[Laser] PSK31 via light - up-convert to RF vs audio amp

[TWOSIG at aol.com]

Kerry
 
Sounds like you have more to tell about the PSK31 on light beacon.
 
In your "squared up" drive on the laser for PSK31, I am wondering if you  
have looked at the received signal on a spectrum analyzer.  And if so,  could you 
see harmonics of the audio frequency.  
 
I am thinking that the square wave should have generated lots of harmonics,  
both of the carrier (audio) frequency and at the frequency of the data rate (  
31.25 Hz ).  The third harmonic should be about 1/3 the power, fifth at  1/5, 
seventh at 1/7, and so on.  I think I have seen harmonics of the data  rate 
above and below, on an 80M signal that was overdriven.  That was  probably 
clean out of the computer sound card, but probably had too  much mike audio gain 
inside the transceiver.
 
If the harmonics are visible, I wonder if they can be decoded?  Would  the 
band width be multiplied by the harmonic number, or would they just be  
multiples of the audio carrier that are modulated at the same rate as the  fundamental?
 
As a side comment which might be useful with light communications, the BPSK  
signal could be produced with a two input Exclusive OR gate.  One input has  
the carrier, which will work equally well as a direct RF carrier or an audio  
frequency carrier that must then be modulated onto RF, or light for our  
systems.  The other input has the data stream.  The gate output  will  act as a 
buffer for data input "zero" and as an inverter when the  data input is "one".  In 
other words the phase is zero or 180  degrees.  If the data transitions are 
not synchronized to the carrier  transistions, there will be glitches.  In the 
PSK31 we are familiar with  from sound cards make these glitches very small, 
probably below the noise  level.  With, or without, the glitches the spectrum 
of the squared  signal should have lots of harmonic content.  The harmonics of 
an RF  carrier would be fairly easy to filter out, but the harmonics of the 
data rate  are not, so the system is not good if you need to conserve bandwidth. 
 For  light systems, the wider bandwidth should not be a problem, unless you 
are  trying to put multiple signals on one beam of light.
 
I have have wondered if the harmonic content could be used as redundant  
sources of data on a weak signal.  Perhaps it is just interesting, but not  of any 
practical value. 
 
Thank you.
 
James
N5GUI
 
 
 
In a message dated 7/31/2006 10:49:29 AM Central Standard Time,  
kbanke at qualcomm.com writes:


Just  as a note - My  simple PSK31 laser transmitter is not linear and  
works well enough.  I basically take the audio output from a laptop  
computer ( or more recently a $6 MP3 player for a beacon)    square up 
the signal and drive the laser  diode full  on/off.
- Kerry  -