Design of RF Solid State Switches

8/6/2019 Design of RF Solid State Switches

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CONTENTS

Page No¶s

Abstract 1Introduction 3

Common Switch Configurations 3

Ideal Vs Practical Switches 4

Types of RF Switches 4

Reflective or Absorptive Switches 4

Advantages 5

PIN Diode Fundamentals 5

RF Switch Considerations and Terminology 7

Classification based on circuit combinations 10(a) Series connected switch 10

(b) Shunt Connected Switch 11

(c) Compound Switch 12

FET Switches 13

(a) Single(Series) FET Switch 13

(b) Series Shunt Switch 14

PIN Vs FET Switches- which one to use? 16

Comparison of PIN Diodes & FET¶s 19

Conclusion 19

Future Scope 20References 20



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Department of Electronics and Communication Engineering,

College of Engineering, Osmania University, Hyderabad-07

DESIGN OF SOLID STATE RF SWITCHES

Switches are widely used in many RF Systems, from low power transmit-receive

switches in time division duplex wireless transreceivers to millimetric wave beam-

forming systems for satellite tracking to high power(often military) phased array

systems. PIN diode operation is reviewed such that the impact of device choice and

characteristics on the design can be understood. This Paper examines basic design

techniques employing PIN diode as switching element and FET switches

concentrates on circuit layout techniques to achieve high isolation. The Paper

concludes with a comparison of PIN and FET based switching components.



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INTRODUCTION

A switch is an electrical component for opening and closing the connection of a

circuit or for changing the connection of a circuit device. An ³Ideal Switch´ exhibitszero resistance to current flow in the ³ON´ state and infinite resistance to current

flow in the ³OFF´ state. A practical switch design exhibits a certain amount of

resistance in the ³ON´ state and a finite resistance in the ³OFF´ state.

The use of PIN diodes as the switching element in microwave circuits is based on

the difference between the PIN diode reverse and forward bias characteristics. At

lower microwave frequencies, f < 2 GHz), the PIN diode (including package

parasitics) appears to be a very small impedance under forward bias and a very large

impedance under reverse bias. It is the difference in performance between forward

and reverse bias states upon which switch operation relies.

Most switch designs to be considered use a difference in reflection, rather than

dissipation, to obtain switch performance. Very little power is dissipated by the

diode itself, thus permitting small devices to control relatively large amounts of

microwave power. Thus, PIN diode switches are reactive networks, where losses are

a second order effect. In subsequent sections, we will see that switch circuits

resemble filter circuits in many ways.

Common Switch Configurations

Switches may be implemented in many configurations. These configurations are

described in terms of the number of poles and the number of throws implemented in

the switch. The number of poles describes the number of signal paths controlled by

the switch. The number of throws indicates the number of potential directions into

which a pole may be placed. For example, the simplest switch configuration is a

single pole, single throw (SPST) switch. This configuration has one signal path

which can either be completed by the switch or interrupted by the switch. A single

pole double throw switch (SPDT or SP2T) can connect a single transmission line to

either of two other transmission lines. The number of poles and throws, and the

combinations thereof, is unlimited in the ideal sense, but has practical limitations



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Ideal vs. Practical Switches

In practical solid state RF/microwave switches, it is not possible to produce neither

perfect open impedance nor a perfect short circuit. Consequently, there is always

some small amount of incident signal that is absorbed by the switch and a bit more

reflected by the switch¶s non-ideal impedance when the switch is in the state inwhich it should ideally pass all incident signal energy. This small reduction in signal

amplitude is known as insertion loss (IL) and is typically described in terms of

decibels (dB). Insertion loss is simply the ratio of the output power to the input

power.

TYPES OF RF SWITCHES

Mechanical

Relays, Rotary Switches, Push Buttons

Electronic Switches

Semiconductor Switches (FET¶s, Diodes)

Reflective or Absorptive Switches?

Reflective Switches

A reflective switch is one in which the incident power at the ³off´ port isreflected back to the source as a result of the impedance mismatch presented by the

PIN diode. In contrast, an absorptive switch is designed to present 50-ohm

impedance in the ³off´ state, and to absorb incident power.

