[time-nuts] Switching regulator replacement for 7805

jimlux jimlux at earthlink.net
Mon Dec 5 13:22:59 EST 2016

On 12/5/16 9:13 AM, Dan Kemppainen wrote:
>> Depending on the application, another possible application is to
>> sync up the DC/DC converter to the "main" clock source. This makes
>> the switching noise then coherent to the system, which either makes
>> it average out completely, or possible to filter it out in the digital
>> domain using a deep notch-filter in receiver applications.
> You are assuming the switching noise is directly caused by the
> fundamental switching frequency, this may not always be the case. Often
> the fast edges can excite parasitic reactances in the board and
> components, which in turn oscillate and radiate at their own frequency.
> This is not necessarily related to the switching frequency.

On big science spacecraft this is pretty common (which have all sorts of 
sensitive science instruments, some of which may not have the best 
EMI/EMC characteristics from both a source and susceptability 
standpoint).  On the other hand, as switching rates have gone up, it's 
not clear that distributing a 20kHz "PWM reference clock" around is a 
good idea.

On a radio I was recently working with, it used to radiate quite nicely 
at about 15-19 MHz, even though the switching rate was in the hundreds 
of kHz range - we figured that this is where the "antenna efficiency" 
(better with higher freq) of the traces matched with the "harmonic 
power" (lower with higher freq)

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