Why is ground loop interference a common interference phenomenon?

The earth loop often appears without a clear trace, leaving no visible shadow but only a faint mark on the oscilloscope. When an electronic device is functioning normally, it may suddenly appear and then vanish. This kind of interference can be tricky to detect and diagnose.

Ground loop interference is a common issue that occurs when devices are connected via long cables. The root cause is usually the presence of ground loop currents. In some cases, disconnecting the safety ground wire of a device can eliminate the interference, as this breaks the loop. However, this effect is more noticeable at lower frequencies. At higher frequencies, the impact of the ground wire becomes less significant.

There are two main reasons for ground loop interference. First, differences in ground potentials between two devices create a ground voltage. This drives current through the loop formed by Device 1 - the connecting cable - Device 2 - and the ground. Due to circuit imbalance, the current on each wire differs, leading to a differential mode voltage that disrupts the circuit. Additionally, if other high-power devices share the same ground line, it can cause higher current flow, increasing the voltage due to the ground line’s impedance.

Second, when interconnected devices are in a strong electromagnetic field, the field can induce a current in the loop formed by Device 1 - the connecting cable - and Device 2, causing interference.

There are two primary approaches to solving ground loop interference: reducing the ground line's impedance to lower the interference voltage, or increasing the impedance of the ground loop to reduce the current. If the impedance is infinite, the loop is effectively broken, eliminating the ground loop. For example, floating one end of a device or isolating a circuit board from the chassis is a simple method. However, for safety or protection reasons, such direct methods are not always feasible. More practical solutions include using isolation transformers, optical couplers, common-mode chokes, and balanced circuits.

Here are some specific practices:

1. [Cause of interference]: The "ground potential loop" adds voltage to both ends of the cable shielding, forming a loop through the 75 ohm resistors at each end, which generates an interference voltage on the load.

2. Cut off the ground loop – this is the most effective and straightforward method, requiring no additional anti-jamming equipment.

3. One of the key principles in monitoring system design is to connect the end device to ground at only one point. The front camera, BNC head casing, and cable shielding must be insulated from the ground (open circuit). Even with the use of anti-interference equipment, the ground loop can still persist because it is unstable. As more power grid equipment is added and power imbalances occur, the ground potential difference may change, potentially leading to equipment damage in severe cases.

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