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Why do oscilloscopes need optical isolation probes?

Category: FAQ

Release time: 2025-03-05

An optical isolation probe, also known as an optical-fiber isolated probe, is a type of measurement probe used with oscilloscopes. In the field of test and measurement, the signal acquired by the front end of a high-voltage differential probe is generally transmitted to the back-end test equipment via a cable. This cable transmission method has the following drawbacks:

Why do oscilloscopes need optically isolated probes?

An optical isolation probe, also known as an Optical-fiber Isolated Probe, is a type of measurement probe for oscilloscopes. In the field of test and measurement, the signal acquired by the front end of a high-voltage differential probe is generally transmitted to the back-end test equipment via a cable. This cable transmission method has the following disadvantages:

1. Non-insulated, no safety in high-voltage situations, electrical isolation cannot be achieved between the test point and the test equipment;

2. The cable has parasitic capacitance, inductance, and resistance characteristics, limiting the bandwidth;

3. It is difficult to simultaneously meet the indicators of high voltage, low voltage, high bandwidth, and signal integrity;

4. Poor suppression ability for high-voltage, high-frequency common-mode interference.

However, high-voltage differential probes also have some advantages:

1. Cost-effectiveness: High-voltage differential probes are typically more economical than optically isolated probes.

2. Simplicity: High-voltage differential probes are relatively simple to use and maintain, and do not require complex fiber optic or optoelectronic conversion technology, making them easier to operate.

Optically isolated probes are also used to measure differential signals. Their principle is based on the photoelectric effect and physical isolation technology. They convert electrical signals into optical signals, transmit them via optical fiber to the other end, and then convert the optical signals back into electrical signals, thereby achieving complete electrical isolation between signal input and output. This conversion not only provides an extremely high common-mode rejection ratio (CMRR), providing excellent common-mode rejection performance across the full bandwidth, but also features high isolation voltage and a wide measurement range.

The working principle of an optically isolated probe can be summarized in the following steps:

1. The electrical signal is converted into a laser signal at the front end of the probe.

2. The laser signal is transmitted to the back end of the probe via optical fiber.

3. At the back end of the probe, the laser signal is converted back into an electrical signal.

The principle diagram is shown in the figure below. This probe mainly includes an attenuator, an electro-optical-electrical conversion network, a lossless transmission line, and an oscilloscope connector.