[time-nuts] Cutler NAA on 24.0kHz....

David I. Emery die at dieconsulting.com
Fri Aug 15 16:31:45 EDT 2014


On Fri, Aug 15, 2014 at 03:09:23PM -0400, paul swed wrote:

> I had discounted it because of the lack of info and totally unclear how to
> get rid of the FSK. MSK is FSK just with a very narrow shift. Also the
> effects of the fsk on the carrier. Hmmm glad I am typing this.
> It may be that the mark or the space is indeed exactly the true carrier and
> that op amp circuit is notching out the shifted carrier. I will bet thats
> the trick!


	MSK is a kind of continuous phase differentially coded PSK in
which the carrier phase smoothly shifts either EXACTLY plus 90 degrees
or EXACTLY minus 90 degrees in EXACTLY one symbol time - in the MSK
variety of PSK the transmitted carrier phase never stays the same as the
it was the last symbol time.   When transmitting steady mark there are a
series of these shifts plus 90 degrees (hi frequency mark) one after the
other, and when transmitting steady space there is a series of minus 90
degree shifts.

	Obviously a steady negative phase shift is what is produced by
a signal below a center "carrier" frequency and a steady positive
phase shift by one above that frequency.   Thus the isomorphism with
filtered FSK.

	Effectively a QPSK (multi-arm Costas type) tracking loop should
be able to track a MSK signal just as if it was a filtered QPSK signal
with only 90 degree and minus 90 phase shifts each symbol time and generate
a phase continuous recovered carrier.





-- 
  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."



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