[time-nuts] need power trans. for HP3325

[Didier Juges]

Dr Bruce Griffiths wrote:
> Didier
>
> SCR's are still used in very high power (Megawatt) high voltage (0.5 
> Megavolts or more) inverters where they replaced grid controlled mercury 
> arc devices (some of these have been in operation for 40 years or more).
>
> I am sceptical that any affordable switcher has an output noise as low 
> as a well designed linear regulator (<10uV pp DC -20MHz).
>
> Bruce
>   
They are not, but system designers, as far as I can tell, have overcome 
this problem by providing local filtering/decoupling/regulation at the 
most sensitive points and therefore do not require the entire power 
budget to a system to be low noise.

Most of our power supplies (we only make custom military supplies) have 
many outputs (one we make right now has 34 outputs, all independently 
regulated with floating grounds), sometimes several outputs of the same 
voltage, so that the customer can isolate various sections of the 
system, and provide as much filtering at the point of load as needed for 
each subsystem. Also, being a 100% military house, few customers call 
our supplies affordable :-)

For instance, on the 5370, a lot of the power is used to drive logic 
circuits, TTL and ECL. For the most parts, these are not very sensitive 
to noise, certainly not as much as the OCXO, the VCOs and the input 
comparators, which probably (I have not checked the manual yet) run from 
a different voltage.

Modern high integration chips have advanced power/thermal management 
functions which turn off parts of the chip when not in use. This results 
in very severe load transients. Often, the noise (periodic and random 
deviation, PARD) on the 5V, 3V and lower rails is driven by the very 
fast load transients of the logic chips more than by the switcher 
itself. A specification we are reviewing right now has a load change 
requirement from 55 to 110A on a 3V rail in less than 1uS at the end of 
a 56 inches bus bar. With that kind of load transient, the ripple does 
not matter much compared to the transient response through the bus bar, 
even assuming a perfect power supply with infinite output capacitance.

Didier