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Help me understand my oscilloscope, please

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LAN_403

IamSmooth

Sun Jan 17 2010, 07:14PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I am tracking a square wave and sine wave on my 2465 tek scope for my inverter. They should be in phase. Sometimes the signal jumps to 180 out of phase. I have determined that when I play with the LEVEL and HOLD knobs I can see that this is a scope problem, and the signals are still in phase.

Why does my scope do this sometimes and what can I do to get a more reliable "lock" on the two channels? WOuld a more modern, digital scope help? ANy recommendations?

Sun Jan 17 2010, 07:26PM

Registered Member #2372 Joined:
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Posts: 62

Whenever you set up the trigger level you should make it trigger on either positive or negative and put it right in the middle of the signal, if you put it near the top or bottom sometimes it will trigger unreliably or off noise or overshoot.

Nicko

Sun Jan 17 2010, 07:38PM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

On the 2465 series, when triggering off a lowish frequency, use the trigger HF filter - its a very fast scope, and may trigger on noise that you can't see unless you look for it specifically. Look in the trigger coupling (top right) and select either noise or HF reject.

Generally, these scopes (I always use a 2465A or a 2430A myself) have excellent & stable triggering and what you are seeing is not a common mode of failure. The 7904A in the photo below has gone to a more deserving home. Its probably user error...

Cheers

roddenberry

Sun Jan 17 2010, 09:14PM

Registered Member #2272 Joined: Wed Aug 05 2009, 01:34AM
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Posts: 7

Oh my, Nick... that is a sweet lab! I have a measly Tek 2215 and a BK 1461 but those are reliable workhorses that i've owned for nearly 20 years... My next piece of equipment I hope to get my hands on soon is a nice, used frequency counter... the only one I have right now is in my Radio Shack DMM and I don't think it's really accurate.

BTW, I also have a really old RCA scope dating back from the mid 40's but it needs repair.

- Robert

Dr. Slack

Mon Jan 18 2010, 08:31AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Experiment with "chop" and "alt" settings for the dual beam. You would normally use chop for slow timebase settings, and alt for high, but there is a broad area in the middle when either would be appropriate, and switching to chop can help diagnose trigger funnies.

Chop does a single sweep, and chops the Y amp between traces. It always shows the two traces in phase, but the relatively low chopping frequency can create artefacts with high frequency components of the signal.

Alt waits for a trigger then does a sweep on one input, then waits for a trigger and does a sweep on the other input. Lucky (unlucky) settings of timebase and trigger holdoff can give you a stable display where one input is always triggering not quite where you thought it should,

...

Mon Jan 18 2010, 09:34AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

To echo what Nicko noted, the problem is due to noise in your signal. The scope is set to trigger in the smallest rising (or falling depending on the slope +/- setting) edge at the trigger voltage--so if there is noise in your signal it can trigger twice and give the double waveform you noted. You may find that increasing the holdoff a bit (the holdoff disables the trigger for a finite amount of time after a trigger is detected, to avoid triggering twice on a waveform), as can playing with the trigger coupling. If you scope doesn't have integrated lf reject or noise reject coupling for the trigger, you could rig your own by running the probe through a bnc T, with a rc filter set to give a time constant a fair bit higher than the frequency of the signal you are looking at, and feed the resulting signal into the trig input on the scope (and set the scope for external trigger). This method is a bit of a hack since you are not able to use the input amplifiers, which reduces the trigger options and loads the circuit more (don't try to use a 10x probe in this configuration, if you need to use a 10x or 100x probe you would need to use a separate tap from the circuit to generate your trigger from).

As to the DSO solving your problem, it might make it a bit more bearable because you can hit the stop button and look at a single trace (which eliminates the issues due to triggering altogether), but you should be able to fix it with your analog scope.

Good luck!

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