How to use an oscilloscope

[jackinnj]

Originally posted by Jennice
Jacki,
Btw., the scope does not seem to have BNC (shielded) input...
Jennice

Needs an adapter -- you can also pick up a HP Preamplifier (465?) 20 or 40dB of gain -- these go for around $20 on EBay (of course, that's more than the scope!)

[Jennice]

I wouldn't count on the scope using standard pin distance, but that's easy to measure...

[SimontY]

a description of your horizontal time axis (ms/div) and sensitivity (vertical, mV/div)

Not managed a new pic. yet, but here are the settings I used to get that wave..:
x/time axis = 1ms/cm
y = 20volts/cm

Appreciate the comments Jennice and jackinnj - however, I do not want to invest much money into this thing. At least not until I know what I can usefully get from it! All I really want is to look at the quality of DC power - and if I can put it on the mains... well that would be swish, but it says 120v AC max


-Simon

[Jennice]

Hi Simon,

If you're certain that Y=20V/cm, then you got a signal of approximately 90 V(peak-peak).

This really doesn't match with your statement about monitoring a 5V psu. Could it be that your division is 20mV/cm instead?

Byw.: It is possible to measure mains voltages using a resistive voltage divider. Either use a probe, or DIY style, but make dure that the resistors can handle the voltage and dissipated power.

In the x axis you have 7 dividers for a cycle. With 1ms/div, that would give a cycle time of approx. 7 ms. This again equals a frequency of 140 Hz.

Considering that the scope is aged, it could possibly be explained as a 120Hz signal, but where would that come from? A harmonic of 60Hz mains?

Please re-check the connections to your circuit. I think the image still looks odd for a psu ripple image.

Besided, chech the oscilliscope for "adjust" buttons or screws, or "strech" buttons. Some scopes have ways to change the scale in steps that deviate from the fixed settings.

Also, it seems that you have a dual trace scope. This means that it has the ability to display two signals at the same time. Many scopes (especially older types) however, have a common signal ground, so make sure you don't create a short circuit.
See if the input terminals are marked A and B, or X and Y. X/Y modes are useful for checking phase between signals, but that's a different story
If it IS a 2 channel scope, make sure that you display the channel that the probe is connected to. Otherwise, you'll most likely end up with a signal like the one on your scope, if it monitors a un-connected channel. See if you have a selection button dealing with "A, B, ALT" or something like that.


Jennice

[SimontY]

Hi Jennice,

If you're certain that Y=20V/cm

Surely it must be mv then. It is 5v, I have measured it at 4.9v with my multimeter.

chech the oscilliscope for "adjust" buttons or screws, or "strech" buttons.

Yep, it has stretching knobs, and I had adjusted with them until I got the nicest looking and clearest wave. Next time I look I will try to leave them in the middle, is this correct?

make sure that you display the channel that the probe is connected to

I was definately displaying the correct one! I 'pushed' the unused trace off the screen - and besides, it had no probes connected and would only display a flat line. Even with a probe waving around in the air, not much comes on screen (maybe it isn't all that sensitive).

Bearing these factors in mind, could I have been displaying a stretched ripple image? I will try again and see what I can get.

I appreciate your patience in helping me!


-Simon

ps "The Hi-Fi set is ok when you don't notice it's there..." -this is a good tag: it reminds me I still have a little way to go!

[Jennice]

Thanks for noticing the tag-line You're the first one to comment on it... and yes, I too have some way to go (if I'll ever get there).

As for helping, I suppose that is what this forum is all about, except for occasional funny mis-haps and sarcasm.

It sounds more likely to be a "mV" range. Still, you surely must have SOME load on it to abtain this ripple?

Regarding the adjustment knobs, there should be quite a few.
You'll probably find a focus and/or intensity adjustment to make the trace as clear as possible. Secondly (and probably near the sensitivity selector) you may find a variable adjustment for input sensitivity.

Near the "Time base" (X direction, speed of trace across the screen) you may find an adjustment knob to adjust it for auitable time base (mainly used to set a "reference" in grid units for verifying a specific time interval or frequency). Usually, such a time adjustment should be left in a "calibrated" state (often the far left or counter-clockwise position. check for labeling).

If you have a signal generator with a known frequency, then it may help you to do a one-time "household calibration" of the time base, after which you leave this knob alone. Otherwise you may want to display the voltage on the secondary of a transformer with a suitably low output voltage.
This will have a frequency ( 1 / "cycle time") which is rather accurate as your reference with 50 HZ or 60 HZ, depending on where you live).

Hope you'll find all my mumbling here useful...

Jennice