[RTTY] Spectrum of RTTY signal

[Bruce Lifter, WT4I rr]

I did my best to take Chen's figure and overlay one of INRAD's
250Hz filters. (I did a screen capture dumped it into paint and then played
"connect the dots" with the data extrapolated from the INRAD graph.)

Here is the link to the filter:
http://www.qth.com/inrad/graphs/186.gif

Here is the link to Chen's figure overlayed with the filter:
http://www.wt4i.com/Demo/250HzFilter2.gif

73, Bruce



> -----Original Message-----
> From: rtty-admin@contesting.com [mailto:rtty-admin@contesting.com]On
> Behalf Of Kok Chen
> Sent: Wednesday, August 29, 2001 11:56 AM
> To: rtty@contesting.com
> Subject: [RTTY] Spectrum of RTTY signal
> 
> 
> Hi Folks,
> 
> I have the spectrum of an RTTY signal posted at
> 
> http://homepage.mac.com/chen/.Public/RTTY/fskspec.pdf
> 
> This is an Acrobat (pdf) file.
> 
> A 45.45 baud, 170 Hz shift signal with 32 uniformly distributed
> random bits is generated, and a Hamming windowed FFT is taken.
> 
> The FSK is generated in a phase continuous manner (the carrier
> phase angle at the beginning of a bit is set to the phase angle
> at the end of the previous bit).
> 
> [i.e., the FSK time waveform is continuous, although the derivative
> of the waveform is not necessarily continuous]
> 
> There is no shaping of the data bits, data transitions are
> instantaneous. 
> 
> 4,000 of these spectra are computed and averaged, equivalent
> to something like 45 minutes of real time signals (but only took
> a couple of seconds of computation on a current portable computer).
> 
> The vertical axis of the plot is on a square root amplitude scale.
> This is so that the sidebands show up well.  A linear power scale
> won't show them at all, as you can imagine, and with a linear
> scale, the details are not clear.  A traditional log plot would
> have overwhelming sidebands.  So, I settled for square root
> amplitude plot.
> 
> The tick marks on the horizontal scale are at 10 Hz intervals.
> 
> The vertical dotted lines indicate the 250 Hz bandwidth limits.
> 
> 
> 73
> 
> Chen, AA6TY