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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