[GreenKeys] Wobbly FSK ?

[Richard Knoppow]

----- Original Message ----- 
From: "Nick England" <navy.radio at gmail.com>
To: "Greenkeys" <greenkeys at mailman.qth.net>
Sent: Tuesday, August 02, 2011 12:43 PM
Subject: [GreenKeys] Wobbly FSK ?


>I recently added some info to my site on the KY-30/GRT, an 
>early Navy
> FSK keyer designed to take RF from a transmitter's 
> oscillator, mix it
> with an FSK signal, and send it back to the transmitter 
> for
> amplification.
> http://www.navy-radio.com/rtty/ky30grt.htm
>
> A description of the TTY input keying voltage which 
> controls the
> keyer's local oscillator says "It may also be wobbled at 
> about 200
> cycles per second by turning on the PHASE MODULATION 
> switch. The
> amount of phase modulation is varied by adjusting an 
> associated dial.
> Phase modulation of the transmission may tend to improve 
> reception
> under some conditions of severe selective fading."
>
> Was this wobbly phase modulation a common thing back in 
> the day?
>
> cheers,
> Nick K4NYW
> www.navy-radio.com

     I guess that there is a technical paper on this burried 
away somewhere. I think the idea is to broaden out each mark 
and space pulse so as to make it less likely it will be 
completely lost by selective fading. As I am sure you know 
selective fading results when a signal is received via two 
paths of nearly the same length so that they cause a comb 
filter effect at the receiver nulling out some frequency. 
Usually, the path lengths are not very stable so the exact 
frequency of null moves around plus one or both signals may 
have regular fading, that is a change in stength over a wide 
band. By phase modulating the pulse at a fairly high rate, 
fast, that is, in comparison to its duration, the spectum is 
widened somewhat which makes it less likely the entire 
signal will be lost in a null. It is, in effect, a sort of 
frequency diversity.  I have no idea of how well this works. 
I have only the vaguest memory of reading something about 
the theory long ago.
     BTW, I fell into your web site last night, what an 
amazing collection of material:-)


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com