[time-nuts] Power supply for time source concerns

[Bob kb8tq]

Hi

Filtering *is* part of all this. If you are distributing a standard around the lab, cable management
*will* be part of it as well. Having a volt p-p pop up due to an unterminated cable (of the right length)
is not at all unheard of.

Bob

> On Dec 24, 2019, at 3:48 PM, Taka Kamiya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> I just found something funny.  I have been thinking "clean" power supplies that connects to AC mains.  Then I thought, what about lead acid batteries??  So I went to my lab and took some measurement.  This is a 12V 7A lead acid sealed battery, the kind commonly found on UPS devices.
> The result?  Surprise?  The battery is oscillating at 5MHz and noise level is 15mV peak-to-peak!!!!
> Of course, not...!  Battery is pure DC and while voltage might drift, this is not that.  For the record, a charger of any kind is not hooked up.  It's one battery all by itself.  Battery is not oscillating but that's what the measurement actually shows.  That brings another point in my quest to "clean" power source.  It's not just the power supply but the whole lab eco system has to be considered.  Having one master 24V source (my original plan) is not the answer if mV level noise is going to be a problem.  
> 
> This "discovery" puts whole new layer to having a nice power supply.....
> 
> Just as a point of reference, I hooked up a common cheap float charger.  The charger itself has 2V p-p noise.  Connected to battery, it still have 100mV p-p noise.  There goes battery = noise sponge theory....
> --------------------------------------- 
> (Mr.) Taka Kamiya
> KB4EMF / ex JF2DKG
> 
> 
>    On Tuesday, December 24, 2019, 3:00:50 PM EST, Bob kb8tq <kb8tq at n1k.org> wrote:  
> 
> Hi
> 
> 
>> On Dec 24, 2019, at 6:40 AM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
>> 
>> --------
>> 
>>> That again depends on topology and control type.  The canned converters 
>>> are almost always optimized to have the lowest number of switches and 
>>> work with cheap magnetics (single coil) without easily entering 
>>> problematic operation modes, noise is only a secondary concern. 
>> 
>> That depends a LOT on which canned converter you decide to buy,
>> if you only go after price, or W/mm³ capacity, then certainly yes.
>> 
>> But for a one-off application like this, any money saved on a
>> cheap model is easily lost many times over in the trouble it will
>> cause.
>> 
>> But returning to the original post:  Has anybody ever characterized
>> how much difference it makes to use two different PSU's for heater
>> vs. electronics sides of telecom Rb's ?
> 
> The “old time” answer was that a poorly regulated / poorly filtered supply was 
> considered “ok” for a heater. For the active electronics you wanted something
> nice and stable / clean. To your point, once you get around to *measuring* 
> this, stability wise that answer often does not hold up. Noise wise, you are right 
> back to “what frequency?” ….
> 
> The somewhat more complex “old time” answer was that you don’t want the 
> honking big current of the heater coming off the supply you have tried so hard
> to super-regulate. ( = it’s the supply that’s the issue not the Rb it’s self). Obviously
> that’s going to depend on how the supply was designed. 
> 
> Of course next layer to the onion is …. where does the ground current go? …. hmm….
> 
> Bob
> 
>> 
>> I'm sure there is a reason why they make it two different pins ?
>> 
>> -- 
>> Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
>> phk at FreeBSD.ORG        | TCP/IP since RFC 956
>> FreeBSD committer      | BSD since 4.3-tahoe    
>> Never attribute to malice what can adequately be explained by incompetence.
>> 
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> 
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