XPost: sci.electronics.design   
   From: pcdhSpamMeSenseless@electrooptical.net   
      
   On 11/19/2015 11:55 AM, John Larkin wrote:   
   > On Wed, 18 Nov 2015 13:17:08 -0500, Phil Hobbs   
   >  wrote:   
   >   
   >> I'm working on an interesting project for a biotech customer, and I   
   >> could use some wisdom from the assembled multitude.   
   >>   
   >> They're building a scanning fluorescence microscope that scans several   
   >> lines at once with a single photomultiplier, using a clever scheme to   
   >> make each line come out at a different RF frequency.   
   >>   
   >> The frequencies are inherently evenly spaced, so third-order IM products   
   >> cause image artifacts.  So they asked me to come up with a way to reduce   
   >> the IMD.   
   >>   
   >> I've been doing some measurements, and have found the usual cubic   
   >> dependence of the IM3 products up above about 50 uA of anode current.   
   >> That's generally a combination of voltage sag in the dynode bias string,   
   >> plus some space-charge effects.  It can be reduced by changing the   
   >> dynode voltage distribution so that the last few stages have more bias.   
   >>   
   >> Interestingly, though, below ~50 uA there's a broad range of anode   
   >> currents (20 dB or so) where the IM3 products go up linearly with the   
   >> signal, so that the SFDR is a nearly constant 60 dB, plus or minus a   
   >> few.  This is without changing ranges on the spectrum analyzer or   
   >> anything like that.   
   >>   
   >> Ahah, you say, the IM3 is in the illumination source, which runs at a   
   >> constant drive level.  So I thought, but it ain't so.   
   >>   
   >> My illumination source is a pair of blue LEDs, each driven from a   
   >> separate amp with separate power supplies.  To get the spurs big enough   
   >> to see, I'm using two synthesizers locked together--one is just a   
   >> frequency doubler hung off my 10 MHz rubidium reference, and the other   
   >> is a PTS1000 locked to the same reference, running at 20.001 MHz.  The   
   >> analyzer is an HP 8566B, locked to the same reference.  That way I can   
   >> use a 10-Hz resolution bandwidth to see the spurs.   
   >>   
   >> Using a photodiode, I've verified that the spurs in the light sources   
   >> are below -80 dBc.   
   >>   
   >> There's mention in the literature of a space-charge effect at the   
   >> photocathode, which would vary less with gain than in the last stages.   
   >> If that's it, the -60 dB would be set at the very beginning, so it   
   >> should depend mostly on the actual light intensity at the photocathode.   
   >>   
   >> However, the linear behaviour occurs regardless of whether I'm using a   
   >> fixed light level and adjusting the anode current with the bias voltage   
   >> pot or leaving the voltage alone and using a variable optical attenuator.   
   >>   
   >> It also doesn't depend on the attenuation of the signal in the RX   
   >> chain--adding a 10 dB pad doesn't change the SFDR in that anode current   
   >> range.   
   >>   
   >> This is, in other words, weird as hell.   
   >>   
   >> Any ideas?   
   >>   
   >> Thanks   
   >>   
   >> Phil Hobbs   
   >   
   > I would offer you help, buy my customer is competing with your   
   > customer. Want a bunch of bad ideas?   
      
   I've got enough of those on my own. ;)   
      
   Thanks for thinking of me.   
      
   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   
      
   --- SoupGate-Win32 v1.05   
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