[Laser] Ramsey Kit range

Sun, 09 Nov 2003 20:42:38 -0500

John,

You are right, but the Ramsey RX runs into some special IC before it 
goes on to the LP filtering, which makes
me suspicious of what is really going on.   Like most folks, I just put 
the parts in the board and soldered it
together, not questioning the magic of the black box.  There is a lack 
of buzz with the lights, which also makes
me think there is more too it, but that might be because the thing is 
absolutely deaf relative to the
PD-based receivers I've built.  3KHz B/W will be a much better S/N than 
100Khz for sure....

Andy K0SM/2

John Matz wrote:

>I just thought I'd mention ... if it really is true PWM, not on a subcarrier
>... then the bandwidth need only be 3 kHz on the receive end.  Recovery is
>simply lowpassing the PWM transmission.
>John Matz KB9II
>----- Original Message -----
>From: "Andrew T. Flowers, K0SM" <[email protected]>
>To: <[email protected]>
>Sent: Sunday, November 09, 2003 7:01 PM
>Subject: Re: [Laser] Ramsey Kit range
>
>
>>This reminds me,
>>
>>It seems there have been several folks who have put together these
>>ramsey kits.  In fact, Dave and I have had the pleasure of using these
>>kits in a radio contest or two.  Does someone want to find a way to "hot
>>rod" these things to get a little better performance out of them?  It
>>seems to me that someone could come up with a "front end" that beats the
>>heck out of the phototrasistor that comes with it, keping in mind that
>>it needs to have a respose up to over 100KHz or so (it's 18KHz PWM, and
>>you need to resolve the waveform fairly accurately I think).  It's
>>pretty clear to me that most of the light is getting thrown away due to
>>the teeny-tiny active area of those devices.  If someone wants to look
>>at this I can show you a schematic what the RX looks like.  It might be
>>a worth doing since many folks seem to be getting interested in laser
>>comm stuff through these kits...and it's also in the a "ham spirit" to
>>figure out how to make a good thing better....
>>
>>Andy
>>K0SM/2, Rochester NY
>>
>>David D. Rea wrote:
>>
>>>On Sun, 2003-11-09 at 15:20, [email protected] wrote:
>>>
>>>>On the other hand, the area of beam, or the power in it, may not be
>>>>
>descibed
>
>>>>by a mathematical relation to the square of the distance.  For "short
>>>>disatances" the intensity of the received beam may be linearly
>>>>
>proportional to these
>
>>>>"short" distance numbers.  If that is the case then the power on the
>>>>
>detector
>
>>>>would be proportional to the area of the lens, so that the useful range
>>>>
>is
>
>>>>factored by the square of the diameter ratio of the lens to the
>>>>
>detector.
>
>>>>Both ways of thinking about range seem to be oversimplification of the
>>>>
>"real"
>
>>>>world, but is either a practical estimating tool?
>>>>
>>>Hi James-
>>>
>>>You're fairly close to on-target with your theory. The received power
>>>will fall off with the square of the distance, but this only occurs in
>>>freespace. Keep in mind that you've got atmospheric nasties to deal with
>>>as well - after you get above 1 Km or so, you'll start noticing the
>>>effects of humidity and purturbations in the air; i.e. you'll lose power
>>>due to absorption and dispersion of water mollecules, and you'll see a
>>>"shimmering" effect as the beam traverses different thermal planes
>>>between the transmitter and the receiver.
>>>
>>>There has been quite a bit of work done in this subject, as I found out
>>>when writing a paper on laser communication during an undergrad
>>>independent study. You can get as deep in the math as you want; there's
>>>no shortage of folks out there who have written PhD theses on this
>>>stuff...
>>>
>>>To add a layer of complexity, remember that no lens is perfect, either.
>>>If you're dealing with a nice AR multi-coated glass lens, maybe you'll
>>>throw away 5% of your signal (if you're lucky). But if you're using a
>>>less expensive (but MUCH larger area) fresnel lens, plan on tossing
>>>about 50% of the inbound light back toward the receiver. This is where
>>>you compensate for reflection with sheer lens size...
>>>
>>>Anyway - hope this helps a bit...
>>>
>>>73 de Dave K2THZ
>>>
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>>
>>
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