The operating bandwidth of the switch is determined by the blocking capacitors

selected, the bias circuitry, and the diode¶s reverse-bias capacitance. Reducing the

diode¶s shunt resistance increases isolation in this type of switch. This reduction is

achieved either by increasing the current or decreasing the diode¶s overall resistance.

Absorptive Switches Multi-throw absorptive switches typically employ the series-shunt approach.

The required 50-ohm terminating impedance is achieved by the series combination

of the diode and terminating resistance to ground. This type of termination has good

high-frequency characteristics, but power-handling ability is limited by the ability of

the diodes and resistors to dissipate RF power. In addition, absorptive switches



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typically exhibit somewhat slower switching speeds. These types of switches are

usually not absorptive at their common port (in the ³all-off´ state) but can be made

absorptive for special applications.

Advantages of Electronic Switches over Mechanical Switches are as follows:

High Switching Speed

Small in Size

More efficient and reliable

Cost effective

The PIN diode finds wide usage in RF, UHF and microwave circuits. It is

fundamentally a device whose impedance, at these frequencies, is controlled by its

DC excitation. A unique feature of the PIN diode is its ability to control largeamounts of RF power with much lower levels of DC.

PIN Diode Fundamentals:

The PIN diode is a current controlled resistor at radio and microwave frequencies. It

is a silicon semiconductor diode in which a high-resistivity intrinsic I region is

sandwiched between a P-type and N-type region. When the PIN diode is forward

biased, holes and electrons are injected into the I region. These charges do not

immediately annihilate each other; instead they stay alive for an average time, calledthe carrier lifetime, t. This results in an average stored charge, Q, which lowers the

effective resistance of I region to a value R S.

When the PIN diode is at zero or reverse bias there is no stored charge in I region

and the diode appears as a capacitor, CT, shunted by a parallel resistance R S.

PIN DIODE

PIN diodes are specified for the following parameters:



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R S : Series resistance under forward bias

CT : Total capacitance at zero or reverse bias

R D : Parallel resistance at zero or reverse bias

VR : Maximum allowable DC reverse bias voltage

: Carrier lifetimeAV : Average thermal resistance

PD : Maximum average power dissipation

Pulse : Pulse thermal impedance

PP : Maximum peak power dissipation

In simplest form the capacitance of a PIN is determined by the area and width of the

I region and the dielectric constant of silicon. This minimum capacitance is obtained

by the application of a reverse bias in excess of VPT, the voltage at which the

depletion region occupies the entire I layer.

Equivalent Circuit of

I-region before punch

Through

Simplified Equivalent Circuit, series

A good way to understand the effects of series resistance is to observe the insertion

loss of a PIN chip series mounted in a 50 � line.

Simplified Equivalent

Circuit, shunt



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RF switch considerations and terminology

Frequency and Bandwidth

Although most of the switching systems do not have a limit on the lowest

frequency of operation, they do have an upper limit. For semiconductor devicesthis is due to the ¿nite time in carrier mobility. The losses incurred from

resistance and parasitic reactances are the main cause limiting the performance of

electromechanical switches at higher frequencies.

Insertion loss

The insertion loss of an RF device is a measure of its efficiency for signal

transmission. In the case of a switch, the insertion loss is speci¿ed only when its

state is such that signal is transmitting or when the switch is in the on-state. This

is speci¿ed in terms of the transmission coefficient, S21, in decibels, between the

input and output terminals of the switched circuit. Usually speci¿ed in decibels,

one of the design goals for most of the RF switches is to minimize the insertion

loss. The insertion loss tends to degrade with increase in frequency for most of

the solid-state switching systems. Compared with these, RF MEMS switches can

be designed to operate with a small insertion loss at several gigahertz. Resistive

losses at lower frequencies and skin-depth effects at higher frequencies are the

major causes for losses.

