The microwave switch, also known as a radio frequency switch, performs the conversion function required to regulate the microwave transmission channel. An RF (Radio Frequency) or microwave switch is a device that allows high-frequency signals to be routed across a transmission line. In microwave test systems, RF and microwave switches are commonly employed for signal routing between the instrument and the component under test (DUT). Let's have a look at the fundamentals of RF switches today.

Ⅰ. Performance Parameter

In the transmission path, RF and microwave switches can efficiently convey signals. Four basic electrical parameters can be used to describe the function of this sort of switch. Despite the fact that numerous parameters affect the performance of RF and microwave switches, the following four are considered crucial due to their significant correlation:

Isolation

Isolation is an index that measures the effectiveness of the switch cut-off by attenuating the signal between the input and output ends of the circuit.

Insertion loss

When the switch is turned on, insertion loss (also known as transmission loss) is the total power loss. Because insertion loss can immediately contribute to an increase in system noise figures, it is the most important metric for designers.

The picture shows a single-pole twelve-throw electromechanical switch using 12 different SMA female coaxial connectors

Switching time

The time it takes for the switch to convert from the "on" state to the "off" state, and vice versa is referred to as switching time. For high-power switching, this time can be measured in microseconds, and for low-power high-speed switching, it can be measured in nanoseconds. The most typical definition of switching time is the amount of time it takes for the input control voltage to go from 50% to 90% of its ultimate RF output power.

Power handling capacity

Furthermore, the power handling capability of a switch is defined as the maximum RF input power that the switch can withstand without deteriorating its electrical performance permanently.

Ⅱ. Detailed classification

Electromechanical relay switches and solid-state switches are two types of radiofrequency and microwave switches. These switches come in a variety of configurations, ranging from single-pole single-throw to single-pole sixteen-throw, which can convert a single input into 16 separate output states, as well as multiple throw variants. The switch is configured as a double-pole double-throw switch. This switch has four ports and two switch states, allowing the load to be switched between two different sources.

Electromechanical relay switch

The electromechanical relay switch has a low insertion loss of 0.1dB, high isolation of >85dB, and the ability to switch signals at millisecond speeds. The key benefit of this sort of switch is that it can operate in the DC to millimeter-wave frequency range (>50 GHz) and is not susceptible to electrostatic discharge. Electromechanical relay switches can also manage higher power levels (up to several kilowatts peak power) without causing video leakage.

However, there are a few issues worth mentioning in the operation of electromechanical radio frequency switches. This sort of switch has a conventional service life of just around 100 times, and its components are more susceptible to vibration. The entire number of switches that an electromechanical switch may complete while meeting radio frequency and repeatability standards are referred to as service life. Electromechanical switches with a high level of quality or dependability are ideal for applications that require a longer service life. This type of switch has extremely high reliability and other performance, as well as service life of up to 1000 times. The longer service life is due to the actuator's more robust design and a better-optimized transmission link in terms of magnetic efficiency and mechanical rigidity. Furthermore, this sort of switch is built to survive tougher environmental conditions and to exceed the MIL-STD-2002 standard's criteria for sinusoidal and random vibration, as well as mechanical shock.

PE71S6064 single-pole double-throw electromechanical switch with an operational frequency range of DC40GHz and guaranteed service life of 10 million times as an example of a reliable electromechanical radio frequency switch.

Solid-state radio frequency switch

Solid-state radio frequency switches, on the other hand, have a smaller package thickness and physical dimension than electromechanical switches since their circuit assembly is relatively flat and does not contain big components. High-speed silicon PIN diodes or field-effect transistors (FET) or integrated silicon or FET monolithic microwave integrated circuits are the switching elements used in solid-state radio frequency switches. Other chip components like capacitors, inductors, and resistors are independently integrated on the same circuit board as these switching parts.

PIN diode-based switching products have higher power handling capabilities, but FET-based switching devices have faster switching speeds. Solid-state switches, on the other hand, have an infinite service life since they contain moving parts. Furthermore, the solid-state switch has high isolation (60>80dB), a quick switching speed (100 nanoseconds), and is stress and vibration resistant.

The insertion loss of solid-state radio frequency switches is another important feature. In terms of insertion loss, solid-state radio frequency switches are inferior to electromechanical switches. Solid-state radio frequency switches, on the other hand, have limits in low-frequency applications. This is due to the fact that the lowest limit of its operating frequency is merely kilohertz, not DC. This limitation is due to the semiconductor diode's inherent carrier lifetime characteristics. Solid-state radio frequency switches are also more susceptible to electrostatic discharge, and their power handling capabilities are influenced by the switch arrangement, connector type, operating frequency, and ambient temperature. Although certain PIN diode switch topologies can withstand peak power of several kilowatts, this comes at the cost of slower switching rates.

The PE7167 single-pole four-throw switch, as an example of a PIN diode switch, has an operating frequency of 500 MHz to 40 GHz and a maximum switching speed of 100 nanoseconds. The PIN diode is essentially employed as a variable resistor in solid-state switches, and its resistance value may be modified by a DC bias current.

