From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 01/20/2014 11:57 AM, haiticare2011@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:   
   >>   
   >>> 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.   
   >>   
   >>>   
   >>   
   >>> 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.   
   >>   
   >>>   
   >>   
   >>> In most cases, better to prototype with a Picaxe and then go to   
   >>> assembler or C for the final product.   
   >>   
   >>>   
   >>   
   >>> 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.   
   >>>   
   >>   
   >> That sort of stuff I usually do with a laptop and a LabJack.   
   >> Pretty good medicine for lots of things.   
   >>   
      
   > Thank you Phil. I have a version of that, the DataQ. On another   
   > subject:   
   >   
   > 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.   
      
   >   
   > 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.   
      
   (In large diodes, transit time and capacitance both go like the square   
   of the diameter, but for different reasons.)   
      
   Having enough light is really the key--that way you can use smaller   
   diodes with shorter transit time.   
      
   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   
      
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