In message 1958104416.2586171.1577043445646@mail.yahoo.com, Taka Kamiya via t
ime-nuts writes:

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.

I have had good results modernizing my HP5065A with a high end
DC/DC converter, in order to reduce the heat, and in this particular
case, improve stability:

http://phk.freebsd.dk/hacks/HP5065A/20150930_dcdc/

Almost no switchmodes (AC/DC or DC/DC) are spec'ed to run into a
serious pi-filter, but so far I have never had problems with it.

The best way to find out if your filtering works, is to switch
the supply between lead-acid (possibly with a linear regulator)
and the switch-mode solution, and see if you can measure any
kind of difference, be it frequency stability or noise spectrum.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@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.

Hi

Each project is a different “experience”. Some things are way more susceptible to switching noise
than others. Some switchers put out way more noise than others. You would not run a linear supply
on a computer. The low noise front end of a radio is going to need a quiet supply …..

There are a lot of linear supplies that come in dissipating more heat than they deliver to the load. You
can also do fancy designs that are up above 90% efficient. (count on using BGA’s for that one ….).

First and easiest question: Does this beast really need to be powered up all the time? Second question:
really ???? :) There aren’t a lot of devices that need to be / should be run 100% of the time …. My
biggest answer - turn it off.

Bob

On 12/22/19 1:18 PM, Poul-Henning Kamp wrote:

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.

Back when I used to accumulate time nuts gear, I also used to accumulate various linear power supplies (mostly surplus / used HP lab supplies.).

Approx 10 years later I should probably should look at the noise levels, regulation etc again.

I also have a 24 volt lead acid battery system for backup in the event of a power failure and for use if I want to avoid using AC powered supplies.

One of my nagging / never resolved issues was how to arrange for long term 24 volt backup power for my BVA OCXO.  The 24 volt backup battery bank is float charged at somewhat more than 24 volts, and I was hesitant to subject the BVA to those voltages.

The short term solution was to run the BVA from an HP linear lab supply that in turn is powered from a UPS.  I never got around to replacing that with something more elegant.

Mark S

mark@alignedsolutions.com
604 762 4099

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.  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@earthlink.net> wrote:  

On 12/22/19 1:18 PM, Poul-Henning Kamp wrote:

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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On 12/22/19 3:16 PM, Taka Kamiya via time-nuts wrote:

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.

Jim wrote:

Achieving 60-80 dB in practice with typical DIY printed circuit layout
and construction (by this I mean amateur PCB layout, typical board-house
fabrication, and typical amateur packaging) is extremely unlikely.  Even
30 dB is highly optimistic.  To get even that far, one needs multiple
shielding (see list member Gerhard Hoffman's construction photos for
ideas) and may very well need PC boards with more than two layers.

Not impossible, but not what the average amateur is used to designing
and building every day.

Best regards,

Charles

Am 22.12.2019 um 20:37 schrieb Taka Kamiya via time-nuts:

[...]

The suggestion to "just use a linear power supply", especially if it was
designed some decades ago is probably not getting the desired results
these days.  There is a lot more conducted noise on the power lines than
there used to be, starting from several kHz right into RF territory.
Linear regulators generally have bad PSRR at higher frequencies (often
starting to degrade in the low kHz region already) and unless you have a
clean input supply you pass any HF noise almost directly to the load.
SPS are not necessarily worse than linear supplies when it comes to
noise, but it generally shows up in different places frequency-wise (the
detailed characteristics depend a lot on the exact topology chosen, so
that's a whole 'nother dimension of things to consider when chosing). It
is also harder to contain the switching noise as you have several
high-current loops typically whose area must be kept as small as
possible to not radiate noise.  Conducted noise can be more easily
filtered, but it gets out both through the input and the output side.
The input side is often neglected a bit, which can come back to bite you
when you have multiple supply rails in the system.  Most switching
topologies (or at least the ones that can be "clean" enough for the
purposes under discussion) will produce a triangular ripple at the
output at the switching frequency.  You can make it smaller and smaller
at the expense of load regulation, but it usually is easier to just deal
with whichever number of millivolts that you're left with by putting an
LDO post-regulator directly at the load (you put it at the load so you
have better regulation and you can also use the supply line between the
SPS and the LDO as part of a Pi filter).  The LDO needs to have good
PSRR at the switching frequency and maybe the first or second harmonic,
the rest of the spectrum should already have been dealt with by filtering.

Late Jim Williams' (RIP) application notes are always a good read:

https://www.analog.com/media/en/technical-documentation/application-notes/an70.pdf

https://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdf

https://www.analog.com/media/en/technical-documentation/application-notes/AN118fb.pdf

--
Achim.

(on the road :-)

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