[Ronja] Vacuum tube in Ronja preamplifier

[Karel Kulhavy]

On Sun, Oct 14, 2007 at 03:25:18PM +0200, Gullik Webjorn wrote:
> Clock,
> 
> 
> >From the Ronja specifications
> 
> Power out 17mW
> Divergence: 1.9 mRad
> 
> Assume: 130 mm lupe, 1400m distance
> 
> If I calculate received power from this, I get
> 
> Spot size = tan(1.9e-3) * 1400 m = 2.66 m
> 
> Receive lens = 0.065^2 * PI = 0.0132 m2
> 
> Rec power = 0.017 W * 0.0132/(2.66)^2 = 31 uW = -45 dBW = -15 dBm
> 
> To me this seems like a lot of power, more than 10-15 dB above
> what is specified for Fiber Optic receivers.
> 
> I guess you have allowed for fade margin, but, is Ronja really noise
> limited?

yes every transmission system if designed properly is noise limited.

> 
> Do you agree with my calculations, or did I miss something?

No the spot calculation assumes that the radiated pattern is a sharp uniform
circle which is not true.

it also ignores 4 glass-air interfaces which reflect power 5% each.
It also ignores the loss of power in the glass itself which is not negligible
in these window-glass Chinese lenses.
It also assumes that the receiver lens is able to concentrate all incoming
light onto the detector which is not true.
Fibre optic system doesn't have ambient light from the sun with the associated
strong noise source.

CL<

> 
> Regards,
> 
> Gullik
> 
> 
> 
> 
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