[time-nuts] Generating a solid PPS from 10Mhz source

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Jan 19 12:55:16 EST 2016


On Monday, January 18, 2016 08:45:20 PM you wrote:
> --------
> 
> In message <29659871.S9XTlaFu4r at linux>, Bruce Griffiths writes:
> >To detect 100Hz modulation due to photocurrents in the LEDs the 
150W
> >incandescent bulb had to be placed within a few cm of the LEDs.
> 
> Incandescent bulbs don't have much "hum" in their light output,
> they're basically heating elements and they don't cool down nearly
> fast enough.
> 
> Try with fluorescent light instead, there's a reason people accuse
> them of flickering.
Its actually quite difficult to find low frequency operated fluorescent lamps 
here. I only have a small 6" one (sans phosphor so strictly not fluorescent) 
I assembled some years ago as a source of the mercury green line for an 
interferometer. I'll unearth it and setup an experiment using forward 
biased LEDs. 


In any case obtaining an LED photocurrent of more than few microamp is 
very difficult. With a forward current of a few mA the resultant change in 
LED voltage will only be a few uV.


Shielding the LED to  ensure that such photocurrent induced modulation 
of the forward voltage drop is at the subnanovolt level is relatively simple.


I used to test the suitability of photomultiplier housings by checking for 
light leaks with a 1 KW incandescent lamp using the photomultiplier as the 
light detector. Labyrinth seals with nested enclosures with everything 
painted matte black usually sufficed.


Biggest problem was light leakage via the BNC/MHV connectors.


Bruce






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