[time-nuts] Power supply for time source concerns

[Achim Gratz]

Am 22.12.2019 um 20:37 schrieb Taka Kamiya via time-nuts:
 > Most commercially manufactured time and frequency sources use
 > switching power supply.
[...]

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 :-)