[Laser] RE : Re: Fundamental energy in square wave

[Garnier Yves]

James,
If both signals have the same average power (averaged
energy during one second) that means the sinus signal
has greater pick amplitude the square one.
If you only want to detect the fundamental frequency
the sinus signal will give greater pick amplitude.
(1.414 more voltage)  
If your signals have the same maximum photons flux,
the fundamental frequency will give the same amplitude
independently the sinus or square wave.
A photo detector converts a photons flux to a current
and then to a voltage.
The voltage min and max values gap defines available
electric signal amplitude because has you say each
photon always gives energy without positive and
negative contribution in the photo detector.
It is quite different with radio wave and the
properties are not the same.
For example for the same output power if you double
the distance TX RX in radio the received power
decreases by 6dB
If you double the distance in optic communication from
the optical source the photons flux decreases by 4,
the photo detector current decreases by 4 and the
detected voltage by 4...
That means the received power decreases by 16 dB!  
73 Yves F1AVY
http://f1avyopto.wifeo.com



>Does it also provide a stronger signal at the
>receiver?  If so, how much? 
>And does that translate into a benefit for detecting
>and decoding the 
>signal?


--- Charles Pooley <ckpooley at sbcglobal.net> a écrit :

> James:
> 
> For understanding of the square wave, refer to any
> math book discussing Fourier transform.
> 
> You mentioned impossibility of negative light.  Not
> so, in a sense.  If you were to use coherent
> transmission, shifting the phase 180 degrees is
> essentially negative light with respect ot the first
> phase.
> 
> But as a practical manner, the application will
> determine whether this is needed.  OOK, pulse
> position are but a few modulation schemes
> available.  For an OOK application see:
> 
>
 http://www.microlaunchers.com/7816/L3/laser/laser-link.html
> 
> Here, on/off keying is appropriate as the count of
> photons is the limiting factor and using CW LEDs
> make that the choice.
> 
> Charles Pooley             Microlaunchers
> 
> --- On Mon, 1/19/09, James Whitfield <n5gui at cox.net>
> wrote:
> From: James Whitfield <n5gui at cox.net>
> Subject: [Laser] Fundamental energy in square wave
> To: "Free Space LASER Communications"
> <laser at mailman.qth.net>
> Date: Monday, January 19, 2009, 2:58 PM
> 
> I have a question that is more math theory than
> practical experimentation.
> 
> What is the spectrum energy in a on off pulsed beam?
> 
> To explain the problem a little better, this is what
> I was doing:
> 
> I took the idea that a 50/50 square wave is
> (theoretically) made up of the 
> sum of the fundamental and all of its odd harmonics,
> each multiplied by the 
> reciprocal of its harmonic number.  To get a better
> "feel" for the
> situation 
> I wrote a spreadsheet to calculate 250 steps of a
> full cycle of a sine 
> function and to add to it the odd harmonics,
> adjusted in amplitude, to the 
> 19th.
> 
> The resulting graph looked as though it would
> converge to a square wave with 
> whose height would be about 87.63 percent of the
> height of the fundamental.
> 
> That being the case it would seem that a square wave
> would then contain the 
> spectrum energy of fundamental sine wave with an
> amplitude more than 14 
> percent higher than the measured height of the
> square wave.
> 
> Trying to relate that to light communication gets a
> little complicated since 
> what we send out is a modulated wave, which in
> effect is adding a bias to 
> the sine or square wave so that the light gets
> brighter and dimmer, but 
> never goes negative.  Even on-off keying represents
> a bias condition.  Off 
> is the negative rail, on is full brightness.
> 
> In practical terms, let's say that I have a light
> communication transmitter
> 
> with a peak instantaneous output of one watt.  I
> bias its output to half 
> power and modulate it with either a sine wave or a
> square wave, both at the 
> same frequency.  Both signals have the same average
> power, although the sine 
> wave may take a linear amplifier that is less
> efficient at controlling the 
> supplied power.  The square wave would seem to have
> an advantage in 
> simplicity and efficiency.
> 
> Does it also provide a stronger signal at the
> receiver?  If so, how much? 
> And does that translate into a benefit for detecting
> and decoding the 
> signal?
> 
> I know this is a math question.  And since I already
> got onto the subject, 
> what would be the pros and cons from a signal
> detection (at the computer 
> sound card ) of sending pulses that were not 50/50? 
> Keying ON for 1/3 of 
> the time and OFF for 2/3 would translate into lower
> average power for the 
> same peak.  So for the same average power, does that
> mean a stronger signal 
> even if the spectrum is not obvious?
> 
> 
> James
>  n5gui 
> 
> 
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