An unusual Oscilloscope phenomenon

Legs.

in my

ted

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That was not my point. The limited low-pass band of the input amplifier and sampling circuit will either completely filter out higher-frequency components, or will alias them down. If you are seeing a nice sinusoidal waveform on an oscillioscope whose fundamental frequency is close to the frequency limit of the scope, chck it with a higher-frequency device.

I've seen the same (or similar) result with a higher bandwidth (300MHz) scope, so the vertical bandwidth limit has nothing to do with it. The TDS-220 is just the one I happened to have available at the time. I filmed some comment on this extra stuff but it didn't make the cut.

Dave.

I wish I could get into the blob, just as a matter of interest. I doubt it would be work the effort, though. Displays don't cost that much to make it worthwhile. The simple hack is to just use the thing on the meter PC board - you can saw away what you don't need. The last one I hacked used the whole meter, minus the switching, just to monitor a supply voltage. There was plenty of room inside the supply cabinet so I just soldered leads to the input and did a little surgery on the front panel so yo could see the LCD. I had a little DC-DC converter that provided the 9V for the meter. The supply ramps the voltage up slowly between set points over time, and the built in meter provides a way to watch the progress without needing to connect an external meter each time.

Ed

I can understand the excitement about your FLIR imager, would love to have one. You could probably open up a lucrative non-electronics side business, "Larkin's Energy Audit Services LLC" or something like that.

Spectrum analyzers can be a mixed bag. I like the old boat anchors better most of the time. New ones often employ data processing such as partial FFT. An example was a case where a client could not find noise with a freaking expensive analyzer. I found the root cause within the hour. Using a computer controlled scanner that cost 600 bucks ...

FLIR just bought Extech, and they have a new low-end imager. I don't know the price.

We got a 3 GHz Aeroflex, which is itself a mixed bag. The software is buggy and it has nasty 120 Hz spurs. Like a lot of imports, the local guys are fairly powerless to get things fixed.

John

Quote "Extech is now making FLIR cameras available in the USA only through its network of leading distributors." Sentences like that usually mean $$$$.

Waiting for that Harborfreight infrared camera ... :-)

My scanner goes to 3.3GHz but sensitivity up there drops painfully. One of these days I'll have to built a preamp for it so I can do some

A serious edge (good) PC-controlled scanners have over almost any analyzer is that you can listen in not just in AM mode but switch to SSB. Finding root causes can be a breeze that way. Client complains about birdie, doesn't have an idea where it's coming from, suspects something is slightly oscillating or whatever. Rig up scanner, switch to SSB, get the little mini-hammer I use for piano tuning, tap, tap, tappa-di-tap ... "weeeeeeooooouieee" ... aha!

Conducted EMI paths can sometimes be found as well. Take a signal injector, switch scanner to SSB so it hears the slightest modulation on the tone, touch this, that and the other thing with the signal injector .... "warble" ... tada!

Well, then I guess using Henkel Pattex glue isn't all that great an idea. ;-)

Use just plain ol' ordinary common off-the-shelf "contact cement", and let it DRY before you start placing the shims. Come to think of it, when I was a video game tech, the shipping guy used to do it to the sides of video games when they'd convert a cabinet from one game to another.

Cheers! Rich

Same in this here lab :-)

Last year I worked on a couple of cameras for the Navy. My main job was a 500x500ish color camera but its mate was a 512x512 FLIR. The Xilinx FPGA in each camera was $3800, which I thought was a bit much for a TV camera, until they told me that each of the three CCDs was $5K and the prism was $40K. A year ago, this week in fact, interviewed for a job at Cincinnati Electronics designing FLIR cameras. They were doing 4096x4096 FLIR cameras.

;-)

I think half of us are talking about low-noise audio frequencies, and half of us are talking about RF. The way I heard it, you leave one end unshielded for audio preamps and stuff, but ground both ends for RF; I think it has to do with wavelength and, well, propagation characteristics. Of course, the thing that was drummed into me while they were teaching me this was "GROUND LOOPS!". :-)

Cheers! Rich

Audio: if one end of a shield is disconnected, the delta-voltage (60 Hz, RF, esd spikes) between chassis is greater. If the signal is single-ended, that increases noise directly. If differential, it increases common-mode noise.

Why would you want to do that?

A 60-Hz (and RF, and ESD) "ground loop" can result from different boxes being plugged into different outlets which have some 60 Hz difference voltage (or form a relative antenna for RF.) Connecting the shield on both ends shorts out some or most of this difference voltage and REDUCES the effect of the "ground loop" on the signal.

Whoever did the drumming was passing on folklore.

John

Trouble is, since we moved from Alexander Graham Bell's wall phone (with crank) to cell phones RF frequently turns into audio after getting into such single-end shielded cables. All it takes is an opamp with bipolar input structure.

Of course, with a good old tube amp you'll continue to be fine :-)

Unless the shield slowly commences a faint red glow, followed by some "amperage scent" wafting through the room ...

Seriously, I've seen that happen.

Yup. OTOH that provides a good business opportunity 8-D

Yup. I've seen some ghastly wiring, some done by licensed electricians.

The most 60 Hz I've experienced (between outlet ground pins, in an office building) was a few volts RMS, and plenty stiff. Messed up some printer signals pretty good.

The real way to pipe signals around is fiberoptics. A kilovolts of common-mode is no problem.

Hey, time for our engineering meeting; Japanese food today!

John

Because that's what it said to do in the instructions that came with the kit.

Cheers! Rich

What's fun is when you tie your local audio system to the cable TV public access feed, and the hum is at -5dBm! Who know WHERE that TV cable is grounded, if ever... ;-)

Charlie

Supposedly, it is bonded to the same ground as the AC lines they share the pole with. Copper thieves steal so much ground wire these days, its almost impossible to maintain good grounds.

I cured all my hum problems with "Ground Loop Isolator - by Xitel".

...Jim Thompson