Computing stuff tied to the physical world

Jumping to conclusions

In Hardware on Jun 12, 2012 at 00:01

Yesterday, I made an effort to remove some glitches which I thought were due to the switching regulator used inside the ±15V DC-DC converter. To be honest: it didn’t really make any sense when I saw this on the scope…

It’s always easy to draw some conclusion – it’s a bit harder to avoid uttering complete nonsense…

Here’s the signal with the power supply turned off, and only the ground cable still connected:


Clearly the same signal – it appears even when the 1 meter ground cable isn’t tied to anything: it’s an antenna!

In other words: I’m probably just picking up one of the FM transmitters in this region. I should of course have turned on the scope’s built-in 20 MHz bandwidth filter. There was no reason to look for frequencies that high with a switcher in the 60..100 KHz region. And the fact that neither a bypass capacitor nor various inductors made much difference should have been a clue. How embarrassing.

Onwards, quickly!

  1. You seem to have quite a bit of fun with your local radio stations. Have you thought about relocating to a Faraday cage?

    • There’s a pretty big and powerful transmitter close to JeeLabs (about 20km away), so it’s not surprising, but it’s not the first thing you’ll think about either. Thanks for sharing this blunder though. It might save me and some others, some time in the future when debugging a circuit!

  2. Completely unrelated: I’m having trouble posting on the forum. I replied to a question about the analogplug, which was withheld for admin approval. I thought it was because of a lot of links in there (to weblog posts and arduino docs) so I composed a short reply with just my suggestion. Posting that got me a blank page with only the text: Service unavailable.

    Is it just me/is there anything I can do about it?

    • A recent forum update (Drupal 7.14) seems to have created this problem. I’ve updated the problematic module twice, in the hope that the problem would have been squashed by now. Could you try again, please?

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