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

Roberto Barrios rbarrioss at msn.com
Tue Feb 7 22:10:48 UTC 2012


50 ohms is a compromise between maximum power transfer and minimum
attenuation, as mentioned in page 9 of Network Analyzer Basics
(http://cp.literature.agilent.com/litweb/pdf/5965-7917E.pdf)

Regards,
Roberto EB4EQA

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Attila Kinali
Sent: martes, 07 de febrero de 2012 21:59
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Why a 10MHz sinewave output?

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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