[time-nuts] Another David Partridge frequency divider question
Charles P. Steinmetz
charles_steinmetz at lavabit.com
Thu May 9 15:08:01 UTC 2013
Azelio wrote:
>Usually the external reference input of measuring equipment is
>internally squared, so if you feed a square or sine wave there should
>be no difference.
Some external reference inputs may be DC coupled and zero-switching,
in which case unipolar logic levels will not work well. I don't know
of any specific equipment to which this may apply, but if you try it
and it doesn't work well it is one thing to consider. An external
coupling cap (or AC coupling the output internal to divider) should fix it.
Also, 74AC logic level (~ 5 Vpp) may be a little hot for some
reference inputs. Many reference inputs present a load impedance
higher than 50 ohms to the source (often ~ 1k ohm), so you may not
get the divide-by-2 attenuation from the open-circuit voltage that
you expect in a matched system. In this case, an external 50 ohm
termination will reduce the level to ~ 2.5 Vpp.
Best regards,
Charles
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