EEVblog #20 – The unusual oscilloscope phenomenonPosted on July 26th, 2009 7 comments
The unusual oscilloscope phenomenon – Part 2!
I was never able to reproduce that on my trusty old HP 54600B.
I’ve never seen it, actually.
I guess I’m not as galvanic as you…
You won’t see it on a 54600B, as it doesn’t have the required single shot bandwidth, it’s only 20MS/s.
You may want to show what happens when the 50 Ohms is connected directly at the scope. I suspect it may be hard to get the signal to completely go away with the scope by itself. But keeping the 50 Ohm and adding and removing the 50 Ohm cable could show a change in magnitude. If it does go down without the cable, the scope designer still deserves some respect for the challenge of having good CMRR at 100MHz. At these frequencies, signal can sneak in anywhere.
Good for you, not being overawed by all the famous names. (Of course the EDN guy should have known better than to try that on an Aussie!)
One possibility I haven’t heard mentioned is an anti-ground-loop circuit in the scope. Ground loops are like gremlins because they’re generally millivolt-level signals with 10-milliohm level impedances. We don’t expect a 1 mV signal with a circulating current of 100 mA, so it looks mysterious when wiggling a cable makes the hum go up or down by 10 dB.
To get rid of the problem, lots of instruments use a parallel 1 ohm/10 nF network in series with the cable shield, with a differential input amplifier. (Horowitz & Hill explain the technique somewhere iirc.)
Although that’s a win for ground loops, it might make the scope vulnerable to pickup. How about disconnecting the coax and just putting a clip lead on the shell of the BNC?
(The humidity level in New York is currently above 80%, so I’d have to wait till November to try it myself!)
Great blog, btw.
Oh, thanks for the info.
Too bad, I had wanted to show my kid that effect.
I have seen all the video blog till now, but I didn’t understand why the single shot sample rate is ten time less then the real time sampling… Somedoy can explain me?
I tried whit an tds210 oscilloscope (it’s the same as tds220 but whit 60 mhz bw instead of 100)
The only thing I can get is a impulse that slew down whitout any oscillation.
I can generate it whit just take the probe and shake it (whitout toching any metallic part) and also using a insect killing device, that generate a 4000V discharge.
No way to generate it if I sit up or down, or moving around the probe. Maybe it’s because right now in Italy the humidity it’s quite high…
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