[eagle] Re: Jim's Phase Noise Number!!

Juan Rivera juan-rivera at sbcglobal.net
Mon Jun 11 14:12:21 PDT 2007


Please refresh me on the function of the watchdog timer.  In the state I was
running, while observing the 3-second jumps, the receiver was shut down but
the CAN-Do module and dongle were both powered up (pins 39 and 40 not
connected to the receiver.)  I thought the module only reset if it detected
no heart beat from the dongle.

I think this evening I will fire up the receiver to recreate the noise I've
been seeing and then start playing with some brass sheet in front of the
inductor as a shield.  I tried probing all the pins with a 10x scope probe
connected to the SDR-IQ but I didn't see anything.  I'll repeat that test
with a 1x probe.  Based on last night I'm currently thinking that the noise
is radiated from that inductor rather than conducted via the 40-pin
connector.  There are receiver components very close to that inductor.



On 6/11/07, Bdale Garbee <bdale at gag.com> 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.
> Bdale
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