[time-nuts] Rb as source for ADEV?

Ed Palmer ed_palmer at sasktel.net
Thu Feb 6 22:21:31 EST 2014


Hi Hal,

More offset is better, but the actual amount is irrelevent. It's easier 
to process the difference frequency (i.e. 1 Hz or 10 Hz or whatever) if 
it's higher. The problem is that low frequency means low slew rate which 
means trigger noise that will be interpreted as jitter which will mess 
up your results.

However, it's not easy to find high quality oscillators that are 
slightly offset from the standard frequencies. For some technologies you 
may not be able to move the frequencies by any significant amount. You 
can use a synthesizer to generate an offset frequency, but that has to 
be done carefully to ensure that the synthesizer doesn't inject a bunch 
of noise into the measurement.

The big boys tend to avoid these issues by using a DMTD (Dual Mixer, 
Time Difference) method, but that's not a reasonable solution for a 
beginner. A typical progression for a new time nut is to start with a 
TIC (Time Interval Counter) and make measurements as described earlier 
in this thread. Maybe upgrade your TIC once or twice. Then, as the time 
nut infection settles into your bones and soul, move on to the mixer and 
eventually to the DMTD as you make measurements at lower and lower 
levels. You also tend to upgrade your references on a more or less 
continuous basis.

Speaking of the infection settling into your bones, a few months ago I 
thought my pulse was oddly low. I looked over at my WWVB analog clock 
and found that not only was my pulse 60 beats per minute, but it was 
also in sync with the second hand of the clock! I've got it bad!

Ed

On 2/6/2014 8:47 PM, Hal Murray wrote:
> Bob Camp <lists at rtty.us>:
>> Offset the local reference one hertz or so and let the 5334 do it’s thing.
> Any hints on how to get a good oscillator that's off by 1 Hz from 10 MHz?
>
> How did you pick 1 Hz?  Is 10 Hz offset better?  How about 0.1 Hz?
>
>




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