Buying and using an oscilloscope

[Bonky]

Thanks; I've just skim read both reviews; which is the better of the two for 'my' kind of work (low voltage stuff/radios/perhaps the odd hi-fi amp)?

Many thanks,

Richard

[Mark Hennessy]

I'd say the AN8002 is the better bet for most people. However, you might consider the BSIDE ZT301 instead. I haven't written the review up yet, but my initial impressions of that are here: http://www.eevblog.com/forum/beginners/o...msg1344853

Back to the 'scope...

I did wonder if you were measuring the calibrator output with a meter, but gave you the benefit of the doubt. It helps to be precise when writing, even if it takes a bit longer.

You say you can't see the output of the calibrator on your scope screen, but can see something when connecting an external oscillator. That suggests the settings of the scope are wrong. Unfortunately, there are many controls to adjust on even a simple 'scope, hence many ways to get it wrong. Ideally, you'd be able to sit down with someone who could talk you through it, but in the absence of that, here's how it breaks down:

There are 4 major sets of controls:

1. CRT
2. Vertical
3. Horizontal
4. Triggering

Of these, triggering is the hardest to explain.

With the CRT controls - which are just brightness and focus on that model - it should be self-explanatory. But don't over-run the CRT - keep the brightness as low as possible. I'm in the habit of readjusting it all the time. In extremis, you could damage the phosphor layer in the CRT. Also, there is a trace-rotate pre-set, which does what the name suggests. That shouldn't need adjust much, if at all once you've located the instrument in its home on the bench.

For the vertical controls, you have the volts per division switch (the large one), the variable gain control (the small one mounted within the main V/div switch - leave in the "C" position unless you've a good reason not to), and the beam switching arrangements. For now, stick with Channel 1, but once you're happy with that, check that Channel 2 behaves the same. Additionally, you have the input coupling control (DC-AC-GND). Leave that at DC for most measurements, but AC will be needed from time to time. More on that in the future... Finally, the Y-POS controls - make sure these work, and leave them somewhere towards the middle for normal use.

The horizontal controls are to set the speed of the timebase. The 'scope does nothing more clever than put a spot on the screen - it only looks like a waveform because it moves quickly. As you turn the TIMEBASE switch anticlockwise, the sweep speed of the spot slows down, and at the slower speeds it's clearly just a dot. This control also has a variable control within, but again needs to be left in the "C" position if the numbers around the switch are to be true.

Like the vertical controls, there is a shift control. Adjust this so the start of the trace coincides with the left hand edge of the graticule (posh word for the grid etched in the CRT). Getting in the habit of doing this is good - a lot of engineers don't bother - because you can see the point at which the trace starts, which helps when trying to figure out the triggering...

Which leads me onto the tricky part...

If you remember old TV sets where you had to adjust the vertical hold control to stop the picture running up and down, then you'll have a head start here. For the younger folk I tend to see at work, that particular anecdote means nothing, but perhaps it helps here?

The 'scope moves the dot from left to right at a smooth speed that you select with the "TIMEBASE" control. It is moved up and down by the signal on the Y input. But, how does the 'scope know when to start moving?

Providing it's in AUTO mode, it'll free-run, producing a nice flat line in the absence of a signal - that's what we want by default. This is controlled by the button on the top-right of the instrument. In one setting of this button, the trace will disappear completely if the signal isn't correctly triggered - occasionally useful, but for now, pick the other position.

If the square wave from your oscillator was a nice, stationary trace - I'm guessing it was? - then congratulations! These 'scopes trigger quite easily, so that's quite likely. Other 'scopes need more help.

For triggering to work, the 'scope takes a "sniff" the signal that is applied to the Y input. Then, when this signal crosses a certain threshold, the 'scope raises the green flag and sweeps. After this, the beam is switched off and the dot is returned to the left of the screen, ready for the next green flag...

