How to Measure Differential Signals Using an Oscilloscope

Reference: https://www.pcbway.com/blog/technology/How_to_measure_differential_signals_using_an_oscilloscope_%5BExamining_the_ADSL2_%5D.html

Single-Ended Transmission

Single-ended signaling is a simple and common way of transmitting electrical signals from a sender to a receiver. The signal is ground referenced (0 Volt) and one conductor (wire) carries the signal and one conductor carries the common reference potential. The current associated with the signal travels from sender to receiver and returns to the power supply through the shared ground connection. If multiple signals are transmitted, the carrier requires one wire for each transmission line plus one shared ground reference. Figure-1 shows the single-ended signaling topology.

Differential Signaling

Differential transmission is less common than single-ended transmission. It uses two transmission lines which sometimes accompanies with a third reference (ground) line. One transmission line of the differential pair is used to carry the signal and another line is used to carry the inverted signal. The receiver extracts the information by calculating the potential difference between the inverted and non-inverted signals. Both inverted and non-inverted signals are balanced, it means they carry the same potential, but with opposite polarity. By this strategy, the majority of the induced external noise on the transmission lines will be subtracted and canceled on the receiver side.

With differential signaling, the sender and receiver don't necessarily share a common ground reference. However, the use of differential signaling does not mean that using a ground reference has no positive effect on the line quality. When several transmission lines are used, it is absolutely beneficial to use a ground reference, in accompany with a pair for each differential signal. Figure-2 shows the differential signaling topology.

Benefits of Differential Signaling

a. No return path

Since there is (ideally) no return current, using a ground reference is less important. The ground reference is essential in DC-Coupled signaling such as RS485 or CAN where the signals must be maintained within the allowed voltage range.

b. Resistance to Incoming EMI and Crosstalk

If EMI (electromagnetic interference) or cross-talk (i.e., EMI generated by nearby signals) is induced from outside, it is added equally to the inverted and non-inverted signals. The received side responses to the potential difference of input signals, therefore the majority of the noise will be canceled. Therefore the bandwidth and SNR factors of the signal will be improved. This noise immunity allows the sender to use lower voltage levels but still maintain an adequate SNR ratio. Also, the SNR value of differential signaling is automatically increased by a factor of two, in comparison with an equivalent single-ended implementation.

Differential signal measurement in practice

To measure a differential signal, we have two options, one is using a differential probe and second is using a two channels oscilloscope. A differential probe is expensive but handles a better accuracy. Using two/four channels oscilloscope is the cheapest method which handles acceptable results.

Just you need to connect oscilloscope’s Channel-1 to one of the differential lines/wires and Channel-2 to another one. Then go to the math function and enable CH1-CH2 which means a difference. Then adjust the oscilloscope to observe the signal. You use the run/stop or a single shot button to freeze the signal on the screen and examine it.

I have examined the ADSL2+ signal on the telephone line using the above-mentioned differential measurement method. You can consider the signal in figure-3.

You can follow the method to examine the USB, CAN or your own desired differential signals/data. My oscilloscope can not trigger the math result signal, so I had to use the Run/Stop button, but your device might have such an option.