I modelled this, generating ET2 Coherent waveforms with gradual phase
transitions and stored as .WAV files, then played back into the Coherent
decoder. With a Raised Cosine phase transition slope and the ability to
adjust the amount of this from practically no slope to a full envelopes
worth of phase transition, decoding results looked quite encouraging -
the bandwidth was reduced considerably and decoding of good signals
worked 100%. Never got the opportunity to test in QRM apart from some
simple tests adding to off air received noise. Other tasks got in the
way so never perused this much further, but the major task putting me
off was to work out a way of continuously phase modulating a transmitter
over a full 360 degree range. This looked at first sight even more
difficult than amplitude modulating - had already made a PWM modulator,
although not big enough for the Decca transmitters, published in the
lastest LF Experimenters book.
Andy G4JNT
-----Original Message-----
From: Rik Strobbe [mailto:[email protected]]
Sent: 2001-03-23 09:55
To: [email protected]
Subject: LF: WOLF (BPSK) modulation
Hello Mike & group,
With Mike's suggestion to cut the signal for a short time
during the phase
tranisitions the cure might be worse than the illness :
WOLF is (currently) used at 10BPS, so you would need to cut the signal
(worst case) 10 times a second for a short time (few
milliseconds). This
will create a lot of sidebands, the signal might be worse
than with 'rude
BPSK'.
Jim's signal is remarkable clean, as could be expected from a PSK-like
signal. The 2 'disadvantages' (in my opinion) are :
- 'high power' electronics involved
- giving away 3dB (by the envelope AM), versus 'rude BPSK'
An alternative way to get a 'clean' signal might be to have a
smooth phase
transition. Instead of a 180 degrees instant phase jump one
could have a
gradual phase change, let's say over 10 milliseconds. At
10BPS this would
leave the signal for 90% of the time in the 'correct phase'.
Advantages would be :
- additional electronics would be at the 'low power end'
- any PA (wether linear or not) could be used without any changes
- signals would be only 0.54dB down versus 'rude BPSK'
Depending on the amount of sidebands that is acceptable the transision
period could be changed. A shorter transisition period would
give a better
SNR, but more sidebands while a longer transistion period would give a
cleaner signal but less SNR.
There would be different ways to implement the smooth phase
shifting, going
from the classic phase modulator to digital techniques.
But before I start any project in that direction I would like
the opinion
of the experts, maybe I have just overseen some facts that
make my whole
suggestion worthless ...
73, Rik ON7YD
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