[time-nuts] Power supply for Thunderbolt

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Aug 5 18:43:03 EDT 2008

David C. Partridge wrote:
> I'm using the Meanwell T30-B, but Tom van Baak's test on this shows a bit of
> phase noise at 1Hz offset which is a tad nasty:
> http://www.leapsecond.com/pages/tbolt/noise.htm
> I'm wondering if this can be reduced with a bit of extra de-coupling on the
> 5V supply, but haven't got round to scoping the 5V supply to see what
> happens every second (presumably as the output is sent to the serial port).
> Cheers
> Dave

If  phase noise at 1Hz offset is a problem you will probably need a 
power supply filter with cutoff frequency well below 1Hz to make any 
discernable difference.
Since the an RC filter will produce a voltage drop in the 5V supply the 
resistor value will have to be kept small and the capacitor value 
correspondingly large (several farads).
It may be possible to stack 3 low ESR super capacitors in series 
(maximum voltage rating is around 2v -2.5V those with higher ratings are 
actually series connected stacks) to do this but this is a relatively 
expensive proposition.

Alternatively a modified version of the Wenzel active power supply 
filter could be used if a series stack of 2 low ESR supercaps were 
substituted for the capacitor connected from the supply to the 
transistor base. The modified circuit stabilises the transistor's 
effective emitter resistance and keeps the active filter dc current low.

It may also be possible to use a variation of Wenzel's opamp augmented 
active filter scheme, however the filter low frequency cutoff will have 
to be well below 1Hz for it to be effective.


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