Detailed Characteristics and Properties of RF Switches

Frequency Coverage

Operating frequency is often one of the first considerations when selecting an RF switch  for a given application. 

Low-frequency performance is another consideration that is equally important as  maximum operating frequency.  

        • The frequency response of the FET and PIN diode switches is also influenced by  the external components, such as RF bias chokes and DC-blocking capacitors.  Even though a switch may be specified to operate at very low frequencies, the frequency  response of DC-blocking capacitors may limit its ability to do so. Careful selection of these  external components is necessary to preserve switch performance.

Isolation 

Isolation is defined as the ratio of the power level when the switch’s path is OFF to the  power level when the switch is ON. In other words, it is the suppression of a signal in  excess of the insertion loss at the OFF port. 

High isolation in switches is crucial in most measurement applications.  Good isolation prevents stray signals from leaking into the desired signal path. High isolation is especially critical in measurement systems where signals are consistently  being routed to and from a variety of sources and receivers through various switch test ports. If these stray signals are allowed to get through, measurement integrity is severely  compromised. 

        • An electro-mechanical (EM) switch or MEMS-based switch achieves isolation by  physically disconnecting the metal-to-metal contact of the conduction path. The non-conducting states of these devices are not perfect, such as parasitic  capacitances that degrade the ability to impede signal flow. 

        • PIN diode switches typically offer better isolation performance at high frequencies  than FET switches due to the FETs’ drain-source capacitance when the FET is  turned OFF. However, this disadvantage can be resolved with the hybrid design by  using shunt PIN diodes for isolation. 

        • PIN diode switch isolation is poorer than FET switches at lower frequencies, around  tens of MHz, due the low frequency limitations of the PIN diodes.  This isolation can be improved by proper selection of the PIN diodes, for optimum  I-region thickness of the diode used in the switch design.  

Insertion Loss 

Insertion loss of an RF switch, expressed in decibels (dB) is determined by measuring the  power loss (difference) of a signal that is sent in through the common port, and the output  signal from the port that is in the ON state. In receiver applications, the effective sensitivity and dynamic range of the system is  lowered by the insertion loss of the front-end switch.  In transmitter applications where the additional RF power needed to compensate for the  loss is not available (RF power amplifiers in particular), insertion loss will be a critical  specification of a switch. Insertion loss in Solid-State switches is generally attributed to the following factors: 

        • Resistance losses due to the finite resistance of series connected components,  particularly PIN diodes and finite Q-factor of capacitors. 

        • Mismatch loss (VSWR losses) at the terminals of the switch, or within the switch.  With proper matching compensation techniques, mismatches can be reduced. 

        • Signal leakage through any OFF path device capacitance. 

        • Conductor or transmission line loss within the switch itself due to the presence of  microstrip, coaxial line, or wave-guide inter-connection lines. 

        • Solid-State RF switches get more insertion loss as the number of arms (or throws) of the switch increases.  

        • Extra insertion loss includes off-arm terminations and video filters.

        • PIN diode RF switches typically have higher insertion loss at low frequencies due to  the sharing of DC biasing and RF paths, while FET switches have higher insertion  loss performance at higher frequencies due to the higher ON resistance of FETs. 

        • To achieve broadband insertion loss performance at high frequencies, the challenge  is in managing the OFF device capacitance. This is commonly done by reducing  device size while accepting degradation in the minimum insertion loss at low  frequencies. 

        • Narrowband tuning techniques can be used to minimize capacitive effects and  recover much of the insertion loss degradation over a 10% to 20% bandwidth. It is good practice in a new PIN switch design to first evaluate the circuit loss by  substituting, alternatively, a wire short or open in place of the PIN diode.  This simulates the circuit performance with “ideal PIN diodes.” Any deficiency in the  external circuit may then be corrected before inserting the PIN diodes. 

