[time-nuts] Another David Partridge frequency divider question

Charles P. Steinmetz charles_steinmetz at lavabit.com
Thu May 9 11:08:01 EDT 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,


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