[Laser] Ramsey Kit Modes

[C. Turner]

With reference to the ongoing discussion, I think that there's a 
disconnect in the thinking, when it comes to suitability of the "PGP" 
receiver and the Ramsey transmitter - or any PWM-based transmitter.

First of all, I would echo what James has said:  The frequency response 
of the front end is irrelevant with respect to the reception of the 
speech, as the integration of the received signal (largely by virtue of 
the frequency response of the detector) recovers the modulation.  As an 
analogy, think along the lines of an old-fashioned diode-type envelope 
detector for AM.

If one uses the 'PGP receiver, one must take into account that it has a 
6dB/octave rolloff and the implications of this for the various modes.  
For speech, this would result in reduced intelligibility, but for 
computer-based modes, such a response curve may be irrelevant - provided 
that the output level at the frequency of interest is sufficient to 
overcome the computer's sound card's noise floor.  (Some modes may balk 
at having a non-flat spectrum input to them, but that's probably not 
important for PSK31 or other single-frequency narrowband modes.)

***

As for the use of the phototransistor on the Ramsey kit, I see two major 
problems with that:

- For very low light levels, phototransistors aren't very good as their 
leakage noise quickly overrides the photonic signals:  Even though they 
are self-amplifying and produce larger signals than photodiodes, the 
resulting signal-noise ratio (which is the important part) is inferior.  
As for their frequency response - for speech applications, they are fine 
- as far as they work.

- The Ramsey manual only mentions the use of lenses in passing but 
should have gone farther in extolling their virtues - that is, 
essentially noise-free signal "amplification!"  Unfortunately, the 
packaging of typical phototransistors makes their efficient use with 
lenses rather difficult - although even very poor coupling of a fairly 
small (magnifying-glass sized) lens into the phototransistor provides a 
radical improvement, which shows how bad the "raw" phototransistor 
really is!

Had I a Ramsey kit that I was trying to use for this sort of thing, I 
believe that the most useful part of it would be the transmitter - 
which, despite some serious flaws, is quite usable.  The receiver, 
however, should at most be considered to be a "starting" point, 
discarding the front-end section and using only the lowpass/amplifier 
sections.

***

Finally, here's an test that one can perform:  Point the receiver at the 
planet Jupiter or Saturn - or even Mars when it's not near conjunction:  
If it doesn't easily detect a lot of hiss, then it's not working well at 
all - either due to the lack of optical gain (poor lens system) or just 
plain "deafness."  For reference, a K3PGP receiver with a modest-sized 
Fresnel (8"x11") will be pretty-much saturated if you do this test and 
it can readily detect hiss from about any "city-visible" star.

73,

Clint
KA7OEI