Ports Return Loss and VSWR 

Return loss, expressed in decibels (dB), is a measure of voltage standing wave ratio  (VSWR) and is caused by impedance mismatch between circuits.  At microwave frequencies, the material properties as well as the dimensions of a network  element play an important role in determining the impedance match or mismatch caused  by the distributed effect. 

        • VSWR is an indicator of reflected waves bouncing back and forth within the  transmission line, which increases the RF losses.  

        • Mismatched impedances increase VSWR and reduce power transfer.  If VSWR is high, higher power in the transmission line also leaks back into the  source, which might potentially cause it to heat up or make the circuit oscillate.  In Solid-State RF switch design, there will be a finite ON resistance whether we choose to  use FET or PIN diode as the series ON-OFF mechanism. This causes an impedance mismatch which results in poor return loss.  

        • Proper design can use matching circuits to improve the VSWR or return loss of the  switch without sacrificing the other specifications of a switch. 

Switching Speed 

Fast switching speed is important in RF applications especially in applications that require  the stacking of multiple switches in series, or where high-frequency transmitting and  receiving rates need to be used. 

        • Switching speed is defined as the time needed to change the state of a switch port  from ON to OFF or from OFF to ON.  Switching speed is often characterized in two ways: Rise/Fall time, and ON/OFF time.  

        • Rise time is the time it takes for the detected RF output to raise from 10% to 90% of  the final value, when a switch arm is changed from an OFF state to an ON state. 

        • Fall time is the time it takes for the detected RF output to drop from 90% to 10% of  the initial value, when a switch arm is changed from an ON state to an OFF state. 

        • Rise and fall times do not include the switch driver delay time.

        • ON time is the time period from 50% of the transition of the control signal to 90% of  the detected RF output when the switch arm is changed from an OFF state  (isolation) to an ON state (insertion loss). 

        • OFF time is the time period from 50% of the transition of the control signal to 10% of  the detected RF output when the switch arm is changed from an ON state (insertion  loss) to an OFF state (isolation). 

        • The ON and OFF times include the switch driver propagation delay. 

Switching time of an RF switch can be measured by using an RF signal generator, a  square law RF detector (e.g. Schottky diode), a fast Rising/Falling edge (~10nsec) square  wave function (pulse) generator for switch control, and an oscilloscope. 

Settling Time 

The widely used margin to final value of settling time is 0.01dB (99.77% of the final value)  and 0.05dB (98.86% of the final value). 

        • The above specification is commonly used for GaAs FET switches because they  have a gate-lag effect caused by electrons becoming trapped on the surface of the  GaAs.  

        • PIN diode switches have excellent rise time and settling time performance, in the  nanosecond range compared to FET or hybrid switches. 

There are two common ways to measure the settling time of a FET switch: 

        • First is the measurement with an oscilloscope, but this method always has some  uncertainties due to the resolution of the oscilloscope as well as the linearity and  response time of the square law detector.  

        • A faster and more accurate measurement can be made using a Network Analyzer  with an external trigger output. The two ports of the Network Analyzer are  connected to the switch input and output ports. The trigger signal from the function  or pulse generator is used to synchronize the switch control input with the Network  Analyzer external trigger. 

In systems where RF switches with a slow settling time are used, designers have to idle  the test program for few milliseconds (or more) before each switching cycle in order to  allow the switches to fully settle. 

Video Leakage 

The word “video” was adopted from television, and refers to signal spectrum starting from  very low frequencies up to few or even hundreds of MHz range. 

        • Video leakage refers to the spurious signals present at the RF ports of the switch  when it is switched without an RF signal present.  These signals arise from the waveforms generated by the switch driver and, in particular,  from the leading-edge voltage spike required for high speed switching of PIN diodes. 

        • Most switches contain video leakage, which can damage sensitive devices. The magnitude can be as low as a few mV to as high as few Volts in a 50Ω system.  FETs or hybrid switches generally offer lower video leakage (< 10mV), while PIN diode  switches have higher video leakage (~1V), because in PIN diode switches, the RF and  DC biasing share the same path. When control voltage is applied to turn the switch ON  and OFF, a current surge will be generated. The DC block capacitor used on the RF path  causes the current to surge along the RF path when the switch is turned ON and OFF.  The current surge level depends on how fast the control voltage changes as well as the  voltage level and the capacitor’s value. 