Isolation

The isolation of a switching system is speci¿ed when there is no signal

transmission. This is also measured as S21 between the input and output

terminals of the switched circuit, under the no-transmission state or when the

switch is in the off condition. A large value (in decibels) indicates very small

coupling between input and output terminals. Thus the design goal is to

maximize the isolation. In RF MEMS switches isolation may degrade as a result

of proximity coupling between the moving part and the stationary transmissionline as a result of leakage currents.



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RF Power handling

RF power handling is a measure of how efficiently a switch passes the RF signal.

This is commonly speci¿ed in terms of a 1 dB compression point, which is

adopted from the ampli¿er characterization industry. It is commonly assumed

that the output power level follows the input power with a linear ratio. But inmany devices there is a maximum power above which this linearity does not

hold. The 1 dB compression point is de¿ned as the maximum input power level

at which the output power differs by 1 dB with respect to linearity. The 1 dB

compression points and the power handling of many devices such as PIN diodes

and MMIC switches are functions of frequency.

Switching time (Rise time & Fall time)

These parameters, fundamental too many designs, are actually composed of several subsets, each one defining the time required for switching to take place

between two states in the switch response (Figure 3a). Rise time is defined as the

period between full ³off´ and full ³on," specifically from 10 percent of this

condition to 90 percent of the square-law-detected RF power. Conversely, fall

time is the period between 90 percent of full ³on´ to 10 percent of full ³off." Rise

time and fall time do not include driver propagation delays.

On time and Off time

The time lapse between 50 per cent of full input control signals from the driver to

90 percent of the square-law-detected RF power when the device is switched

from full ³off´ to full ³on´ is called the ³on´ time. The ³off´ time begins when

the 50 percent point of control signal occurs, to the point when it achieves 10

percent of its square-law detected RF power and the unit is switched from full

³on´ to full ³off." On and off times include driver propagation delays. This is

sometimes referred to as "Modulation Time."

Switching Transients

Switching transients are the exponentially decaying voltage spikes at the input,

output or both of an RF signal path, due to a change in the control voltage. These

switching transients are often called sidebands due to switching, and it shows

important indications of the performance of a switching system. It is often

required to monitor the output RF spectrum during the design of an RF system,



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and hence components of the RF chain, such as ampli¿ers and switches, must be

tested with a known stimulus. Both electromechanical and electromagnetic

transients exist during the switching process. While the electromechanical

transient is due to mechanical motion (wherever present) of the switch element,

the electromagnetic transient is due to energy exchange between electric ¿eldsand magnetic ¿elds of the electric equipment in the network.

It may be noted that these transients arise from nonlinearities in the

network. The switching transients in PIN diode switches are due to the stored

charge in the intrinsic region being quickly discharged by the control voltage. In

balanced Schottky barrier designs, the charge stored by the diode is very small

and the majority of the transients are caused by the mismatch within the drive

circuits. However, the switching transient mechanism of the gallium arsenide

¿eld effect transistor (GaAs FET) MMIC circuits results when the rapidlychanging gate voltage is coupled to the switch output through the gate-to-channel

capacitance of the FET, thus experiencing a greater feed through because of its

faster switching speed.

Classification based on the circuit combination:

Series Combination

Shunt Combination

Compound Combination (Series-Shunt)



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Series Connected Switch:

Figures below show two basic types of PIN diode series switches, (SPST and

SPDT), commonly used in broadband designs.

Principal operating parameters of switch are calculated as follows:

(A) Insertion Loss (dB) =

(B) Isolation (dB) =

(C) Power Dissipation (Forward Bias)

For , this becomes

(D) Peak Current

(E) Peak RF Voltage

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Shunt Connected Switch:

Figure below shows two typical shunt connected PIN diode Switches.

The Principal equations describing the operating parameters of shunt switches are

given by

(A) Insertion Loss

(B) Switch Isolation

(C) Power Dissipation (Forward Bias)

(D) Power Dissipation

(E) Peak RF Current

(F) Peak RF Voltage

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Compound Combination (Series-Shunt):

Compound Switches are series-shunt switches used in combinations to improve

overall switch performance. The broad band Insertion Loss of the series switch is

combined with the broad band Isolation of the shunt switch in a number of

combinations to follow. Here are few combinations:

FET Switches:

FET Switches

The control field effect transistor (FET) or switching FET functions as a three port

device, where the channel between source and drain ports forms a conduction path

for the RF signal and the gate port, controls whether an RF signal is blocked or may pass. A DC control voltage applied between the gate and channel is required to

create this function. Most control FETs use a depletion mode configuration, which

means that the channel is normally in its low impedance state with no control

voltage applied and in its high impedance, pinched off state when a negative voltage

with respect to the channel is applied (thus the term ³pinch off voltage´).



