Oscilloscope coupling and effects on the measured signal

Question

I have a basic understanding of the of coupling settings on an oscilloscope, but I need some help to understand what I am seeing on scope.

I have a signal generator, with an arbitrary function set, a 20 us pulse HIGH with 80 us LOW. Amplitude is set to 10 V, and the signal generator allows me to set between -511 to +511 points to shape the signal. I set the first point at +511 (for 20 us) and the remaining at 0 (for 80 us).

When I connect to the oscilloscope to measure the signal, depending on the coupling chosen I get different outputs.

• DC 1 MOhm, shows me the pulse approximately at 10 V peak.
• AC 1 MOhm, shows me a similar approximately 10 V peak, but shifted down -2 V.
• DC 50 Ohm, shows me approx. 5 V peak pulse

When I connect the output from the signal generator to my circuit, (I am using the signal to control a MOSFET,) if the DC coupling is ON on the scope, the voltage is not high enough to turn on my MOSFET, but if I us the DC 1 MOhm coupling then it works fine.

I would like to understand what is happening with the different coupling settings. Particularly why signal is approximately halved from the 1 MOhm to 50 Ohm. And why it effects my circuit in the way it does?

And any advice, in terms of coupling choice, when wanting to make measurements of signals and voltages across parts of a circuit, would also be appreciated?

Answer 1

The signal generator has an output impedance of 50\$\Omega\$, so connecting it to an oscilloscope input with 50\$\Omega\$ impedance gives you half the the signal across the input. Your signal generator may have a 50\$\Omega\$ output impedance selection that does not really change the output impedance but delivers double the set amplitude in anticipation of a 50\$\Omega\$-terminated load, when that option is selected.

Normally you will want to use high impedance (usually with a 10x probe) on your oscilloscope in order not to disturb the signals. The 10x probe will give you 1/10 the loading on the signal. You will also normally want to use DC coupling. AC is useful when you want to look at a small signal on top of a large DC bias (for example, ripple on a power supply). When set to AC coupling, the average value of the signal becomes zero, so a small duty cycle positive-going pulse will be slightly below zero when low and well above zero when high.

You would use the 50\$\Omega\$ scope input primarily for high-frequency signals where you are dealing with a source that is intended to be terminated with 50 ohms (and you would use 50\$\Omega\$ coaxial cable to transmit the signal, usually with BNC connectors on most oscilloscopes).

You may wish to use x1 probes when your signal is small in relation to the noise floor of your oscilloscope and you can tolerate the additional loading (typically some tens of pF and 1M\$\Omega\$).

Answer 2

The DC coupling mode shows your signal as it is. When you use AC coupling the signal will shift by the DC level of your signal. Since your signal is 10 volts for 20 usec and 0 vols for 80 usec. its average DC level is 1/5*10 + 4/5*0 or 2 volts which is what you observed. When you use the 50 ohm setting on your scope, your signal will drop in half due to the 50 ohm output impedance of your generator which acts as a voltage divider. In general you should use the 1M DC coupling mode. The AC mode is useful when your signal has a large DC conmponent and a relatively small AC component. This mode will prevent the signal from going off the screen when you try to observe the AC component. The 50 ohm mode should be used when you need to terminate a 50 ohm source.

Answer 3

It sounds like your signal generator has a 50Ω output, which is common. With the 50Ω setting on your scope, you are creating a 2:1 voltage divider.

Typically you want to use as high input impedance as you can get away with on your scope (probe set to 10X) so you don't load your circuit.

Answer 4

Scope settings

• DC - Directly Coupled - Input is fed directly to scope.
• AC - Alternatively Coupled - Input is passed through a capacitor to remove any DC component before being fed to scope.

Since you are feeding a pulse into the AC coupled scope, this will explain your shifted results.

Easier seen if you create a sinewave with an offset from function generator. DC will show the offset sinewave. AC will show the sinewave at 0V.

A shifted square wave will look good in DC, but the back end of each pulse will fall away in AC coupling as the capacitor tries to filter out what it perceives as the DC.

The 1MΩ and 50Ω settings are for matching the scope to target circuit.

Any measuring device will impact the circuit it measures. The 1MΩ high impedance setting will allow the scope to have minimal impact on the circuit being measured.

The 50Ω setting is for matching impedances to connect the scope to 50Ω loads (coaxial cables). In your case, the 50Ω function generator and 50Ω scope form a voltage divider, so signal is half, or 5V.