Jun 2010 12:01 On Jun 4, 4:41 pm, George Herold <gher...(a)teachspin.com> wrote: > On Jun 4, 10:07 am, Phil Hobbs > > > > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > > On 6/3/2010 11:12 PM, George Herold wrote: > > > > langw...(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 > > > I was just janking your chain--I had no idea that anybody had actually > > tried that. Phototransistors are so horrible on so many levels that > > it's pretty amusing to see someone trying to make fast pulses out of them. > > > 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 - > > Oh...(Silly me) Well so how 'bout a photo-transistor with three > terminals (one where the base has a lead connected to it.) I'll short > the base and collector together and then reverse bias the EB > junction. With the bias voltage set just below where the thing wants > to zener. Then a photon could set the whole thing off? > > I thought I had a photo transistor around here someplace, but I > couldn't find it. (The original prototype of optical puming used a > phototransistor, until I convinced every one that a photodiode was, > more sensitive (larger area), less noisy and faster.) > > Ahh the other silly thought that this generated was using an avalanche > zener diode as a photo detector. I know the Zener's I use are > sensitive to light, so again if I was to park them just below the knee > voltage and then hit them with light could I get some gain out of > them? Kinda a poor man's APD. You might want to search on single photon avalanche photo-detection. http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-3-2232 I haven't looked at the full paper but the list of references includes some useful stuff. -- Bill Sloman, Nijmegen
From: Phil Hobbs on 4 Jun 2010 16:40
On 6/4/2010 12:01 PM, Bill Sloman wrote: > On Jun 4, 4:41 pm, George Herold<gher...(a)teachspin.com> wrote: >> On Jun 4, 10:07 am, Phil Hobbs >> >> >> >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >>> On 6/3/2010 11:12 PM, George Herold wrote: >> >>>> langw...(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 >> >>> I was just janking your chain--I had no idea that anybody had actually >>> tried that. Phototransistors are so horrible on so many levels that >>> it's pretty amusing to see someone trying to make fast pulses out of them. >> >>> 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 - >> >> Oh...(Silly me) Well so how 'bout a photo-transistor with three >> terminals (one where the base has a lead connected to it.) I'll short >> the base and collector together and then reverse bias the EB >> junction. With the bias voltage set just below where the thing wants >> to zener. Then a photon could set the whole thing off? >> >> I thought I had a photo transistor around here someplace, but I >> couldn't find it. (The original prototype of optical puming used a >> phototransistor, until I convinced every one that a photodiode was, >> more sensitive (larger area), less noisy and faster.) >> >> Ahh the other silly thought that this generated was using an avalanche >> zener diode as a photo detector. I know the Zener's I use are >> sensitive to light, so again if I was to park them just below the knee >> voltage and then hit them with light could I get some gain out of >> them? Kinda a poor man's APD. > > You might want to search on single photon avalanche photo-detection. > > http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-16-3-2232 > > I haven't looked at the full paper but the list of references includes > some useful stuff. > > -- > Bill Sloman, Nijmegen Geiger-mode APDs are mostly a crock unless you need timing accuracy better than, say, 10 ns. Their dark count rate is a good six orders of magnitude worse than a PMT of the same area, and their dead time is 10 times longer. On the other hand, they don't die if you put them in a helium atmosphere, and they last longer than 5 years. 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 |