Before detecting the circuit signal, it is necessary to know what circuit the circuit under test is and what signal the signal is under test. Blindly testing or using incorrect measurement methods can lead to incorrect waveforms or even damage to the instrument.
1. What is a differential signal?What is a single-ended signal?
Differential transmission is a technology of signal transmission, which is different from the traditional practice of one signal line and one ground wire, differential transmission transmits signals on both wires, and the amplitude of these two signals is equal, the polarity is opposite, and the phase difference is 180 degrees. Then, the signal transmitted on these two wires is a differential signal. The nature of differential transmission means that differential signals are signals that occur in pairs. At the same time, because of the existence of pairs, the two signal transmission lines of the differential signal can be used as reference points for each other, or systematically as reference points in the circuit system. Therefore, it is important to accurately measure the amplitude, phase, and frequency of the differential signal.
Single-ended signal refers to a signal that is transmitted by only one wire or one line, and generally takes the circuit system as its voltage reference point. This can also be understood as the potential difference between a single-ended signal and the system ground that is transmitted on the same line.
2. Why do some signals use differential transmission instead of single-ended transmission?
Compared with single-ended transmission, differential transmission has stronger anti-interference ability. Because the two lines of differential transmission are close to each other, the interference noise is loaded on the two signal lines almost at the same time, and we can regard the difference between the interference signals received by the two lines of differential transmission as 0, that is, the noise has no effect on the logical significance of the differential signal. Since the reference point of single-ended transmission is systematic, the effect of this interference noise is directly fed back to the signal receiver.
The differential transmission method reduces the potential for electromagnetic interference (EMI). Since the two signal transmission lines are close together and the signal amplitude is equal, the amplitude of the coupled electromagnetic field between the two signal transmission lines and the ground wire is also equal, and their signal polarity is opposite, so that the electromagnetic field generated by them will cancel each other out. Therefore, there is also less electromagnetic interference to the outside world.
The timing positioning of the differential transmission mode is more accurate. The receiving end of the differential signal can be used as the point where the positive and negative jumps occur according to the difference between the amplitudes of the two signal transmission lines, as the point where the logic 0 1 jumps are judged. The single-ended signal usually uses the voltage threshold as the hopping point of the signal logic 0 1, and the single-ended transmission is greatly affected by the ratio of the voltage threshold to the signal amplitude voltage, which is not suitable for low-amplitude signals.