LF: Re: CMSK Beta released

["James Moritz" <james.moritz@btopenworld.com>]

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