[Laser] 5mw laser tranceiver kit

Wed Sep 22 10:08:09 EDT 2010

Already done!

Point 1/ Clint KA7OEI has exactly that type of tone-based anti-scint option on some of his transceivers. We used it on the occasion on which we communicated across Yucca Vallay in Southern California in February 2008, when I manned one station and Clint joined Bob Legg W6QYY on the other. However it is desirable to maximise the tx and rx beam aperture to reduce scint optically, even before applying these electronic "cosmetic" systems to the signal. If the signal completely goes to zero, as it can with coherent systems where total cancellation can occur, no degree of AGC can recover anything from zero signal.

Point 2/ Our current fold-up design will employ two PhlatLight LEDs working through two co-aligned Fresnel lenses, placed either side of a receiving Fresnel. By splitting the source in this way, one achieves the maximum possible aggreagate tx aperture (spatial diversity) without recourse to separate optical units demanding separate alignment on the target. You will note by reading our account of our Tasmanian test in 2005, reproduced here:

http://www.bluehaze.com.au/modlight/

...that we indeed had TWO complete optical transceivers on Mount Barrow to receive our signals over 168 km. Listening to the signal, effectively, in stereo, with each receiver separated from the other by a couple of metres, we could easily hear that the interpretation of signals from both was more intelligeable when clouds intervened than it was in listening to either one. A metre or two is all it takes, possibly even less for effective spatial diversity.

Best wishes,

Chris Long VK3AML.

======================================

> Date: Wed, 22 Sep 2010 05:40:09 -0700
> From: toasty256 at yahoo.com
> To: laser at mailman.qth.net
> Subject: Re: [Laser] 5mw laser tranceiver kit
> 
> 
> Hi Tom
> 
> Yes, i believe there might be a couple ways to partially cancel 
> out atmospheric scintillation by using a full duplex link. I 
> think it would amount to a fast ALC at the transmitter using 
> feedback from the distant receiver "target". Or a fast AGC at 
> the receiver which uses a pilot subcarrier tone from the 
> transmitter to control it. A kind of real-time active feedback 
> of channel conditions from one station to the other.  
> 
> 1.
> The distant receiver supplies the "feedback" by sending out a 
> pilot tone, say 1 khz, with constant amplitude. While traveling 
> through the atmospheric path, the tone is amplitude modulated 
> with the attenuation that is occuring at that instant. The 
> transmitting station uses that data to "pre-adapt" its own 
> modulation amplitude according to the current conditions of the 
> channel. If the channel is fading, the transmitted audio is 
> increased. If the channel is peaking, the audio is reduced. 
> The result is an outgoing audio stream that tries to pre-
> compensate for channel conditions in real time. Since the 
> transmitter and receiver both share aproximately the same 
> atmospheric path, signals coming from either station would 
> contain nearly the same path-loss data. 
> 
> This ties up one of the duplex paths of course. On the reverse 
> communication, the roles are reversed and the other station 
> transmits the tone signal. So whichever station is listening, 
> sends out an unmodulated tone for the other station to use as 
> feedback. 
> 
> To restore the full duplex, the feedback pilot tone could be 
> moved out of the baseband, to a higher frequency where it would 
> not interfere. Like a 20 khz tone. This amounts to an unmodulated 
> subcarrier. At the transmitter where the tone is used, it is 
> detected and the low frequency envelope is used to drive the 
> transmitter ALC. So, each receiver has two channels, one for the 
> tone and the other for regular audio. Idealy if the the channel 
> fades 10 dB, the transmitter increases amplitude by the same 
> amount, 10 dB. 
> 
> 2.
> Another way to do this would be for the transmitting station to 
> send the reference tone signal piggy-backed on its own audio as 
> a subcarrier. The receiver would use this pilot tone to drive its 
> AGC, so that it's receiver gain is changed to compensate for the 
> transmitted tone scintillations (and thus the received baseband 
> audio signal since it shares the exact same path literally). So 
> the receiver station modulates its receiver gain in accordance 
> with the pilot tone variations. In this case the transmitter sends 
> plain audio that is uncompensated and the pilot subcarrier. It is 
> at the receiver where the scintillations are cancelled. This is 
> really the tradishional receiver AGC although it is not derived 
> from the base band audio but the pilot carrier or subcarrier 
> signal instead. This second method may be a bit better than the 
> first since it uses a reference tone that travels in the same 
> path through the entire atmospheric path, whereas the first 
> method is offset a little from the same path and so is a bit 
> different. A combination of both a receiver subcarrier AGC and 
> feedback ALC at the transmitter could increase the combined 
> effects. In this case both stations are supplying a reference 
> signal the other uses. 
> 
> Other ways to do this are diversity reception and actually using 
> the whole signal instead of only part of it. Diversity reception 
> amounts to using more than one small piece of the light beam. An 
> extreme case of diversity reception is: the receiver lens captures 
> the entire light beam from the transmitter, where it will be 
> averaged on its photodetector (if it is big enough). This is the 
> type of thing Yves talks about when he removes scintillation by 
> looking at the beam spot reflected by some object. The entire beam 
> spot is focused on the detector. And so most of the scintillation 
> averages out and goes away. 
> 
> Maybe this type of thing is practical to do, i dont know for sure 
> since i havent tried it. Maybe it has already been tried before by 
> those who know more about it than me. When the observitories use 
> the artificial star method, and the complex adaptive optics, they 
> are compensating for the direction the light is coming from in 2
> dimensions x-y. A third channel would be the amplitude but i think 
> the detector itself is integrating the amplitude directly (with 
> film or a ccd) and they are not worried with amplitude changes in 
> the signal for the most part.
> 
> 
> > 
> > I wonder if scintillation can be corrected, akin to a
> > terrestrial 
> > observatory using a laser to produce an artificial star
> > whose motion is 
> > removed from the objective image?  Could, for
> > instance, twin paths - or 
> > full-duplex colinear paths - be useful to remove the
> > effect?
> > 
> > Tom
> > http://www.ustream.tv/channel/bowcam
> > http://www.ustream.tv/channel/cape-coral-marine-radio
> > VHF
> > http://67.207.143.181/vlf9.m3u Lightning, spherics
> > 
> 
> 
> 
>       
> ______________________________________________________________
> Laser mailing list
> Home: http://mailman.qth.net/mailman/listinfo/laser
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Laser at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html