oscilloscope signal channel has 2 impedance levels- 50 ohms and 1 megohms.
When do you use 50 ohms and when 1 megohms?

Thanks

 

50 ohms when you're working on radio circuit lines that have a 50 ohm impedance, othewise leave it at the 1M (probably 10M) setting.

 

Usually you want the measuring device (voltmeter, oscilloscope etc) to have the least effect on the circuit you are measuring. A low impedance measuring device will 'load' the circuit being measured causing an innacurate reading. In those cases use a high impedance setting, if there is a choice, such as on your 50 Ohm or 1 megOhm scope. Usually you don't have a choice on a voltmeter. It is what it is.

With an oscilloscope where you are viewing the waveform you neither want to load the circuit nor do you want to change the shape of the waveform (which is actually capacitive loading). The impedance of the scope input can both load the voltage same as a voltmeter causing a lower than actual measurement and it can change the waveform especially in high frequency circuits such radio (rf) circuits as Marshall said.

What can be helpfully instructive is to measure with two devices simultaneously to see how one measuring device affects the voltage or waveform being measured with the other. Use two scopes or a voltmeter and a scope and by connecting and disconnecting one of the measuring devices while monitoring with the other observe how the voltage or waveform changes. Remember this - Whenever you attach a measuring device to a circuit to measure something, you change the circuit being measured affecting the actual measurement more or less. Hopefully less so you can observe an accurate measurement.

 

Oh, and don't forget to calibrate the probe. Most scopes have a calibration output that's merely a square wave and the probes have a tiny screwdriver slot for adjustment. Tweak that screw to get the best square wave.

 

When using a probe, it is important to have the correct terminating impedance. Using the wrong impedance will give incorrect results. A lot of passive high-impedance types are intended for an input impedance of 1MΩ in parallel with something like 15pF. Probes for 50Ω use are rarer; unless you work with equipment meant for really quite high frequencies you may not encounter them.

Leaving probes aside, you must avoid selecting the 50Ω impedance if the point you are monitoring can have a large voltage on it. Due to the low impedance, anything more than a few volts is likely to constitute an overload: 5V gives 0.5W. The relatively big current drawn can also damage the circuit you are expecting to measure.

 

Thank you very much for your valuable suggestions.
Regards