Test Equipment: Oscillocopes

[TinselKoala]

Yes, Brian, when the trigger mode is not in "AUTO" the scope's display blanks and shows nothing when the waveform is below the trigger level. This is done to save the phosphor. Like if you aren't actually using the scope but you have it on to keep it warm, there is no point in having a straight line burning the phosphor, so the scope blanks the display in the Normal trigger modes. This is not a problem.  I usually use "Auto" to set baselines and to look at very small signals, then switch to Normal so that the display blanks when not actually viewing a signal or the signal is below the set Trigger Level.

You adjust the trigger Holdoff a lot during the video and I kept wanting you to adjust the Trigger Level instead. Also you need to make sure the trigger Source is on the channel you want to trigger on! If you are triggering on CH1 and you turn that channel off by selecting Ground on the input coupling switch, the scope can't trigger!  I can't see the scope's controls clearly enough in the video to see what's going on but it seems to me that the thing is behaving fairly normally as far as I can see.

It may be that there is some extra noise coming in that is causing it to be unstable unless HF or LF filtering is on. You are changing so many things and I can't see the critical settings well enough to diagnose this problem. It may be that the Holdoff control doesn't need to be perfectly on the "normal" line  to get a stable trigger; this may be a calibration issue or a result of the noise on the signal.

Instead of using "Chop" please use "Alt" on the Vert Mode, connect both probes to the Calibrator, then select either CH1 or CH2 on the Trigger Source instead of "Norm". Then use the Trigger Level control to try to get a stable display. Check the difference between Auto and Norm trigger Modes as you vary the V/div setting on the channel you are triggering on. In Norm Mode the screen will go blank when the trigger level is set out of the range of the signal amplitude. This will also depend somewhat on the vertical position of the trace, you can take the trace out of the trigger level by moving it up or down on the screen and if you are in Norm Mode the screen will blank when you do that. This is what you were seeing in part of the video where the traces would disappear as you moved them up or down with the Vert Position control.

[Pirate88179]

Yes, the AllAmericanFive radio guy is fantastic and knows what he is talking about.  I find his videos fascinating and very educational.  He actually did a video and mentioned my Tripplett meter as he has one as well.  He came up with the battery hack that I use because they no longer make those old 30 volt batteries.  Great guy.

Geeze, after watching that guy, and Dave at EEvlog, and listening to TK, MH and Mark...you would think that I would have learned a lot more than I have by now.  Some of the info sinks in, and some of it goes right through and I have to learn it again..ha ha.

But seriously, check out that guy's videos Brian...they are indeed great.

Bill

[Pirate88179]

Yes, the AllAmericanFive radio guy is fantastic and knows what he is talking about.  I find his videos fascinating and very educational.  He actually did a video and mentioned my Tripplett meter as he has one as well.  He came up with the battery hack that I use because they no longer make those old 30 volt batteries.  Great guy.

Geeze, after watching that guy, and Dave at EEvlog, and listening to TK, MH and Mark...you would think that I would have learned a lot more than I have by now.  Some of the info sinks in, and some of it goes right through and I have to learn it again..ha ha.

But seriously, check out that guy's videos Brian...they are indeed great.

Bill

[Brian516]

Ah, now I feel stupid.

I should have thought a little bit more about what exactly the different settings do and what they change.
Also, for some reason I was thinking that when part of the trace was going off-screen it wasn't affecting anything, that it was just simply going out of the viewing range.  Now I know that's not the case.

As for the trigger level adjustment, I must have been thinking that it was as sensitive as most of the other controls. So far, now that I know WHEN to use it, it is the least sensitive control of all of them. Scratch that, it can be very touchy at times also. Starting to get the hang of things now..

I will be watching a lot more instructional videos in the coming days so I can not only avoid feeling stupid, but stop bugging everyone with stupid questions and false problems!! That AllAmericanFiveRadio channel is definitely a good place to start.



Brian

[picowatt]

Brian,

Here is a real quick and simple explanation of what the trigger and trigger level do.

Let's say you have the horizontal sweep set to 1ms per division.  The trace effectively sits and waits off screen on the left side of the display until the scope receives a trigger pulse, which, like a starting pistol at a race, makes the trace race across the screen toward the right at the horizontal rate you selected (the 1ms per division).  Once the trace gets over to the far right side of the screen, the trace is temporarily shut off (blanked) and at very high speed races back over to the left side and again just sits there and waits for a trigger pulse.  And so on and so on...  No trigger, no racing off to the right.

Now, let's say you have a sine wave input to channel 1 that goes from +1 volt to -1 volt.  The trigger level is used to determine at what voltage a trigger pulse is generated.  Note that your trigger level control is labeled as positive or negative, with zero being straight up.  If you adjust the trigger level for +0.5 volts, when the incoming sine wave exceeds +0.5 volts, the scope generates a triggerpulse and sends the trace off and running across the screen.  When the trace returns to the left side, it will again sit and wait, but only until the incoming sine wave again exceeds +0.5 volts, which produces another trigger pulse and again sets the trace off toward the right.  This causes the trace to scan across the screen, overlaying a new image of the sine wave over the still fading out previous sine wave image.  The images overlap perfectly because the trace is always starting out from the left side at the same point in the cycle of the sine wave (the point where the sine wave reaches the +0.5 volts that the trigger is set to).

Now, if you turn down the level of the sine wave being fed to the scope so that it only goes from +300mv to -300mv, and you don't readjust the trigger level, which is still set at +0.5 volts, the scope will never generate a trigger pulse because the incoming signal never exceeds the +0.5 volts that the trigger level is set to.  This is why the trigger level control is important and needs to be adjusted for the most stable display of every waveform you display.

Stable repetitive waveforms such as those coming from a function generator or the calibrator are fairly easy to get a solid trigger from and produce stable displays that exactly overwrite the previous scan.  Complex waveforms, such as music, are a bit more difficult to trigger on, but using the HF reject and attempting to set the trigger level so that the trigger is generated from the typically larger low frequencies (bass notes) will produce a usable display, particularly at lower sweep speeds.

Turn your trigger holdoff knob to "normal" (which is minimum holdoff) for now and just leave it there!!  You won't be needing any holdoff for repetitive signals (FG, calibrator, etc)

PW