[time-nuts] Effects of noise on EFC line? - Resolved

Bob Camp kb8tq at n1k.org
Thu Aug 7 19:11:25 EDT 2014


Hi

Your EFC line is probably bypassed internally to the OCXO. A 3db modulation bandwidth beyond 1 KHz is unlikely. A modulation bandwidth below 100 Hz is quite possible. 

Next thing to consider is that the EFC does FM on the OCXO. Phase noise is PM modulation. FM is 1/Fmod relative to PM. If I go up a decade in frequency with constant FM, my PM sideband will go down by 20 db. Yes that’s for small modulation indexes. That’s very likely the case if we are dealing with noise. 

You can calculate exactly what PM sideband in dbc you will get from a 1 Hz tone at 1 mV p-p on your EFC. From that you can pretty quickly work out what this or that number of microvolts will do at this or that frequency.  The answer is normally that the noise you have from a reasonable regulator or op-amp isn’t a big deal. 

Bob


On Aug 7, 2014, at 6:58 PM, Hal Murray <hmurray at megapathdsl.net> wrote:

> 
> bob at evoria.net said:
>> So, I may throw another cap on it, but it seems to be clean down to what I
>> can measure at the OCXO on my old Tek 455 with an X10 probe. 
> 
> Another thing to consider when chasing that sort of problem: How much are you 
> picking up with your scope probe and/or its ground wire?
> 
> The classic home brew better-probe is to use a chunk of 50 ohm coax feeding 
> into scope with a 50 ohm terminator.  (You get 2 of them if you cut a readily 
> available connectorized cable in half.)  Insert 950 ohms at the probe end to 
> get higher input impedance at the cost of a 20:1 divider.
>  http://www.signalintegrity.com/Pubs/straight/probes.htm
> 
> It usually requires soldering at the DUT.  For me, it also requires a mental 
> shift.  You have to think of your scope probe as a consumable rather than 
> something that won't wear out if you take care of it.  A chunk of coax will 
> last a long time.  Just cut off another 1/2 inch when the end gets too 
> mangled.
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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