XPost: sci.electronics.design   
   From: robertbaer@localnet.com   
      
   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   
   >   
      Am too ignorant in that to give direct help, but stupid enough to   
   give possible "work-around".   
      Since they have a  "clever scheme to make each line come out at a   
   different RF frequency", have them not harmonically related (like the   
   DTMF frequencies as used by Ma Bell).   
      Maybe there is really no such thing as "inherently evenly spaced";   
   that it just looks that way..   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)