Solid-state radio frequency switches are more dependable, have a longer service life, and switch faster than electromechanical switches. Solid-state radio frequency switches should be preferred in applications requiring higher switching speed and reliability; electromechanical switches should be preferred in applications requiring wide frequency band coverage as low as DC and low insertion loss, and solid-state radio frequency switches should be preferred in applications requiring long service life. High-reliability switches are suitable for applications with strict criteria.

Other properties of the above-mentioned varied switching products, such as 50-ohm resistive loads, should be understood by designers. Any idle open transmission line in the switching circuit could resonate at microwave frequencies. Under normal operating settings, this resonance can reflect electrical energy back to the radio frequency source, inflicting damage to it. Because of the considerably reduced isolation, the above-mentioned damage will be more severe for systems with an operating frequency of 26 GHz or higher. As a result, most transmission lines are constructed with a 50-ohm impedance, resulting in very little reflected energy at the RF switch once a 50-ohm resistive load is added.

Electromechanical RF Switch

There are two types of electromechanical radio frequency switches: terminated and non-terminated. When all channels are terminated with a 50-ohm load in the termination switch, the selected channel is closed, cutting off or isolating all currents. The incident signal's energy will be absorbed by the termination resistor and will not be reflected back to the RF source in this way. Because the non-terminated switch is configured to a 50 ohm load, other portions of the system must accomplish impedance matching to reduce energy reflection. The non-terminated switch has the advantage of having a low insertion loss.

The armature relay mechanism is another significant component of the electromechanical radio frequency switch. The induced magnetic field causes the armature coil to move when the coil is energized, opening or closing the contact. When the electricity is not flowing, the non-latching switch is equipped with a spring or a magnet that keeps the switch in a typically closed state. When the power supply is stopped, this sort of switch is ideal for applications where the switch must be restored to a known condition.

The PE71S6064 SPDT switch uses several typical electromechanical radio frequency switch components, including a DC 28V latching actuator, several independent contact pieces connected to the switch status indicator, and a 50 ohm set at the idle port Terminal matching resistance, as shown in the schematic diagram above.

Because the lockout switch has a lockout mechanism and no default position, it keeps the last state before the power is turned off. Because the latching relay switch's contact coil consumes power only when the relay is turned off, it's ideal for applications where power dissipation is an issue.

There are also fail-safe operation modes for some other sorts of switches. The radio frequency channel returns to the power-off state when the voltage delivered to the coil disappears in this mode. However, compared to a latching switch, the switch in this mode has a shorter mean time between failures because just continuous voltage is given to the coil to keep it activated.

A series of auxiliary DC contacts connected to the radio frequency channel switching coil is another notable element of the electromechanical radio frequency switch. These auxiliary contacts are normally used to control indication or signal lights that indicate the state of the RF channel. Furthermore, these contacts can be used to communicate status information to external control systems.

Solid-state RF switch

There are two types of solid-state radio frequency switches: absorptive and reflecting. In order to obtain a lower voltage standing wave ratio (VSWR) in both on and off states, the absorption switch has a 50-ohm terminal matching resistance at each output port. The above-mentioned output port's terminating resistor can absorb the incident signal energy, but the port that is not linked to the terminating matching resistor will reflect it. The above-mentioned open port is detached from the terminal matching resistance when the input signal must propagate through the switch, allowing the signal's energy to be totally propagated from the switch. Absorptive switches are ideal for applications where the RF source's echo reflection must be minimized.

The reflecting switch, on the other hand, does not have a terminal resistor to reduce the insertion loss of the open port. The reflecting switch is ideal for applications that aren't affected by the port's high voltage standing wave ratio. Furthermore, various components other than the port achieve impedance matching in a reflective switch.

The drive circuit of solid-state switches is another key component to consider. Input control voltage drivers are integrated with some solid-state switches. These drivers' input control voltages can achieve certain control functions, such as providing the appropriate current to ensure that the diode can receive a reverse or forward bias voltage, depending on the logic state of the input control voltages.

Most coaxial switch products with operating frequencies up to 26GHz use SMA connectors; up to 40GHz use 2.92mm or K-type connectors; up to 50GHz use 2.4mm connectors; up to 65GHz use 1.85mm connectors. Electromechanical and solid-state RF switches can be made into a variety of products with different packaging specifications and connector types.

A single-pole double-throw radio frequency switch using a PIN diode as a switching element and a passive device that divides the radio frequency and DC bias signal channels is shown in the diagram. The public RF terminal can be connected to the system antenna in a typical application, while RF terminals 1 and 2 are attached to the transmitter and receiver, respectively. The resistance value of the PIN diode is controlled by the forward DC bias current of the diode, which is employed as a radio frequency resistor. In general, the DC bias current can change the RF resistance of a conventional PIN diode by three or more orders of magnitude. When the above-mentioned diode is bias-off, its impedance is as similar to that of an open circuit as possible.

Switches with waveguide ports have the lowest insertion loss, thus they're common in the microwave and millimeter-wave frequency bands for high-power transmission communications. Power handling capabilities are higher in coaxial switch products that use big N-type or TNC connectors (can handle up to hundreds of watts of continuous wave power). Furthermore, products for various purposes can be packaged in a variety of ways, ranging from a "commercial-grade" container that is not isolated and sealed from the environment to a "high-reliability" package that is well sealed and can endure extreme environmental conditions.