[eagle] Re: Jim's Phase Noise Number!!
greencl at mindspring.com
Mon Jun 11 11:38:17 PDT 2007
I've been quiet as most things I might have added have already been
stated. Regarding the power from the spacecraft, think automotive power
standards (really ugly). The power will likely come from switching
sources (at least some of the time) and is not shielded so subject to
everything else running around in the wiring harness and stray stuff
from transmitting antennas, etc. There is some filtering of this in the
CAN-Do! so each application will have to decide if any added filtering
Robert McGwier wrote:
> Not to mention that ALL power supplies to the oscillators were specified
> to have adequate filtering and noise immunity. We need to revisit that.
> There is NO power supply coming from the spacecraft bus where you may
> assume it is nice, flat, and quiet. Assume it is ugly, raw, and must be
> Bdale Garbee wrote:
>> On Sun, 2007-06-10 at 20:04 -0700, Juan Rivera wrote:
>>> To my surprise the module jumps briefly to about 57 kHz at regular
>>> intervals of about 3 seconds.
>> Yes, that's the watchdog timer firing and causing a software reset. I
>> have no doubt that the reset cycle causes a short-term variation in the
>> current consumption profile on the board itself. If the frequency is
>> power consumption dependent, note that it's likely to move around a bit
>> during normal operation... might want at some point to look at
>> conditions with the CAN-Do! running at a "normal" 20ms update rate.
>>> My next plan is to probe all 40 pins by using a 10x scope probe. I’ll
>>> start another log called CAN-Do Noise and start posting there as I
>>> start on this task.
>> As Lyle pointed out in an off-list email, the power supply on the
>> CAN-Do! widget was designed to be efficient, not necessarily to be
>> quiet. Switchers always have noise issues... we've flown a lot of them,
>> and this isn't the first time some filtering or shielding has been
>> required to keep everything happy. You're doing the right things to
>> characterize what's going on, and we'll all help if/as needed.
>> We (the CAN-Do folk) are following your progress with great interest.
>> Augmenting the CAN-Do! documentation with some summary of your findings
>> and mitigation approach to aid future users is something we'll look
>> forward to doing.
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