Single-Ended Transmission

Single-ended signaling is a simple and widely used method of transmitting electrical signals. It relies on a ground reference (0 Volt), with one conductor carrying the signal and another carrying the common reference potential. The signal’s current flows from the sender to the receiver and returns to the power supply through the shared ground connection. If multiple signals are transmitted, each requires its own wire along with one shared ground reference. Figure 1 illustrates the single-ended signaling topology.

Figure 1: Single-Ended Transmission Topology

Differential Signaling

Differential transmission is less common than single-ended signaling, but it offers several advantages. It uses two transmission lines and sometimes a third reference (ground) line. One of the differential pair lines carries the signal, while the other carries the inverted signal. The receiver extracts the information by calculating the potential difference between the two signals. Both the inverted and non-inverted signals are balanced—meaning they carry the same potential but with opposite polarity. This technique helps cancel out the majority of external noise induced on the transmission lines.

In differential signaling, the sender and receiver do not necessarily need a shared ground reference. However, using a ground reference can still improve line quality. When multiple transmission lines are used, it is highly beneficial to use a ground reference along with a pair for each differential signal. Figure 2 shows the differential signaling topology.

Figure 2: Differential Transmission Topology

Benefits of Differential Signaling

• a. No Return Path: Since there is ideally no return current, a ground reference becomes less critical. However, in DC-coupled signaling such as RS485 or CAN, the signals must stay within an allowed voltage range, which makes the ground reference important.
• b. Resistance to EMI and Crosstalk: If electromagnetic interference (EMI) or crosstalk from nearby signals affects the transmission lines, it will be added equally to both the inverted and non-inverted signals. Since the receiver calculates the difference between the two signals, most of the noise will be canceled out. As a result, the bandwidth and signal-to-noise ratio (SNR) will improve. This noise immunity allows the sender to use lower voltage levels while still maintaining an adequate SNR. Additionally, the SNR of differential signaling is automatically doubled compared to an equivalent single-ended implementation.

Practical Measurement of Differential Signals

To measure a differential signal, there are two primary options:

• Using a differential probe, which provides better accuracy but is more expensive.
• Using a two-channel oscilloscope, which is a more affordable option that still yields acceptable results.

To use a two-channel oscilloscope, connect Channel 1 to one of the differential lines and Channel 2 to the other. Then, enable the math function and select CH1-CH2 to calculate the difference. Adjust the oscilloscope settings to observe the signal, and use the Run/Stop or single-shot button to freeze the signal for analysis.

I have used this method to examine the ADSL2+ signal on a telephone line, as shown in Figure 3.

Figure 3: ADSL2+ Differential Internet Signals (Telephone Line)

This method can also be applied to examine USB, CAN, or other differential signals. Note that some oscilloscopes may not be able to trigger on the math result signal, in which case you can use the Run/Stop button to capture the signal. Your oscilloscope may have different options for this functionality.

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