The threshold is determined by the "TRIG. LEVEL" control. Leave it in the middle for now, but when you next look at a sine wave, adjust this control to see the effect (remember what I said above about the importance of seeing the start of the sweep, so turn the "X-POS" control clockwise if you can't see it). You should find that you can get the trace to start at pretty much any part of the sine wave. However, if you turn the control too far, the trace will become "unlocked", and won't be stable. Alternatively, you might find that the trace disappears completely - if so, you've got the "AUTO/NORMAL" switch in the wrong place (try it). Also, try the effect of the "+SLOPE/-SLOPE" switch.

Before the "TRIG. LEVEL" control is some more circuitry to determine where exactly this trigger signal is coming from. In a lot of cases, channel 1 is the right answer, but there are alternatives. There is a BNC connector and switch for an external trigger, and you can also choose to trigger from channel 2 rather than channel 1. Or the 50 mains waveform. Explore these options later, but for now I'd recommend simply triggering from channel one. For this, make sure the following controls are set this way:

TRIGGER SELECTOR: AC
EXT. TRIG: Out
CH.1 TRIG/CH.2 TRIG: Out

Finally, make sure all these buttons are out:

SWEEP/X-Y
COMPONENT TESTER
MAGN.X1/MAGN.X5

Between this and what's included in the manual, there's hopefully enough here to (a) get you started, and (b) verify that the 'scope is working properly (more checks can come later). I'm not sure how long the warranty is, if any, but given the price you paid for this, it'll be nice to test as much as we can as soon as possible.

Happy twiddling!

Mark

[Refugee]

If you are really stuck there is the possibility of a trip over to Worksop.
I can show you the basics of using a scope. If you don't want to see a fancy scope yet you will need to bring yours with you.
I can check it out for you and make sure the calibration is at least reasonable.

[Bonky]

Dear Refugee,

A very kind offer - many thanks. If I do manage a trip it will have to be after Christmas...is that OK?

You really are a very helpful bunch; most impressed.

Thanks,

Richard.

[Refugee]

I was aware that it would be some time later as you are away.
That will give me an excuse to paint over the patch where the roof leaked over a year ago.

[Mark Hennessy]

I think that's an excellent outcome - it's so much easier to do it face-to-face

In the meantime, do have a fiddle. You can't break anything by mis-adjusting the controls - just remember to keep the brightness down to a reasonable level.

[pwdrive]

Keep the input switched to AC for the first few goes otherwise stuff might disappear and you'll be wondering where it's gone.

Lawrence.

[Nick]

Keep the input switched to AC for the first few goes otherwise stuff might disappear and you'll be wondering where it's gone.

Lawrence.

And start with the Y sensitivity wound down to something like 20V/div so that you don't trash the inputs and can find the trace...

[ppppenguin]

Keep the input switched to AC for the first few goes otherwise stuff might disappear and you'll be wondering where it's gone.

Lawrence.

And start with the Y sensitivity wound down to something like 20V/div so that you don't trash the inputs and can find the trace...

Reminds me that I could really do with a 100:1 probe when working with valve kit. The sort of posh scopes I have only go to 5V/div so 50V/div at the tip of a 10:1 probe isn't always enough.

[Bonky]

Just ordered the AN8002;many thanks.

Now another question...(Mods..does this need a new thread?). I'm looking at a bench-top power supply... I'm sure with all my latest acquisitions there will be some wondering why bother when I can't fly the other bits of equipment yet properly. I was looking at something like: https://www.ebay.co.uk/itm/DC-Power-Supp....l4275.c10

One of my many hobbies is model railways and I have a NCE DCC system. It will soon be time for me to want a 5A booster such as: http://www.digitrains.co.uk/ecommerce/se...ation.aspx

...and this needs a 5A supply at about 14V...so I'm really trying to kill two birds with one stone I suppose.

I would prefer a linear supply (not SM) and without a fan noise all the time. I'm aware that the model I posted above does have a fan. I can't justify anything other than a well-regarded Chinese unit although I am aware of a good UK model on the 2nd-hand market.

Any advice very welcome as usual; thanks.

Richard