[Laser] fundamentals

[James Whitfield]

Glenn

I think that you are asserting that for the same transmitted signal that
there is no benefit to one audio carrier frequency over another.   I am
pretty fuzzy about this point because it seems that others in this group
said there is.  Maybe we have been talking apples and oranges, and I just
could not keep track.


To (hopefully) clear things up I would like to get comments about the
following two scenarios:

1.   A laser is amplitude modulated with an audio tone ( could be sine wave,
but if it is simpler to build assume 50 percent duty cycle square wave ).
The tone is also modulated, chose either MCW or BPSK and a data rate so that
any change of the data always occurs on the rising edge of the tone.  ( This
prevents a spike if the data changed in the middle of a tone pulse. )  The
tone frequency is approximately 300 Hz.

2.  Set up an identical system except that the tone frequency is five times
higher.


Both signals are received on the same equipment and fed into a computer
sound card for processing.  The receiving equipment does not attenuate or
amplify one of the frequencies more than the other.


Now does the signal using the 300 Hz tone have an advantage over the signal
using the 1500 Hz tone for being used for communication?

I remember trying to suggest that I though information theory suggests that
the two have the same data bandwidth and should therefore be processable
with filters for equivalent SNR.  Seems to me it was suggested that the AM
signals had 600 and 3000 Hz bandwidth and would necessarily have five times
the noise in the wider signal, and further that no audio processing could
remove the noise.  If the discussion had included a caveat that there was a
filter between the detector and the sound card, I missed it completely.

I took from the the discussion that there is a mantra for optical
communication: low frequency carrier tones are better than higher frequency
tones.


James
N5GUI