Introduction to Circulators, Diodes, Transistors, and Microwave Devices

Circulator

A nonreciprocal device, means transfer function cannot be calculated inverse (output to input). The characteristic of this device depends on which direction the applied signal. It is based on magnetic ferrite and uses a ferrite core.

MW Semiconductor Devices

• Diodes: can be used for detecting MW signals. (Until the 1960s).
• -Schottky pin diodes are passive. Varactor, tunnel are active diodes. Gun impatt, trapatt. You can't use diodes with wires because this wire becomes inductive and stops MW signal. Connect diode chips with wire bonds (without connectors.)
• Schottky barrier diode: there are no minority carriers, so there is no delay (only majority). The operation speed is high, so it can be used in MW applications as a detector.

Cutoff Frequency or Corner Frequency

Cutoff frequency or corner frequency is the frequency either above or below which the power output of a circuit, such as a line, amplifier, or electronic filter, has fallen to a given proportion of the power in the passband.

Transistors

A directional coupler is a 4-port device that is used to sample a small amount of input signal power for measurement purposes. As seen in the diagram below, Port 1 is the input port, port 2 is the output port, port 3 is the coupled port, and port 4 is the isolated/terminated port. It is a passive device that divides and distributes power. The power measurements include incident power, reflected power, VSWR values, etc.

The coupling factor of a directional coupler is the ratio of coupled power to the input, measured in dB.

Coupling = 10log(p3/p1) --- p1 = input signal, p3 = coupled port.

Directivity is the ratio of the power to the decoupled port to the power at the coupled port. 10log(p4/p3). Good quality (-20 dB to -40 dB).

Insertion loss: The ratio of incident power to transmitted power, in dB terminology, is the insertion loss. The ratio of incident power to the reflected power, in dB terminology, is the return loss.

Insertion loss: 10log(p2/p1) p2 = throughput transmission power to port 2.

Isolation of a directional coupler can be defined as the difference in signal levels in dB between the input port and the isolated port when the two other ports are terminated by matched loads.

Travelling Wave Tube (TWT)

TWT is an elongated vacuum tube with an electron gun (a heated cathode that emits electrons) at one end. A voltage applied across the cathode and anode accelerates the electrons towards the far end of the tube, and an external magnetic field around the tube focuses the electrons into a beam. At the other end of the tube, the electrons strike the 'collector,' which returns them to the circuit.

Klystron

Klystron is basically a vacuum tube designed with cavity resonators to produce velocity modulation of the electron beam for amplification purposes.

Magnetron

Combination of a simple diode vacuum tube with cavity resonators and an extremely powerful permanent magnet.