[Laser] MORE XE2AT PART 2

[Art]

At 08:46 PM 2/10/04, you wrote:

>>Reply-To: laser@mailman.qth.net
>>To: laser@mailman.qth.net
>>Subject: Re: [Laser] NO RX XE2AT
>>Date: Tue, 10 Feb 2004 18:15:42 -0500
>>
>>
>>Hi Art
>
>>
>>
>>
>>
>>On Tue, 10 Feb 2004 15:03:34 -0500 Art <artky1k@uninets.net> writes:
>> > Hi Al,
>> >
>> > I have a more basic question....
>> >
>> > You report both QSO partners could see each others beam.........
>> > But, you
>> > only comment on your receiving attempt and what you saw from your
>> > end of
>> > the path.
>> >
>
>
>
>> > I wonder if you can give more details please.
>> >
>> > Both stations were able to see each others laser???
>> >
>
>      YES BOTH CAN SEE THE LASER.
>
>> > Both stations were unable to hear the others signal, or was there
>> > only one
>> > attempt to receive?
>> >
>      WE ( BOTH ) MADE SEVERALS ATTEMPT TO HEARD THE LASER SIGNAL BUT BOTH 
> CANOT HEARD NOTHING, NIL LASER NIL CITY LIGHTS, JUST THE LIGHTS CLOSE TO 
> ME ( ABOUT 500 METERS) SO THE RX IS ALIVE.
>
>> > If both stations were attempting to receive, did both stations use a
>> > 4 inch
>> > lens???
>> >
>      YES BOTH HAVE 4 INCHES LENS IN A TUBE..
>
>
>
>> > ------------------------------------
>> >
>> > My guess would be that the Ramsey transceivers aren't quite up to
>> > the task
>> > of distances that great. They use a carrier to send the information
>> > on, so
>> > your receiver has to be responsive at the carrier frequency-which
>> > means you
>> > give up a VERY LARGE degree of sensitivity due to this overhead.
>> >
>> > The noise is proportional to the square of the relative bandwidth.
>> > Since
>> > your receiver has to be sensitive to the carrier frequency (which I
>> > think
>> > was 30 KHz), you have a greatly reduced ability to copy weak
>> > signals.
>> >
>> > Comparing 2 identical systems, one receiving at 30 Khz and the other
>> > at 1
>> > Khz.... The transmitter for the 30 Khz link would have to be 900
>> > times
>> > stronger to give the same quality of reception.
>> >
>> > I think your results would be very much different if you used a
>> > shielded
>> > Burr Brown opt101 receiver chip with the bandwidth set to 1 Khz by
>> > the
>> > external feedback resistor. Your transmitter would be switched at
>> > (about) 1
>> > Khz, and it's a much simpler circuit. This would send a tone only,
>> > unless
>> > you modulated the laser with audio from a microphone.
>> >
>> > ------------------------------------
>> >
>> > You did not hear any interference from the city lights because your
>> > carrier
>> > frequency was far removed from the 50 or 60 Hz noise emitted by the
>> > city light.
>> >
>> > You probably did not overload the photodiode itself, they have
>> > incredible
>> >  range! It is possible that you overloaded the receiver electronics
>> > though. I don't have a schematic for these Ramsey units. But you can
>> >
>> > improve the receivers electronics rejection of  50/60 Hz overload by
>> > using
>> > appropriately sized coupling cap. You want to insure that the
>> > receiver
>> > carrier can pass from the photodiode to teh receiver without much
>> > attenuation, but the 50/60 Hz must be attenuated.
>> >
>      DEAR ART, I USE FOR TX A K5PGP CIRCUIT FOR CW SIGNAL , I NEVER USED 
> THE VOICE DUE THE DISTANCE..
>
>
>> > Does anyone have a schematic of these units??
>> >
>> > Art
>> >


Hi Al,

James, N5GUI sent me the receiver schematic via email.

The 'LED' in the receiver IS NOT an LED. It looks like one, only because of 
the packaging! It is actually a phototransistor. The dark plastic package 
that it comes in acts as a modest sized lens, which gathers some light and 
focuses it onto the active area of the phototransistor.

However, the vary large (1 Megohm) R1 will very severely limit the 
frequency response, and my guess is that you are loosing much sensitivity 
due to the Q1. I think some minor changes to U1A and replacing Q1 with a 
photodiode would produce spectacular improvements in the sensitivity 
(without spending alot of $$ or time).

I think the phototransistor is good for demonstration purposes over short 
ranges and that you need to use something with more sensitivity for 
distances more than a few miles.

I asked James to send me the transmitter schematic. I want to be sure what 
they are sending BEFORE suggesting a modification.

Standby till I get the transmitter schematic from James.

Regards,

Art