[time-nuts] Another David Partridge frequency divider question

Azelio Boriani azelio.boriani at screen.it
Thu May 9 16:06:47 EDT 2013

Yes, it is best to check the type of input before connecting the
reference. Even if the manual is missing maybe a visual inspection can
reveal if the input is AC coupled or not and the possible input level.

On Thu, May 9, 2013 at 5:08 PM, Charles P. Steinmetz
<charles_steinmetz at lavabit.com> wrote:
> 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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