        • If the load’s DC input impedance is much higher than 50 ohms, the load will suffer  higher video leakage than a load with lower input resistance. 

Off-Port Termination 

        • An RF switch can be reflective or absorptive.  

        • With reflective RF switches, the RF signal at the OFF port is reflected back to the source due to the load mismatch. These switches have a simpler design, a slightly lower cost, and can handle higher  power than absorptive switches.  

        • Absorptive RF switches provide a matched termination at the inactive ports. Because they absorb the RF signal, they are limited by the power-handling  capability of the terminations. These switches are slightly more complex in their design. 

Phase Tracking 

        • Phase tracking is the ability of a system with multiple parts, or a component with  multiple paths, to closely reproduce their phase relative to each other.  A phase tracking requirement is best achieved by first equalizing the time delay between  arms of a multi-throw switch. This requires a tightly controlled physical length of the arms  from the input port to the output port.  

The difference in phase from one unit to another within a product line should be  minimized. Since the switch is made up internally of many elements, (diodes, capacitors, and chokes) with their accompanying mounting parasitic reactance and losses, it is  necessary to control the uniformity of parts and assembly techniques to achieve the best  phase tracking possible. 

A good multi throw RF switch features a phase difference of less than 10 degrees, in a  frequency range from very low frequencies up to microwave frequencies. 

Linearity, Harmonic and Intermodulation Distortion

        • Linearity assesses the ability of the RF switch to faithfully transfer a signal without  distortion. Harmonics and intermodulation distortions are strongly influenced by the RF power which  is applied to the diode or to the FET, and by the lifetime of a PIN diode.

        • Harmonic distortion is a single-tone (single-frequency) distortion product caused by  device nonlinearity.  When a non-linear device is stimulated by a signal at a single frequency f1, spurious  output signals are generated at the harmonic frequencies 2f1, 3f1, 4f1,...nf1.  Harmonics are usually measured in dBc, (dB below the carrier or fundamental signal). • In PIN diode RF switches if use anti-series diodes topology can substantially reduce  even order harmonic distortion products. 

        • Intermodulation distortion appear when the nonlinearity of a device or a system with  multiple input frequencies, causes undesired outputs at other frequencies, causing  the signals in one channel to interfere with adjacent channels.  It is common practice to limit the analysis to two tones (two fundamental  frequencies), f1 and f2, which are normally separated by a small offset frequency of  about 1MHz.  For example the 3rd order intermodulation products (IP3) of the two signals, f1 and f2,  would be: 2f1 + f2, 2f1 – f2, 2f2 + f1, and 2f2 – f1.  The amplitude in dBm of the intermodulation products will increase in power three  times faster than the carrier signal.         • To improve linearity in a FET switch the transistor should be biased to maximize  saturated power. Increasing the FET’s gate width also improves linearity. 

P1dB Compression Point 

Another nonlinearity of a system or device (RF switch in our case) is measured by the  compression point.  

In the linear region, a 1dB increase in input power to the RF switch will correspond to 1dB  increase in the output power of the switch. 

        • When an RF switch is driven into compression it will generate harmonics and  insertion loss will increase. 

        • P1dB is the RF input power level at which the switch insertion loss increases by 1dB  over its low-level value (linear region). P1dB is a measure of how much power the  on switch can handle before it will distort or compress the signal. 

The nonlinearity effect becomes apparent when the output power starts to increase less  than the input power. 

Power compression in switching FETs has separate mechanisms in the ON-case and the OFF-case.  

        • For the ON-case FET is about the peak RF current (Ipk) flowing through the device  which causes compression.  Once the RF current swing is high enough (Ipk > Idss) to cause the RF voltage-drop  across the FET to exceed the knee voltage, the switch starts to compress. 

