[time-nuts] Oh dear

Jim Lux jimlux at earthlink.net
Mon May 7 15:11:53 UTC 2012


On 5/7/12 7:39 AM, Burt I. Weiner wrote:
> A friend of mine signed me up for a catalog from the "Audio Advisor". He
> said I deserved this - I was afraid to ask what he meant by that! Spend
> a few minutes looking over this site: http://www.audioadvisor.com/ Be
> sure to check out their "Power cords" at:
> http://home-audio.audioadvisor.com/search?w=Power+Cords
>
> Burt, K6OQK
>
>

Well.. this is where folks on this list can do the world a service..

The whole thing about timing, stability, phase noise, Allan deviations, 
etc. *is* complex, and it's tricky to come up with easy to understand, 
short, descriptions of "why using a Rb for your CD player is BS".

We've all had to learn this stuff, and we do it in different ways, so 
maybe the collective hive-mind is a good way to come up with decent 
responses (after the initial wave of "can you believe it")

It's like explaining RF exposure limits.  There's a certain amount of 
physics you have to know in order to understand how the limits work.

Most people do understand what's BS and what's not, once they understand 
why.

-> the recent GPS filtering thing.. it took a YEAR for someone in the 
PNT community to finally come up with a good, simple explanation of why 
L^2 arguments were invalid.  And it comes down to the fact that GPS 
isn't a communication link, so you can't use that conceptual model to 
analyze it.  Once you get that, then people go "oh! That's why we can't 
do that and have it still work"


And, on a more technically sophisticated level, there's lots of 
engineers who are still wrapping their heads around the duality of time 
domain (ADEV) and frequency domain (Phase noise) measurements, and when 
you might use one or the other.  I've found a lot of good stuff on this 
list for explaining it (and improving my own understanding.. nothing 
like needing to explain it to someone else to test your own conceptual 
understanding)

Interestingly, setting someone up with a counter, timelab, and a not so 
hot function generator and letting them record and play for a couple 
days (or over the weekend) is a great way. You see things like diurnal 
variation, the HVAC cycling on and off, the sun shining through the window.

The spectrum analyzer does the phase noise thing fairly well (although 
not for "close in"), and concepts like reciprocal mixing from a noisy LO 
gunking up your narrow band signal are pretty obvious.

After that it's practical applications..

Just how bad can the noise be for a particular application?  Are you 
interested in integrated jitter?




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