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Transit Time Microwave Devices
Published in Mike Golio, RF and Microwave Semiconductor Device Handbook, 2017
The IMPATT diode, transferred electron device (often called a Gunn diode), and tunnel diode are the most commonly used two-terminal active devices. These devices can operate from the low microwave through high mm-wave frequencies, extending to several hundred GHz. They were the first semiconductor devices that could provide useful RF power levels at microwave and mm-wave frequencies and were extensively used in early systems as solid-state replacements for vacuum tubes. The three devices are similar in that they are fabricated from diode or diode-like semiconductor structures. DC bias is applied through two metal contacts that form the anode and cathode electrodes. The same electrodes are used for both the DC and RF ports and since only two electrodes are available, the devices must be operated as a one-port RF network, as shown in Fig. 3.2. This causes little difficulty for oscillator circuits, but is problematic for amplifiers since a means of separating the input RF signal from the output RF signal must be devised. The use of a nonreciprocal device, such as a circulator can be used to accomplish the task. Circulators, however, are large, bulky, and their performance is sensitive to thermal variations. In general, circulators are difficult to use for integrated circuit applications. The one-port character of diodes has limited their use in modern microwave systems, particularly for amplifiers, since transistors, which have three terminals and are two-port networks, can be designed to operate with comparable RF performance, and are much easier to integrate. Diodes, however, are often used in oscillator circuits since these components are by nature one-port networks.
Microwave Processing of Materials
Published in Amit Bansal, Hitesh Vasudev, Advances in Microwave Processing for Engineering Materials, 2023
T. Lachana Dora, Radha Raman Mishra
Microwave energy reflects back to the microwave source while irradiating bad microwave absorbers; consequently, magnetrons get often damaged. The circulator protects the microwave instrumentation by behaving as a sort of a diode in an electrical circuit. The circulator could be a three-port waveguide part with nonreciprocating options in terms of forward and reflected wave treatment. Microwaves will solely travel in a way via the circulator. The three ports of the circulators are connected with the microwave source, the applicator, and a faux load. The arrangement deflects the reflected microwave power, and the mirrored power gets absorbed by a dummy load that is sometimes water [1, 5].
Microwave Transmission
Published in Stephen Horan, Introduction to PCM Telemetering Systems, 2017
Circulator and Diplexer Designers use circulators to isolate signals or force signals to flow in a particular direction. Figure 11.8(a) shows the circuit symbol for a circulator where the signal flow goes from port 1 to port 2, port 2 to port 3, and port 3 to port 1. For example, designers insert the circulator between the transmitter and the filter to keep reflected signals from flowing back into the transmitter and damaging it, as in Figure 11.8(b). Designers also use circulators to direct signal flow between the antenna and the transmitter and receiver components, as in Figure 11.8(c).
A review of microwave testing of glass fibre-reinforced polymer composites
Published in Nondestructive Testing and Evaluation, 2019
Zhen Li, Arthur Haigh, Constantinos Soutis, Andrew Gibson, Ping Wang
For an antenna, it should be mentioned that there is a radiative near field (Fresnel region) between the reactive near field and the far field. In this region the reactive fields do not dominate. However, unlike the far-field region, the shape of the pattern varies appreciably with distance. Depending on the values of R and the wavelength, this field may or may not exist. Hence, the radiative near-field region is not in the scope of this paper. To perform a far-field inspection, as shown in Figure 2, either monostatic configuration (a transmitter and receiver sharing one antenna with a circulator), Figure 2(a), or bistatic configuration (a pair of transmitting and receiving antennae), Figure 2(b), can be used. Each kind has inherent advantages and disadvantages. In the monostatic system here, the circulator is a passive non-reciprocal three-port microwave device, which routes microwave signals from the transmitter to the antenna and from the antenna to the receiver and prevents signals from passing directly from the transmitter to receiver. Compared with the bistatic counterpart, the dimensions of the monostatic system are more compact, while the lower isolation can result in a significant leakage [32]. Additional measures must be undertaken to reduce or suppress the undesirable leakage. The bistatic configuration has a high isolation between transmitting and receiving channels. During in-service inspections, commonly only one side of the object under test can be accessed, so the reflection mode is more advantageous.