                 - Increasing the size of the FET increases the Idss and so increases its  compression point and power handling ability.

        • In the OFF case, the switch FET is high impedance and there is little current flow  through it. Compression occurs when the RF voltage swing causes the gate-drain breakdown voltage to be exceeded on the positive half cycle, or when it moves the  FET out of pinch-off on the negative half cycle. 

                 - A simple rule of improving the compression performance of OFF-case switching  FETs is to use higher gate control voltages.         

        • The mechanisms determining IP3 and P1dB of RF switches are different.  IP3 is measured at low input power levels, while P1dB is measured at high input  power levels. 

Maximum Power Handling 

Maximum power handling for RF switches is the power level that will correspond to the  onset of gain compression. Once you exceed the calculated power handling, the insertion  loss of your switch starts to go up. 

        • PIN diode RF switches are inherently higher power, lower distortion devices than  FET switches. In pulse mode some PIN diodes can switch kilowatts of RF power.  

        • The maximum RF power that a PIN diode switch can handle is limited by either the  diode’s breakdown voltage, or by its power dissipation capability.  In a 50Ω system, the power dissipation is usually the limiting factor. 

        • The power handling capability of FET switches tends to be reduced for low  frequency operation. This reduction occurs gradually with decreasing frequency. 

        • Power handling for FET switches in the ON state is calculated using the maximum  current the device can pass. The maximum current is proportional to the periphery of the device. Thus a 1mm  periphery device will have four times the power handling of a 0.5mm device. 

        • The maximum power handling for FET switches in the OFF state is mainly a  function of FET breakdown voltage. The greater the difference between FET pinch-off and breakdown, the higher is the  maximum power handling. 

        • In both, PIN diode and FET switches, the RF voltage shall not exceed the  breakdown voltage limit with turned OFF switching elements. 

Reliability, Lifetime, and Hot-Switching

Reliability and lifetime are critical when selecting switches for applications that require  repeated switching or expose the device to harsh or uncontrolled environments.  

        • Electro-mechanical switches have limited lifetimes due to their mechanical switching  mechanisms.  Normal wear and tear over time will cause these mechanical structures to degrade.  Solid-state switches provide the unique advantage of never wearing out by simply being  switched under normal operating conditions, since no moving parts are used. For these switches, life expectancy is determined by overall time in operation, not the  number of switching cycles.

        • When used within specifications, Solid-State switches can exceed 500 million  cycles, whereas EM switches typically endure less than 10 million operations. Switch lifetime can degrade when the RF switch is operated under conditions that  introduce additional stresses, such as high operating temperature, thermal fluctuations, or  exposure to excessive RF signal power. Another potential stress is hot-switching, which occurs when the device is switched while  an RF signal is being applied.  

        • Although switches are specified for a maximum power handling, this can degrade  when hot-switched.  

        • PIN diode switches are typically more robust when switched at high powers,  whereas switched FETs transition through a region during which high amounts of  signal power may be dissipated. 

        • MEMS switches are typically more sensitive to hot-switching, as micro-welding  between contacts can occur, resulting in reduced lifetime. Micro-welding could be eliminated (or reduced) in MEMS switches if there is a  circuit to block (or limit) the input signals during the switching transition period. 

Electrostatic Discharge – ESD

RF switches can be susceptible to electrostatic discharge (ESD), which occurs when  static charge is suddenly transferred between surfaces with differing voltage potentials.  This can subsequently damage sensitive devices. 

        • Electro-mechanical switches are almost immune to ESD. 

        • PIN diode switches have a moderate sensitivity to ESD. 

        • GaAs switches are generally more sensitive to ESD, with typical ratings of 200V.  

        • MEMS switches are similarly sensitive to ESD as GaAs switches.  

Noise Figure 

        • Noise Figure of an RF switch (in dB) is considered equal to its insertion loss (in dB).