# Oscilloscope question

• posted

Hi to all,

I connect my oscilloscope to an Diode-RL circuit to observe the wave form. The wave form appears completely different when I set my probe attenuation switch x1 and x10. When I set x1 I observe the expected wave function. When I set to x10 the probe and the scope I see an attenuating sinusoidal wave function.

The question is:

-Why I see deferent thinks in each case ???

-Which one is the correct wave function.

Brgds

• posted

Using a x1 probe loads your circuit by 1Mohm + cable and scope capacitance. It's in the pf's.

Either way, your probe can become part the circuit and affect it's behavior. A pita with high impedance sources when the frequency is high enough.

The x10 setting has a better chance of being believable.

Fet probe even better.

D from BC British Columbia Canada.

• posted

Normally, a switchable 1:1/10:1 probe will put about 10 megohms in parallel with perhaps 5 pF at the node you're measuring, when in the

10:1 position, and in the 1:1 position, the scope's 1 megohm input resistance in parallel with whatever the scope probe cable capacitance plus the scope input capacitance is. I'd expect between about 50pF and 100pF. Normally, I'd expect the higher capacitance to cause ringing in an LC circuit, more than the lower one. But probes always affect the circuit they are probing to some extent. Understanding that, you could say that both observed waveforms are correct, for the total system you are observing, or that neither one is correct, since the probe always will change to some extent the thing you're trying to measure.

It's possible that something is wrong with the probe. Does it work OK on known waveforms? It's possible that the probe you have doesn't conform to my expectations about how they usually are made, too.

Can you give more details of the circuit? If you simulate the circuit, complete with a decent model of the probe, what waveforms do you see?

Cheers, Tom

• posted

I am not sure I understand your question, but if the input impedeance of the "scope is 1 Megaohm in the X1 position then it puts a 9 Megaohm resistor in series with the input in the X10 position. So, the signal will seem to be

1/10 as big. Physically, the switch shorts out the 9 Meg resistor when it is in the X1 position. There will usually also be a small capacitor in parallel with the 9 Meg resistor. In addition, there will also be a small adjusting screw which you adjust in the X10 position while observing a square wave signal.

Tam

• posted

A x10 probe for oscilloscopes likely has the same probe impedance, it just adds shunt R and C (trimmable) across the scope side.

The x10 setting can be calibrated, and it can also be broken.

RL

• posted

That's rubbish. Don't know what I was thinking

The shunting bit is not so bad.

The breaking bit is OK. Scope probes can be internally intermittent on either an R or C element, giving odd frequency/amplitude response.

A common villain is the spring clip assembly - which can put probe into capacitive contact with the measured location.

A common victim is the trimmer component, which gets abused mechanically.

RL

• posted

sounds like cheap low BP probes.

```--
"I\'d rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:```
• posted

Yep, that's what I had in mind. He may also be using a probe that doesn't match the scope.

Tam

• posted

What frequency? What voltage? What impedance? I'm astonished by all the people who can answer your post without this basic information. Also, how do you think we can answer without that information?

• posted

Perhaps GIGO is better than GINO.. 'Garbage in- garbage out' is better than 'garbage in- nothing out'.

Question: What's wrong with my lemons? Answer: It can be fixed with oranges..

D from BC British Columbia Canada.

• posted

What frequency? What voltage? What impedance? I'm astonished by all the people who can answer your post without this basic information. Also, how do you think we can answer without that information?

D from AC already answered it correctly, you don't have to be too picky.

• posted

Not once did the words frequency, voltage or impedance figure in that answer. It's one of those "all the information is there, but no knowledge is transferred" posts.

• posted

The results are the same to frequencies from 150Hz to 600Hz and voltages from 20V- 180V

• posted

Hello,

When the frequency of the ringing is maximum about 25% of the probe bandwidth you are using, the *10 setting would give the best reading on your oscilloscope.

It will not mean that it is a correct reading. For example when (for the frequency of the ringing and during the ringing) the output impedance can be regarded as 10 KOhms in parallel with 20 pF, your probe will give significant loading. Assuming ringing frequency above about 500 kHz.

When the ringing is in the MHz range, also ground connections may play an important role. If possible use (each after each other) the spring clip ground and ground lead with clip and change ground connections. When results are the same, grounding will probably be no issue.

There are nice probe docs on the web. Try for example

and go to the technical library, do a search on "probes" and you will find a tutorial on probes.

Note that special *10 probes for high frequency (for example 100 MHz) tend to show a low omhic load at high frequency that may damp ringing at MHz frequencies. This behavior comes from the measures to avoid reflections on the probe cable.

I don't know your circuit, but as the ringing occurs after the turn- off of the rectifier, the point where you are measuring may show high output impedance, hence it is likely that the input impedance of the probe distorts the measurements.

Best regards,

Wim PA3DJS

(Dutch).

• posted

Hmm, is that anything like What's better than having roses on the piano ? Tulips on the organ? :)

```--
http://webpages.charter.net/jamie_5"```
• posted

It's a switched 10:1scope probe feeding a scope. This specifies expected impedance levels. The frequency issue is answered by the OP. Voltage levels are irrelevant, as the source doesn't change from one setting to the next.

RL

• posted

Right, so a 100V 100MHz sine wave riding on a 1000V level, which exceeds most probe's ratings, is of no importance whatsoever. Got it, thanks. I learn a lot in here.

• posted

People that work with signals like that usually know their probe limitations. There's a high probability that newbies are working with slower lower level stuff. If the OP comes back and writes 'whaaaaaaa!...It still doesn't work' Then something freaky is going on and the weird and wacky stuff gets considered next.

D from BC British Columbia Canada.

• posted

You have to show a bit of patience, if you're going to be able to help any of the first-time posters of problems on this news group. Often, they'd have been better off to post elsewhere, if the issue is basic.

I think you'll find that anyone who is working on high voltage or at high frequency, will also be the first to point this out. As the OP has pointed out, his signal frequencies are below 1KHz.

RL

• posted

Say what poser?

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