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Single FET Switch

Figure below shows a single FET configured as a simplified single pole, single

throw (SPST) switch. The first challenge in fabricating a switch using a GaAs FET

is to isolate the DC gate control from the RF path. This is done by using a 5 k� to 10

k� resistor in series with the gate of the FET. This is a very simple bias network thathas many advantages which will be discussed later. The next step is to DC block the

RF source and drain ports using a capacitor with adequately low capacitive reactance

at the desired frequency of operation. This creates an SPST switch. The insertion

loss of the ³ON´ path of the switch will be affected by the channel resistance.

Likewise the isolation of the ³OFF´ path is limited by the capacitance created by the

source and drain spacing as well as FET physical size (periphery). Hence a balance

of channel resistance (RS) and off capacitance (Coff ) must be met. The following

equations show the relationship of the RS and Coff as expressed in insertion loss (dB)and isolation loss (dB) for a single series FET SPST switch.

Assume the FET has the following characteristics:

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Series-Shunt Switch:

To improve the isolation at higher frequencies, a shunt FET can be used following

the series FET. In this position the FET must be ³ON´ in order to increase isolation

and ³OFF´ to put it in the insertion loss state.

This requires two separate control voltages for the switch. Shunt FET has no

appreciable reactive component when it is biased to its ³ON´ state, isolation will

stay relatively unchanged over frequency. The negative gate voltage required for this

switch can be viewed as a drawback, since in most applications only positive voltage

supplies are available. Also in this series/shunt configuration one FET must be ³ON´

while the other FET is ³OFF´. A solution to this problem is the addition of two

capacitors applied to the shunt FET which provide DC blocks to the RF path and

allow the gate port of the shunt FET to be grounded. An additional reference voltage

port must also be added. This technique also allows for a single positive voltage to

be applied to the switch. The selection of the capacitor value is critical to insure

proper frequency operation and bandwidth. The values can be quite small (5 to 15

pF), since the series FET can provide most of the isolation at the lower frequencies.



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This SPST switch is reflective, which means that the output when in the isolation

state will have a substantial VSWR. A class of switches, known as ³matched´ or

³absorptive´ will terminate the output into a 50 � load when switched to the

isolation state.

PIN vs. FET Switches ± Which One to Use?

PIN diodes and FETs have relative advantages and disadvantages to each other for

use in switching applications. The performance attributes which should be compared

when the selection of one of these technologies is undertaken are more numerous

than one might think at first glance. Table below lists the foremost of these

attributes, but is by no means intended to be complete.



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Attribute PIN Diode FET

³Integratability´ with

other componentsPoor Excellent

Power HandlingVery High

(greater than 1KW CW)

Moderate

(10 Watts or Less)

Switching Time

A few tens of

nanoseconds to several

microseconds

Tens to a few

hundred of

nanoseconds

Control Current Up to 100 milliampsLess than 100

micro amps

Distortion PerformanceI/P IP3 are in the range of

+45dBm or higher

I/P IP3 are in the

range of +30dBm

Integratability:

The wafer processing required for modern FET structures such as pHEMTs is

largely lateral with respect to the top surface of the wafer; it lends itself quite well to

the inclusion of passive component structures -- such as metal-insulator-metal

(MIM) capacitors, spiral inductors, and thin film resistors -- all of which can be

formed on or near the topmost layer of the wafer. The bottom-most layer of a FET

wafer is semi-insulating material, which inherently isolates one FET structure fromanother. In comparison, PIN diode wafer processing is vertical with respect to the

top surface of the wafer; the majority of the physical thickness of a PIN diode wafer

is the cathode of the diode. It is very difficult to isolate the cathodes of PIN diodes

that are on the same die. Consequently, with the exception of common-cathode pairs

of PIN diodes, they are inherently discrete devices.

