On Oct 14, 2013, at 4:57 PM, Jim W7RY wrote:
> With all of this flying around, I was wondering if we are actually
> taking/making the "measurements" with accurate "test equipment".
Keep in mind that there are multiple issues with FSK timing -- one is with Stop
bits that are not 22ms, 33ms or 44ms. For that, the 2Tone stop bit display
reports a reasonably accurate stop bit duration as long as the data and start
bits average out to be 22ms each.
A different timing issue has to do with jitter (both data bit jitter and stop
bit jitter). Since 2Tone aggregates the start and data bits, bit jitter (such
as from EXTFSK) will not show up in the 2Tone "stop bit" display.
To see jitter, you need a scope that is synch'ed to the character frame, or
some equivalent instrumentation in the software modem to see the data "eye" --
see the last three figures here for one example of such instrumentation:
http://www.w7ay.net/site/Technical/RTTY%20Sidebands/sidebands.html
When there is jitter, the zero crossings will move around from one character
frame to the next.
(Notice that if the transmission uses 45.0 baud instead of 45.45 baud, 2Tone
will "measure" a stop bit that is stretched by a little more than 1 ms, when it
really has stretched only by about a quarter to half of a millisecond).
73
Chen, W7AY
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