Electronics > Beginners
Old analog oscilloscope questions
james_s:
I wouldn't worry too much about whether the scope is grounded or not. If it originally had a grounded plug which has had the ground cut off or been replaced with a 2-prong plug then it should be replaced with a grounded plug. If it was never grounded to begin with then don't worry about it, it's ok that it's floating. When people speak of "floating a scope" they often mean deliberately disconnecting the ground in order to use the scope to measure a high voltage non-isolated device, this is dangerous, not the fact alone that the scope is floating. So long as you are staying away from the mains side of whatever gear you're working on then there should be no problem. A good rule to follow is if you wouldn't touch something with your bare finger, don't connect a scope to it.
Now as for probe calibration, you don't need anything fancy there. A simple square wave oscillator capable of reasonably fast edges is all that is required. Something like this should make a pretty good probe calibration oscillator.
http://www.epanorama.net/circuits/tdr.html
tedsorvino:
Thanks guys for all the advice and effort.
I'm using an android app. Keuwlsoft Dual Channel Function Generator. It's pretty basic and quite simple.
I know how to set a particular frequency of any shape and amount but I don't know how to set the voltage of its amplitude, since the generator only measures the amplitude percentage.
Could you please help me on how to set it. Is it an appropriate signal generator or do I need something else, even for other proper measurements?
james_s:
It depends on what you want to do. The volume control will set the amplitude, it should be adequate for starting out at least.
tedsorvino:
I understand that amplitude it's volume relative but what exactly do I need to do in order to calibrate the probes as good as possible?
After some audio tests I realized that my cheap probes function quite well, in both settings ( x1 - x10) and in all available voltage divisions. But I need a good reference to be as sure as possible, with my measurements.
Also some of the audio things I need to measure demand a particular peak to peak amplitude voltage.
I think I need a different signal generator as it seems more reliable to work it from a computer soundcard than an android phone (there is a lot of extra noise). One capable to set the voltage too. Any suggestions?
And another question is how does a set amplitude work along with volume? I mean if you set the amplitude to 1V p/p with the volume up to 30% and then you lower the volume down to 10% isn't the new amplitude around 330mV? And what is the real 1V p/p (or whatever amount) amplitude that it's demanded ? How does it really work?
Sorry if I confuse you, it's all new to me.
Gyro:
As previously indicated, the actual voltage and frequency don't matter - as long as it's somewhere in the region of 1kHz and the output voltage is high enough to give you a reasonable sized trace... and it's a squarewave.
tggzzz used the wrong word - it is not probe "calibration" it is probe compensation. The probe automatically has a DC divide ratio of 10 (and 1 in the other switch position), the compensation trimmer in the probe is there to match the probe to the input capacitance of your particular scope. When compensation is correct, you will see a nice square wave with square leading corners, the tops of the squares should be nice and flat (not tilted). A bit of noise shouldn't stop you being able to do this.
The probe compensation trimmer is only needed for the X10 position. In the X1 setting, the probe is basically just a straight-through 'bit of wire', the circuit sees the full scope input capacitance in this case. The X10 position divides the probe input capacitance (not by the full 10 because there are stray capacitances in the probe and clip) and increases the input resistance from the scope's 1Meg Ohms to 10Meg Ohms. It does this at the expense of dividing the voltage by 10 at the same time.
The X10 position is normally the best position to use (particularly at higher frequencies) because it reduces loading on your circuit and helps to protect your scope from accidental overloads.
The probe compensation adjustment needs a squarewave with reasonably fast edges, your PC soundcard or Android app might be ok (try them), if you are not seeing any reasonable visible difference when you adjust the probe compensation trimmer then you will need to make something with faster edges, such as the circuit that james_s linked.
As for voltage calibration of the scope itself - you should be able to check this using a variable DC source and your DMM. Remember that a scope is not a precision measurement instrument - it shows you what waveforms look like and gives to a reasonable indication of amplitude - probably around 5% accuracy for your scope. Only adjust its voltage calibration if it is significantly off.
For checking the timebase accuracy (us/div - ms/div etc.), your PC soundard and Android app should both be fine - their frequencies are derived from a crystal oscillator, so will be accurate. Again, expect your scope to be within a few %.
I hope this clarifies things for you.
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