[time-nuts] Parallel voltage regulators
SAIDJACK at aol.com
SAIDJACK at aol.com
Fri Oct 26 18:39:31 EDT 2007
In a message dated 10/26/2007 14:59:29 Pacific Daylight Time,
bruce.griffiths at xtra.co.nz writes:
>> That is a recipe for disaster if one wants a really low noise oscillator.
>> This is particularly true if one is a beginner.
>> Reducing the output noise of a switching regulator to 100uV rms or less
>> is neither easy nor simple.
I Agree, it get's even worse when several independent switchers are on the
PCB, with similar switching frequencies. This can generate beat frequencies
that vary over time, temperature and from board to board.
NEC Tokin makes a ferrous sheet material that is very good at shielding
switching regulator noise - both electro, and magnetic components.
When trying to get noise floors approaching -160dBc/Hz the RS-232
transmitter on the Fury GPSDO can be used as an example of a potential interference:
The RS-232 transceiver chip generates the RS-232 level signals from a 3.3V
DC input rail using switched-capacitor voltage doublers. There are no
inductors or flyback diodes involved. The switching currents involved are very small
(orders of magnitude smaller than a switchmode power supply), 0.1uF caps are
used as charge reservoirs.
Even so, the current spikes generated in the RS-232 transceiver are enough
to create a couple of low measurable spurs at around -120 to -130dBc/Hz on the
output spectrum of the 10MHz sine wave. This happens even though we have
allocated a separate 3.3V regulator for the RS-232 chip, and have extensive
power supply filtering in front of the regulator and before the transceiver chip.
There are no switching regulator power supplies at all on the Fury PCB for
To completely remove the spurs we added a mode to disable the transceiver's
DC-DC switching regulator ("Quiet Mode"). That takes care of the spurs.
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