[Laser] Ramsey Kit Modes

[James Whitfield]

I would like to add a comment to the exchange between Les and Art.  Specifically from this quote by Art:

" I DO NOT share your optimism regarding the suitability of the PGP front end with the Ramsey system. Basically, the Ramsey system needs a 12 KHz receiver bandwidth in order to pass the pulses that come from the distant transmitter. With the PGP front end, you will peak in the 20 Hz to 100 Hz range, depending on the type of photodiode you use. "  

I concur that the "standard K3PGP" circuit would not be a suitable front end to a Ramsey receiver.  What I disagree with is the assertion that such a front end needs 12 KHz bandwidth or that it need pass the pulses.  I believe there have been adaptations of the K3PGP circuit sufficient for voice communications.

While it is true that the Ramsey transmitter is PWM, the optical detector of the receiver needs only to respond to the lower frequency component of the beam it intercepts.  The modulation of the transmitter has a pulse rate that is not well controlled, but is in the range of 16 KHz to 24 KHz.  It could just as easily be in the 100 KHz range or the MHz range for that matter, because the receiver does nothing with the pulses.  The photo detector serves as an integrator for the period of the pulse, which results in the modulated signal being recovered.  If there is any of the pulse frequency spectrum that is passed through from the front end, it is elimiated by the band pass filter that immediately follows.

The device has no pulse processing.  I have been using my Ramsey receivers for more than five years to catch signals from amplitude modulated LED transmitters.  PWM of a laser pointer and AM of an LED are completely in-distinguishable at the Ramsey receivers so long as they have equivalent signal strength.  I can readily identify the difference between the Ramsey transmitter and my own AM LED transmitter, but that is because the Ramsey has automatic gain circuitry which my design does not.  When the user is not talking, this will cause the background noise to increase to a level that disturbs me, so I very deliberately decided to go with a manually set gain.  It was also much easier to build.

As for the bandwidth needed to use a Ramsey receiver, its band pass filter will chop off anything above 6 KHz, but that is not the design factor that should be used.  If your system does not need to handle 6KHz, don't bother to design a front end that can handle it.  Amateur radio has operated successfully with 300 to 2700 Hz voice bandwidth.  I have even seen settings of 500 to 2400 Hz.  Decide what you need, and build accordingly.  If you don't need voice, decide what you need.  If you want to send PSK31, you might get by with 200 to 300 Hz.  If you want MCW, you might even consider building a front end with its bandpass around the tone frequency you intend to use.  If that is 700 Hz, then 650 to 750 Hz should do.

If we were dealing with a radio receiver, or for that matter a theoretical communications channel, the "bandwidth" of the system might need to be twice the highest modulating frequency, since after all we are using a rather classic AM scheme.  However, once the light energy, and I will not get into the debate whether it is waves or particles, reaches the light sensor causing an electron to get knocked loose, we are dealing with the frequency content and power distribution that results inside the sensor.  Any optical phase or frequency information that was in the beam is gone, that is, unless someone is experimenting with equipment very different from Ramsey or K3PGP.

I would like to make one additional comment that relates to the Ramsey receiver.  Its standard front end is a phototransistor.  This is quite adequate for the bandwidth of a voice capable system.  I have read and heard a lot of criticism of phototransistors in a light communication system because they simply cannot respond as fast as a photodiode.  Whatever other short comings they may have, the choice between using a phototransistor and a photodiode for a voice frequency range system should not be made because the phototransistor is "too slow".

James
 n5gui