Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 59

While most passive components are linear and bilateral (that is, the forward loss equals the reverse loss), a particular class of devices based on the ferromagnetic effect doesn't follow this rule. These devices comprise circulators and isolators. A circulator is a 3‐port device, with low loss in one direction between ports, say from port 1 to 2, port 2 to 3, and port 3 to 1. But it has high loss (called the isolation) in the reverse direction, from port 2 to 1, 3 to 2, or 3 to 1. An isolator is a special case of a circulator with a good load applied to port 3, such that it becomes a 2‐port device. Circulators pose a particular measurement difficulty as the isolation between ports depends upon the match applied to a third port. Thus, for good measurement quality, the isolation measurement requires a good effective‐match on the ports.

Further, circulators are often tuned by magnetizing a permanent magnet attached to the circulator, and it's desired that the measurement system can determine the isolation of all three ports in a single connection step, and with good speed. Thus, multiport (more than two ports) systems were developed to simplify the connections, and multiport calibration techniques were developed to satisfy the need for high‐quality correction.

Even though they are passive devices, circulators and isolators are sometimes tested for their high‐power response, such as compression and IMD. The ferromagnetic effect has hysteresis properties that can produce IMD and compression when driven with sufficiently high powers. Figure 1.36 shows an isolator (left) and a circulator (right). Figure 1.37 shows the signal flow for a circulator. In the figure, the isolator on the left has an internal load mounted at the top, and the circulator on the right has three ports, with an SMA connector in place of the isolator load.

Figure 1.36 Isolator (left) and circulator (right).

Figure 1.37 Schematic representation of a circulator.