LF: Frequency Shift keying types

To:	[email protected], [email protected]
Subject:	LF: Frequency Shift keying types
From:	Andy Talbot <[email protected]>
Date:	Mon, 7 Feb 2011 23:21:54 +0000
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Wolf brought up the subject of coherent Frequency Shifting earlier...

See http://www.g4jnt.com/FSK_Types.pdf for spectral plots comparing both types of FSK. These were generated using the PIC DDS source with both software versions in turn. The waveform is made by switching between tones of 1200 and 2200Hz using 25WPM CW from a beacon keyer module to simulate random data.

The lack of any structure for the narrowest frequency resolution using phase continuous keying is very obvious. This is the more usualy type of FSK seen in practice, usually generated by altering an oscillator frequency with the input data. The disadvantage when anyone tries to receive one of the tones using ultra narrow band techniques is very obvious. The slightly higher out of band levels due to the hard switching of the coherent waveform are just about noticeable, but don't appear that much worse.

Phase coherent (or Frequency Exchange) Keying is more complicated to do in hardware, requiring two oscillators to run continuously and to select one or the other with the input data. However, if implemented using DSP probably the same complexity for either, with two simultaneous NCOs, and selecting one or the other for the tone output but removing the need to change the frequency word regularly.

Andy G4JNT

... !

One suggestion for VLFers: A PIC doesn't cost a fortune. Build two of these generators, and clock them with the same source (GPSDO or OCXO). For the FSK simply flip between both generators. This has the *big* (cannot emphasize this enough..) advantage that if the signal is to weak to be seen as DFCW etc, we can still dig it out through a long FFT because each of the two carriers remains coherent, and can be coherently averaged as long as necessary (much longer than a single DFCW symbol). I noticed this on Stefan's signal today : With the very slow settings, all you see is a rounded hump in the spectrum because the signal isn't coherent. No chance to detect it for "DX" receivers.

Of course flipping the (FSK-) switch may cause a heavy click then (depending on the momentary phase shift between the two carriers) but I think a robust PA will survive this.

Have a nice sunday,
Wolf .

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