[Laser] Ramsey Kit range
Andrew T. Flowers, K0SM
[email protected]
Tue, 11 Nov 2003 21:50:40 -0500
James,
Sounds like you should be able to put something like a K3PGP front end
or possibly the OPT310 diode/amp configuration into the LPF in the
receiver. This should give you the oportunity to use a PD with a
significant acitive area which should allow you to use all of the light
captured by your lens. You'll be able to keep the transmitter as-is,
which gives you the luxury of using voice--not the easiest thing in the
world to put on a laser. If you're clever, you could make the PGP
front-end detachable to use with a different baseband RX or to plug into
your soundcard should you ever want to do that....come to think of it,
I'll give that a go here tonight if I get time....You won't get the
greatest "weak signal" performance out of this thing, but I don't think
anyone has missed LOS laser Q's because the signal wasn't strong enough,
and it may be good enough with a large lens to work "bounce paths" off
of objects. Lots of potential fun here :-)
Andy
K0SM
[email protected] wrote:
>Thank you all for the many comments.
>
>Allow me to give some more details on the LBC6. The receiver consists of a
>phototransistor with a 1M collector resistor capacitively coupled to an
>inverter amp with a gain of 2 and a tensy bit of capacitive feedback for stability,
>then to a sharp cut off 6kHz low pass filter (the identical LPF they use on the
>transmitter) to a pot for gain control and an audio amp (LM386). This is a
>simple light sensor with DC blocking and filtering above the desired audio
>band. The receiver has no trouble "hearing" amplitude modulated light, such as my
>700-800Hz pulse laser circuit or the hum from a farm light.
>
>The transmitter starts with an electret mike and an amp with a gain of 100,
>the afore mentioned LPF, to a microcontroler chip. I can't say from the
>circuit diagram, but the manual says that the chip provides automatic gain control,
>pulse width modulation(The advertisements say 16kHz, the cover of the manual
>says 20kHz, and the text of the manual says 18kHz. I have not checked. It
>works.), and a test tone function, as well as driving a BS170 FET, which turns
>the laser pointer on and off.
>
>There are several places you can break into the circuits. For the receiver,
>I would think that you would want to build an entirely separate light sensor
>circuit, mounted to the optics, an use the LBC6 receiver for what it is, a low
>pass filter and audio amp. The transmitter is probably limited in drive
>current, but it works, for that laser pointer. I have a circuit using two 555
>timers that I think is a lot cheaper and easier to modify, but I haven't tested it
>yet. The LBC6 has what I think is a good mike, amp, and LPF.
>
>In short, the LBC6 does what it claims, and might be souped up. I don't
>think I would recommend it for someone that planned to mod the transmitter.
>
>When I built the units, made a few "improvements" like changing the TX power
>switch from push-on-pull-off to pull-on-push-off to reduce the hazard of
>draining the battery if the switch is accidentally bumped. If I were going to
>build another one, I would consider using the power switch to change from tone
>mode to mike input, and use a power cord to external batteries. I was really
>annoyed when the RX combination audio gain pot and power switch did not fit in
>the mounting holes - I have the tools and experience to modify the circuit
>board or the switch but others may not.
>
>Both the TX and RX units are mounted in 2" plastic pipe (which the manual
>says they are sized for, but does not give even a hint of how to do it. It took
>longer to work out the first installation with cut and try than it took me to
>solder all 4 units.
>
>I have 50mm and 100mm lenses mounted repectively in 2" and 4" plastic pipe,
>with black paper to reduce stray light. The RX unit will fit to the pipes, but
>I have not attempted focusing. Work on the optics and electronics were put
>on hold while I work on two table mounts. They will stand about 4 feet high on
>three 2X4 legs with an 18" diameter "table" made of kitchen counter top
>pressed board with a hole in the center for 1/4" bolt for an azimuth axle. The
>optics and electronics will mount on a board 18" X 48" which is made from vinyl
>clad pressed board. The "board" rests on the "table" with a disk of felt as a
>separator for smooth rotation 360 degrees. The three legs use 3/4" bolts to
>adjust the height individually. Once the "board" is pointed close to the
>target, it is clamped. The optics then are mounted to the "board" with a fine
>adjustment mechanism which I have not worked out yet. I plan to use an 8-32
>threaded rod and a pivot distance of at least 31.25" for on milliradian angle
>change per revolution of the rod.
>
>I think I better close for the night.
>
>James
>
>
>
>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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