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 [email protected] 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.