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

[Kerry Banke]

James - I should have time to start playing again with the laser 
system next week. I'll look at the harmonic situation and will send 
you some screen captures using Spectrum lab. I'll also try decoding harmonics,
  - Kerry

At 07:22 PM 7/31/2006, you wrote:
>
>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  -
>
>
>
>_______________________________________________
>Laser mailing list
>Laser at mailman.qth.net
>http://mailman.qth.net/mailman/listinfo/laser