How does an oscilloscope respond to signals on a shielded line?

Oscilloscope probes have two wires, one for connecting the test circuit to the vertical amplifier of the oscilloscope (called the sense wire) and the other for connecting the oscilloscope chassis ground and the digital logic ground of the local circuit (called the shield wire). Usually, we only need to consider the oscilloscope's response to the sense wire voltage. This section analyzes how the oscilloscope responds to the signal on the shield wire.

Any voltage difference between the oscilloscope's chassis ground and logic ground can induce current in the shield. In Figure 1, the shield current flowing through the shield resistance Rshield creates a voltage drop, Vshield. The center conductor of the probe cable, the sense wire, does not conduct shield current, so no voltage drops across it.

When both the sense and shield wires are connected to the ground of the working circuit, the different voltage drops on the two wires will be reflected as a voltage difference between them on the vertical amplifier of the oscilloscope. We have no way of knowing whether this voltage difference is caused by the actual signal at the far end of the probe cable or by the shield current. Although we hope that the oscilloscope will show no voltage, it can show the shield voltage.

The oscilloscope responds to the shield voltage as if it were a real signal.

The shield voltage is proportional to the shield resistance, not the shield inductance. This is because the shield conductor and the center conductor are magnetically coupled. Any changing magnetic field generated by the current flowing through the shield loop surrounds the shield conductor and the center conductor together, inducing the same voltage on both conductors. The induced voltage exists on both conductors at the same time, while the resistance voltage drop only appears on the shield.

The shield voltage is easily observed:

1. Connect the probe tip of the oscilloscope to the ground wire.

2. Move the probe near the working circuit without touching anything. At this time, only the magnetic induction detection voltage from the induction loop of the probe can be seen.

3. Wrap the end of the probe with aluminum foil and short-circuit the detection contact directly with the metal grounding sheath of the probe. At this time, the magnetic field detection voltage is reduced to near zero.

4. Now put the oscilloscope probe to the logic ground of the test circuit. You should only see the voltage on the shield line. If the shield voltage is very small, it can be ignored.

Shield noise can cause trouble for digital systems that control high-power devices . The huge 60Hz AC current in the device will induce voltage on the digital logic ground, which in turn causes shield noise. If the shield voltage is causing trouble, there are 9 ways to overcome it.

1. Reduce the shield resistance. If the probe is purchased, this is more difficult to do. If you use a homemade coaxial cable probe, then choose a thicker coaxial cable. Change from RG-174 to RG-58, or from RG-58 to RG-8. The hardness of the thicker coaxial cable makes this method unrealistic, but it solves the problem of instrumentation.

2. Add a bypass impedance between the oscilloscope and the logic ground. This causes most of the noise current to flow through the bypass impedance, while a small amount of current flows through the shield. This method is usually not practical, especially for high frequencies. It is almost impossible to pick a good ground point from the test circuit board and connect it to the oscilloscope ground through a conductor with low enough inductance to obtain a significant improvement.

If the bypass conductor is the same length as the probe lead, there is no conductor of large enough diameter that will improve the problem (inductance varies with the logarithm of diameter). If the bypass conductor is much shorter than the length of the oscilloscope probe lead, it may work.

3. Power off the test circuit board, or partially power off. This method is only effective when testing a local circuit. If you suspect that the problem comes from shielding current noise, this is a good test method. This can be used to determine whether the noise is generated by the test circuit or caused by other interference sources.

4. Connect a large inductor in series in the shielding loop, use a large high-frequency magnetic core, and wrap the probe around it 5 to 10 times. This will increase the inductance of the probe shielding layer, thereby reducing the current. This method works better in the range of 100KHZ~10MHZ. Below 100KHZ, a large inductor is required to improve the problem. Above 10MHZ, the effect of the magnetic core will decrease.

5. Redesign the circuit board to reduce electromagnetic field radiation. Change the two-layer board to a four-layer board and add a complete ground plane. Reducing electromagnetic field radiation is the primary way to reduce the tendency of the ground plane to generate noise voltage.

6. Disconnect the safety ground wire of the oscilloscope. Disconnecting the safety ground wire of the oscilloscope invalidates the safety feature of the oscilloscope's AC power system. Once the power supply part of the oscilloscope's power system is connected to the chassis, the oscilloscope's chassis becomes connected to the 110V power supply voltage, which is an unsafe voltage. Normally, if a fault occurs, the safety ground wire bypasses most of the AC power current to the ground and triggers the circuit breaker protection switch of the circuit. It cuts off the power and may well save your life at a critical moment.