[time-nuts] Why a 10MHz sinewave output?

Attila Kinali attila at kinali.ch
Tue Feb 7 20:59:18 UTC 2012

On Tue, 7 Feb 2012 12:19:11 -0800
Chris Albertson <albertson.chris at gmail.com> wrote:

> Related question:   Assuming I'm using 10MHz sine wave.   What's the
> best physical cable to use?  Is there any good reason to use 50 ohm
> cable?   What about 75 ohm?   I looked at a schematic of my counter
> and it looks like the 10MHz signal hits some high impedance chip
> inside.    RG6 seems like the way to go.   It's double shieled and
> lots of cable TV parts could be used.

There is no particular advantage in one or the other, at least not
for most applications. It's tradition that measurement and (most) RF
gear uses 50R, while TV and radio uses 75R. (there was once a reason
for this, but i don't know it).

When it comes to low frequency stuff (ie everything below a couple 100MHz)
i would stick with 50R cable and connectors. Cheap cables aren't too
bad for lab stuff. Although for time-nutty needs you might want to choose
the ones with better shielding.

When you go to higher frequencies (especially above 1GHz) i'd rather
use 75R sat cables + F connectors. These are available in good 
qualities at low price.

What you should not do is mix different impedances, as this will result
in energy reflected back to the source, which might or might not damage it.
But you will definitly have increased jitter due to the reflections.

			Attila Kinali
Why does it take years to find the answers to
the questions one should have asked long ago?

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