On Monday, January 20, 2014 4:56:57 PM UTC-5, Phil Hobbs wrote:   
   > On 01/20/2014 11:57 AM, haitica1@gmail.com wrote:   
   >   
   > > On Thursday, January 16, 2014 11:44:28 AM UTC-5, Phil Hobbs wrote:   
   >   
   > >> On 1/16/2014 11:35 AM, hai2011@gmail.com wrote:   
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   > >>   
   >   
   > >>> Probably everyone knows this, but the Picaxe is a Pic chip with a   
   >   
   > >>> Basic interpreter on board - about $3 for the 8 bit 32 mhz   
   >   
   > >>> version. It is the easiest way to prototype for those who want a   
   >   
   > >>> quick and dirty prototype.   
   >   
   > >>   
   >   
   > >>> The interpreter is slow, so you will end up with a 10 bit A to D   
   >   
   > >>> conversion that takes 300 microseconds. And an interpreter treats   
   >   
   > >>> every oommand in isolation, so every command has to set up all he   
   >   
   > >>> registers and flags over again. In other words, the price for   
   >   
   > >>> convenience is lack of speed. But if you don't mind the horse and   
   >   
   > >>> buggy speed, it's the easiest way to go. And the Picaxe has quite   
   >   
   > >>> a few features - ADC, DAC, PWM, digital counter, settable clock,   
   >   
   > >>> I2C, etc.   
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   > >>   
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   > >>>   
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   > >>   
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   > >>> Compare that with the pic24fj128gc010 chip for speed. This 16 bit   
   >   
   > >>> chip does 10 MSPS 10 bit conversions per sec, or 100 ns. per   
   >   
   > >>> conversion. The documentation is over 200 pages long.   
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   > >>   
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   > >>>   
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   > >>   
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   > >>> In most cases, better to prototype with a Picaxe and then go to   
   >   
   > >>> assembler or C for the final product.   
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   > >>>   
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   > >>   
   >   
   > >>> My apologies to anyone for stating the obvious here. Another   
   >   
   > >>> interesting are is the dedicated, "user friendly" boards for   
   >   
   > >>> optics measurement. Arduino, Raspberry, and BeagleBone. I haven't   
   >   
   > >>> seen much ADC capability on them.   
   >   
   > >>>   
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   > >>   
   >   
   > >> That sort of stuff I usually do with a laptop and a LabJack.   
   >   
   > >> Pretty good medicine for lots of things.   
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   > >>   
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   >   
   >   
   > > Thank you Phil. I have a version of that, the DataQ. On another   
   >   
   > > subject:   
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   > >   
   >   
   > > For flashing an led, I have several options. A programmable square   
   >   
   > > wave generator or an mcu putting out highs and lows. This obviously   
   >   
   > > can turn on a mosfet switch with back diode protect. I am thinking a   
   >   
   > > cap reserve helps if a surge needed. I notice there are 1,3, and 10   
   >   
   > > Watt leds at low cost now. Does the wave center shift during a pulse   
   >   
   > > of electrons, due to their temperature?   
   >   
   >   
   >   
   > Depends.  Phosphide LEDs have very stable wavelength vs drive current,   
   >   
   > at least until they start to heat up.  Nitride LEDs tune a lot further.   
   >   
   >   If you can keep the average dissipation constant, heating effects get   
   >   
   > less important at high frequency.   
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   > >   
   >   
   > > If you can actually get a MhZ-scale BW out of a generic Si PD, as my   
   >   
   > > reading of your paper suggests, then could you get 100,000   
   >   
   > > measurements per second done with a fast TIA and reasonably fast ADC?   
   >   
   > > I wonder if that has any practical use?   
   >   
   >   
   >   
   > 100 kHz isn't fast--you can go way faster than that, if you have enough   
   >   
   > light.  I have a photodiode box whose 3 dB bandwidth is over 30 GHz (a   
   >   
   > Tektronix SD-48).   
   >   
   >   
   >   
   > Of course that's in InGaAs, with very small diodes.  Ordinary silicon   
   >   
   > PIN diodes excel at having low capacitance, as low as 50 pF/cm**2,   
   >   
   > whereas InGaAs comes in about 100 times higher than that.   
   >   
   >   
   >   
   > The down side is that the low capacitance comes from a very thick   
   >   
   > depletion zone, so silicon diodes have much longer transit times, and   
   >   
   > that's what limits their speed at low impedance.  Large diameter diodes   
   >   
   > are limited by the diffusion time in the epi, which doesn't get depleted   
   >   
   > and hence doesn't have that helpful E field.  My fave BPW34 (Osram, not   
   >   
   > Vishay) tops out at around 10-20 ns rise and fall times.   
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   >   
   >   
   > (In large diodes, transit time and capacitance both go like the square   
   >   
   > of the diameter, but for different reasons.)   
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   >   
   >   
   > Having enough light is really the key--that way you can use smaller   
   >   
   > diodes with shorter transit time.   
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   >   
   >   
   > Cheers   
   >   
   >   
   >   
   > Phil Hobbs   
   >   
   >   
   >   
   > --   
   >   
   > Dr Philip C D Hobbs   
   >   
   > Principal Consultant   
   >   
   > ElectroOptical Innovations LLC   
   >   
   > Optics, Electro-optics, Photonics, Analog Electronics   
   >   
   >   
   >   
   > 160 North State Road #203   
   >   
   > Briarcliff Manor NY 10510   
   >   
   >   
   >   
   > hobbs at electrooptical dot net   
   >   
   > http://electrooptical.net   
      
   All right, allow me to re-phrase that. Why would you want to have high speeds   
   of photodetection, say mhz or above?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)