Power handling

The vertical structure of a PIN diode is a relative advantage for power handling. The

heat that is generated by Joule heating within the I layer of the diode can easily beconducted downwards through the diode¶s cathode layer to the system heat sink. RF

FET structures such as pHEMTs and MESFETs are typically fabricated from III-V

materials, such as GaAs, that has lower thermal conductivity than Si, which is the

material utilized for most switching PIN diodes.



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Switching Time

The FET is a majority carrier device whose drain-source impedance is controlled by

the thickness of a depletion layer that extends into the channel from the gate-source

interface. The thickness of this depletion layer can be modulated very rapidly in

response to a change in the gate-source control voltage. By contrast, a PIN diode

stores minority carriers in its I layer when it is under forward bias. These charge

carriers must be primarily conducted out of the I layer to change its impedance from

low to high. This process is inherently slower than the change in drain-source

impedance for a pHEMT or MESFET device.

Control Current

The FET is a voltage-controlled device. In a practical pHEMT or MESFET, the only

current that flows into the control port of the transistor is the reverse leakage current

of the gate-source junction, which is very small, typically less than 10 micro amps.

On the other hand, a PIN diode can require a significant injection of charge carriersinto its I layer to lower its impedance to the required level. The typical bias current

for a PIN diode in a switch is 10 to 20 milliamps.

Distortion Performance

The PIN diodes produce nearly ideal, very linear series resistance when the amount

of charge in its I layer -- as a result of DC forward bias current -- is at least ten times

that of the charge that is alternately injected and removed by the RF signal. When

the diode is nonconducting, it presents a very high resistance in parallel with its

junction capacitance. This junction capacitance is independent of applied voltage for

signals with sufficiently high frequency (typically greater than 100 MHz).Consequently, the PIN diode produces excellent distortion performance.

An FET structure responds very quickly to the magnitude of its applied gate-

source voltage, because the gate-source junction in a MESFET and a pHEMT

structure is a Schottky diode. The Schottky diode¶s very nonlinear impedance with

respect to applied bias conditions can be a comparatively efficient distortion

generation mechanism, which results in input third order intercept for a FET switch

that is roughly an order of magnitude or more lower than that of a PIN diode switch.



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Comparison of PIN Diode and FET Characteristics:

FEATURE PIN SWITCH FET SWITCH

Insertion Loss

(typ. 1GHz)

Very Low (0.4-

0.6dB)

Very Low

(0.4-0.6dB)

Isolation

(typ. 1GHz)

Excellent

(typ. 50dB)

Very Good

(typ. 45dB)

Current

consumptionHigh (typ. mA)

Very Low

(typ. Micro Amps)

Switching speed Fast (typ. 100ns) Very Fast (3-10ns)

Operation DC No Yes

Power Handling Excellent Good(typ. +25dBm)

Circuit Size Small-Moderate Very Small

Design

FlexibilityYes N/A

Conclusion:

(1) RF switches may comprise PIN diodes or FET structures such as MESFETs

and pHEMTs. These two approaches to the design of the switch offer advantages

and disadvantages.

(2) A PIN diode switch typically can handle greater power and produce less

distortion, at the expense of longer switching time and much larger control current

requirements.

(3) The very low bias current needs of MESFET and pHEMT switches makes

them very well suited for battery powered applications. These devices can also be

integrated into complex, multi-throw integrated circuit switches.

Future Scope:

Devices with more operating life span and extremely small in size can be fabricated

such as, MEMS based switches and in near future NEMS based switches.



References:

1. ³Design with PIN Diodes´, Skyworks Solutions application note.

2. ³PIN Diode Circuit Designers Handbook´ Micro-semi Corporation.

3. A.M.Street ³Designing with PIN Diodes´, Practical RF/Microwave Design

Course, university of oxford, International Electronics and CommunicationsProfessional Development course programme, Jan 2000.

Documents

Design of RF Solid State Switches