Microwave Engineering Components and Semiconductor Devices

Microwave Circulators

A circulator is a nonreciprocal device, meaning its transfer function cannot be calculated inversely (from output to input). The characteristics of this device depend on the direction of the applied signal. It is based on magnetic ferrite and utilizes a ferrite core.

Microwave Semiconductor Devices

Microwave Diodes

Diodes are used for the detection of microwave (MW) signals. Historically, these were the primary components used until the 1960s.

• Passive Diodes: Schottky and PIN diodes.
• Active Diodes: Varactor, Tunnel, Gunn, IMPATT, and TRAPATT diodes.

Standard wires cannot be used with these diodes because the wire becomes inductive and blocks the microwave signal. Instead, diode chips are connected using wire bonds without standard connectors.

Schottky Barrier Diodes

The Schottky barrier diode has no minority carriers, which eliminates delays by utilizing majority carriers only. Its high operation speed makes it ideal for microwave applications as a detector.

Cutoff Frequency

The cutoff frequency (or corner frequency) is the threshold above or below which the power output of a circuit—such as a line, amplifier, or electronic filter—falls to a specific proportion of the power in the passband.

Transistors

Transistors serve as fundamental active components in microwave circuitry for amplification and switching.

Directional Couplers

A directional coupler is a 4-port passive device used to sample a small amount of input signal power for measurement purposes. It divides and distributes power to measure incident power, reflected power, and Voltage Standing Wave Ratio (VSWR) values.

Port Configuration

• Port 1: Input port
• Port 2: Output port
• Port 3: Coupled port
• Port 4: Isolated or terminated port

Key Performance Metrics

• Coupling Factor: The ratio of coupled power to the input, measured in dB. Coupling = 10 log(P3 / P1).
• Directivity: The ratio of the power at the decoupled port to the power at the coupled port. Directivity = 10 log(P4 / P3). High-quality devices typically range from -20 dB to -40 dB.
• Insertion Loss: The ratio of incident power to transmitted power. Insertion Loss = 10 log(P2 / P1), where P2 is the throughput transmission power.
• Return Loss: The ratio of incident power to reflected power.
• Isolation: The difference in signal levels (in dB) between the input port and the isolated port when the other two ports are terminated by matched loads.

Microwave Vacuum Tubes

Traveling Wave Tube (TWT)

A Traveling Wave Tube (TWT) is an elongated vacuum tube featuring an electron gun (a heated cathode that emits electrons) at one end. Voltage applied across the cathode and anode accelerates electrons toward the far end, while an external magnetic field focuses them into a beam. The electrons eventually strike a collector, which returns them to the circuit.

Klystron

A klystron is a vacuum tube designed with cavity resonators. It produces velocity modulation of the electron beam for amplification purposes.

Magnetron

A magnetron is a combination of a simple diode vacuum tube with cavity resonators and an extremely powerful permanent magnet.