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LF: Re: CMSK Beta released

from [James Moritz] [Permanent Link][Original]
To: <[email protected]>
Subject: LF: Re: CMSK Beta released
From: "James Moritz" <[email protected]>
Date: Fri, 6 Aug 2010 18:52:12 +0100
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Dear Andy, LF Group,

G4JNT wrote:

True theoretical proper MSK is rare.  Many of the LF submarine broadcasts
are a superb example, and if you get the chance, capture one of these, mix
down to zero frequency and display on an X-Y vector scope. If you can slow 
the plotting rate right down, watch how the vector rotates round first in
one direction for one data polarity, then back again for the other, speeding 
up and slowing down its rotation rate.  Its a rare sight to behold :-)
The Old GBR signal on 16kHz now defunct was fascinating to watch like
this..
Years ago, I made such a "vectorscope" for looking at signals in the LF range and below - the schematic can still be found at http://www.wireless.org.uk/XY.htm. It is a simple I/Q down-converter which mixes the RX audio output with a stable carrier at whatever the BFO offset is, effectively removing the carrier frequency, and the RC filtered I/Q baseband outputs are then fed into the X and Y inputs of an oscilloscope. It was interesting to look at different types of modulated signal with this device - a CW carrier gave a spot on the screen that slowly orbited around at the difference frequency if the carriers were not exactly matched. The BBC R4 carrier could be seen to be phase modulated, with the spot wobbling to and fro around the arc of a circle. BPSK and FSK signals made their own distinctive displays. As Andy says, the MSK signals were interesting to watch - made your eyes go funny after a while. Quite educational ;-) 

Cheers, Jim Moritz
73 de M0BMU
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