Modulated Light DX


Posted on Feb 5, 2014

Which type of modulated light receiver should one use A photodiode straight into a high gain, high impedance amplifier A photodiode into a current amplifier A back-biased photodiode FET input or bipolar IC or discrete How does one minimise noise while maximising amplification Which circuit provides the best linearity of light flux input/voltage output The


Modulated Light DX
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answers are complex and conditional. For modulated light detection at visual wavelengths, the best and simplest options are silicon PIN or avalanche photodiodes for orange, red and IR; and photomultipliers for violet-blue-green. Phototransistors are far too noisy, as they amplify their own internal noise - we estimate around 15 to 20 dB worse s/n than an average Si photodiode/amp of similar sensitive area. LDR`s are generally too slow, silicon solar cells are noisy and slow, thermionic photocells are mostly too low in quantum efficiency - and if they`re gas filled they can also be too slow. This narrows down the options. The options narrow further when one considers the special requirements of atmospheric optical communication. The red end of the visual spectrum has the greatest immunity to scattering by smoke and fog, and is subject to less atmospheric turbulence scintillation than blue or violet light. Red-sensitive silicon photodiodes typically retail for between US$2 and US$30 while photomultipliers or avalanche photodiodes can cost fifty times that amount. Photomultipliers with GaAs photocathodes have good response to red light but they`re rarely available on the secondhand or surplus market. The vast majority of photomultipliers are only sensitive to violet, blue and green light (S4 and S11 photocathodes). Even with the S20 photocathode, photomultiplier quantum efficiency at the 630 nM wavelength of the red Luxeon...




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