[time-nuts] Distribution amps and slew rate
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Sat Nov 3 23:39:38 UTC 2012
david wrote:
>Given that slew rate is so critical, why do we distribute sine waves
>and perform the zero-crossing detection at every target instrument?
Magnus made some good points in response to your question. To
elaborate a bit: it is much easier to provide a friendly transmission
environment for a sine wave (single frequency), and sine waves are
less sensitive to imperfections in the transmission environment
(impedance discontinuities and mismatches, noise ingress,
etc.). Reflections in the transmission environment will put funny
steps in what started life as clean square waves or pulses, and
differential phase shifts will also mis-shape square waves or
pulses. This can even be a problem with sine waves -- see, for
example, the NIST paper on the timing effects of distortion in sine
wave sources for an example of the sensitivity of sine wave systems
to harmonics (Walls and Ascarrunz, The Effect of Harmonic Distortion
on Phase Errors in Frequency Distribution and Synthesis) -- but it is
much worse with square waves or pulses.
Sine wave systems are also much less prone to radiating
noise. Anyone who operates one or more frequency standards as well
as sensitive RF receivers can testify that sine waves are much less
of a hassle.
Best regards,
Charles
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