[time-nuts] Power supply for time source concerns

[Bob kb8tq]

Hi

If you dig into the app notes on the LT304x parts after a lot of extolling the 
wonders and virtues, they eventually get into magnetic coupling between the
“upstream” and “downstream” bypass components. Taking care of that with 
proper layout is possible, but non-trivial.

Is milivolts of noise “ok”? Maybe it is. There are a lot of devices out there that
run with 10’s of milivolts of noise on the supply line. Do you need <100uV p-p 
over 10KHz to 100 MHz? There are devices that do. If that’s what you need, 
it’s going to be a struggle. There is no one single “good enough” number. 

The power supply world still stops at pretty low frequencies. Very common IC’s
these days are quite happy to produce crud at the “many GHz” level. There are
a few folks who carry around cell phones that put out signals up there as well. 
Regulator IC’s will only do part of the job, filtering in one form or the other (likely 
several forms) is still needed. 

Past that system layout begins to get into the act. You can spend a couple of fun
weeks in intro level classes on this sort of stuff. You can be the prof presenting the
class and pretty quickly get to “that would require some research” as the answer
to this or that seemingly basic question. It’s not in any way a trivial topic. 

So what to do?

Bottom line is still that test equipment is your friend. Spectrum analyzers that cover
a wide range of frequencies (and have good sensitivity) *are* part of the “kit” that 
allows you to keep things quiet. You can only go just so far ( = not very far) before
you are into the “verify” stage of things. Indeed sound cards and SDR’s can collect 
some of this data so there *are* a range of tools you can use. 

Checking cabling / grounding / cross talk is very much part of setting up any RF
bench. It always has been. Tracking down that stupid whatever on the other side 
of the room that is putting out a ton of noise *is* part of the drill. Repeating the 
process from time to time also is part of the fun. Looking at the data you collect to
spot “strange” stuff *is* part of the deal …… There is work involved and you need
to understand what you are doing. 

Bob

> On Dec 22, 2019, at 8:29 PM, jimlux <jimlux at earthlink.net> wrote:
> 
> On 12/22/19 3:16 PM, Taka Kamiya via time-nuts wrote:
>> Yes.....
>> I considered float charging a battery bank and switching power supply + linear regulator combination as well.
>> Concern with the first is safety in charging.  I was intending to use GEL cell lead acid battery.  24V chargers are plentiful but there is no telling what they actually do.  Adding a series regulator won't work because of the required voltage drop with most of the 3 terminal kind.  I still want to try this as it will double as UPS as well.
>> Switcher + linear regulator sounds promising.  One unknown is, how much high frequency ripples/noises those regulators actually remove. 
> 
> LT3042 removes 60-80 dB up to 10MHz. 20V, 200mA
> https://www.analog.com/media/en/technical-documentation/data-sheets/3042fb.pdf
> 
> CHeck out also the LT3045 (500 mA)
> LT3093, LT3094 (negative voltages, 200, 500mA, 0.8 uV noise)
> 
> 
> I guess I'll just have to try this out.  Ferrite sleeves, low pass filters, chokes, etc, etc, etc are under consideration.
>> I have a very nice lab grade switching power supply but unfortunately, this stupid thing resets to ZERO every time power cycled and starts up in current limiting mode.  Meaning if I forget to switch modes, it can go as high as 60 volts.  By then, all devices are GONE!
>> My main usage will be just for time standard for my lab.  Stable 10MHz and 1 pps is all I need to sync everything up.  I am not going to multiply output to GHz range or do anything else exotic.  I wonder how much ripple will actually affect?
>> ---------------------------------------
>> (Mr.) Taka Kamiya
>> KB4EMF / ex JF2DKG
>>       On Sunday, December 22, 2019, 6:00:32 PM EST, jimlux <jimlux at earthlink.net> wrote:
>>    On 12/22/19 1:18 PM, Poul-Henning Kamp wrote:
>>> --------
>>> In message <1958104416.2586171.1577043445646 at mail.yahoo.com>, Taka Kamiya via t
>>> ime-nuts writes:
>>> 
>>>> First question to the group is, how do YOU manage this problem?
>>> 
>>> It used to be that there were only one kind of switching power-supply: The noisy ones.
>>> 
>>> That is no longer true by definition, but there is no easy road to this particular Damascus.
>> My strategy is DC/DC converter to get close, and high PSRR LDO linear
>> regulator to the final voltage. Close attention to the DC/DC converter
>> input and output so you don't couple to the ultimate output through
>> radiated or parasitic conducted paths.
>> The LT3042 series regulators have high PSRR, are available with negative
>> voltages now, and can be paralleled for more current capacity.
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