Literature Pindiode

7/29/2019 Literature Pindiode

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PIN DIODE CONtrOl PrODuCts

APPlICAtION NOtE

www.nardamicrowave.com

NAR29037SwitchBrochure.indd 1 2/2/11 1


2/20

As a business unit o L-3 Communications, Narda Microwave-East has served the militar and commercial communication

markets with outstanding products bearing the world-renowned Narda name or nearl 60 ears. With a 150,000

square-oot plant and our dedicated team o sales, design and production proessionals, Narda is read to develop,

design and deliver high-perormance products to address our needs.

With the development and manuacture o state-o-the-art RF and microwave components, Integrated Microwave

Assemblies, and subsstems, Narda has positioned itsel and maintains its position as a technolog leader b oering

advanced products in the requenc range o DC to 100 GHz or both commercial and militar applications. We maintain

the worlds largest inventor o RF and microwave components or rapid deliver o our products to our customer base.

Products manuactured at our production acilities include IMAs, couplers, power dividers, attenuators, RF switches and

power monitors that are suitable or a mriad o RF applications. The Narda brand also includes a ull line o RF saet

products that characterize emission levels or RF workers and the general public.

2

Contents

INTRODUCTION ................................................................ 3

TABLE OF FIGURES ............................................................ 3

PIN-DIODE SWITCHES ....................................................... 4KEy PIN DIODE SWITCH PARAMETERS .......................... 6

Video Leakage .................................................... 6

Harmonics and Distortion .................................. 6

Minorit Carrier Lietime ................................... 6

Switching speed .................................................. 7

Perormance Trade-Os ............................................ 7

Power vs. Frequenc ........................................... 7

Power vs. Switching Speed ................................ 8

Frequenc and Bandwidth ................................. 8

Reective Switches .................................................... 8

Multi-Throw ReFlective Switches ...................... 9

Absorptive Switches ........................................... 9

Transmit/Receive (T/R) Switches ......................... 9A Word On Drivers .................................................. 10

PIN-DIODE ATTENUATORS ............................................. 11

lntroduction ............................................................. 11

Notes on Attenuator Perormance ................. 11Phase Shit and Attenuation ........................... 11

IMD and Harmonics .......................................... 11

Power-Handling Abilit .................................... 11

Monotonicit ..................................................... 12

Mean Attenuation ............................................ 12

Attenuation Flatness ........................................ 12

Attenuation Accurac....................................... 12

Comparison o Attenuator Characteristics ...... 12

Digitall-Linearized Voltage-Variable Attenuators vs

Switched-Bit Attenuators ...............................................13

Digitall-Linearized Analog Attenuators .............13

Digitall-Controlled Switched-Bit Attenuators ...13

PIN-DIODE LIMITERS ....................................................... 15Additional Limiter Considerations ......................... 17

Recover Time ................................................... 17

Power-Handling Abilit .................................... 17

GLOSSARy ..................................................................18-19

This booklet presents an overview o Nardas RF & Microwave PIN Control products. Details and specifcations can beound at www.aamicwav.cmor in the latest printed catalog...the most comprehensive in the industr.



3/20

RF and microwave components based on PIN diodes

have been essential tools in the designers toolkit or

decades. Their unique characteristics make them the

best choice or a wide variet o control applications,

such as switches, attenuators, phase shiters, limiters,

and modulators.

PIN diodes are undamentall similar to standard

diodes, but have an RF impedance that is determined

b an externall- supplied bias current. Their versatilit

makes them excellent building blocks in a wide variet

o confgurations within each product categor, which

allows diverse sstem requirements to be served. In

short, the PIN diode enables all sstems rom the least

complex to the most sophisticated to achieve their

intended missions, while requiring ver little space,

power, or cost.

There are man resources that provide both practical

and theoretical inormation about PIN diode theor,

characteristics, and incorporation in modules and

subsstem designs. However, practical inormation

about choosing the proper PIN-diode-based product

or a specifc application is conspicuousl absent.

RF and Microwave PIN Control Product Application

and Selection has been created to fll this void. It

includes basic discussions o PIN diode characteristics,

the most commonl used PIN-diode based products,

and the trade-os encountered in designing products

around them. The merits o various tpes o control

products within a specifc categor (analog and digital

attenuators, or example) are discussed as well.

3

IntroduCtIon

tAble o Iguresn. dcipi

1 ...... Simple SPST PlN Diode Switches

2a .... Shunt Diodes Located Quarter Wave Length romCommon Junction

2b .... Series Diodes at Common Junction

3a .... Detected RF Power, Rise Time and Fall Time

3b .... Port-to-Port Switching Time

4 ...... Series and Shunt Reective Switches

5 ...... Multi-Throw Reective SPDT Switch

6a .... Asorptive SPDT Switches, Shunt Termination

6b .... Asorptive SPDT Switches, Series Termination

7 ...... T/R Switch

8a .... Reective,Voltage-Variable VVA

8a .... Non-Reective VVA

9 ...... Switched-Bit Attenuator

10 .... Limiter Operating Characteristics

11 .... Tpical Limiter Pulse Response

12 .... Spike Leakage

13 .... Tpical Limiter Perormance vs PIN Diode ILaer Thickness

14 .... Basic Two-Diode Limiter

15 .... Limiter with Schottk Barrier Diode LightlCoupled to Line



4/20

Switches that control the path o RF power rom ver

low requencies through the low millimeter-wave

range are the most common application or PIN

diodes. The level o DC bias applied to the diode

determines its impedance. In the case o a PIN diode

mounted in series with a transmission line, when

the bias changes the impedance rom a low value

to a high value, the circuit acts as a switch. That is,

the switch is in the on state when orward biased

(low impedance), and in the o state when zero

or reverse biased (high impedance). The attenuation

produced b the diode switch is called insertion loss

(II) when the switch is in the on state, and isolation

when in the o state.

In a simple SPST PIN diode switch (Figure 1), the

diode can be either series or shunt connected. The

series-connected PIN diode confguration can provide

reasonabl low insertion loss over a multi-octave

requenc range, but with lower power-handling

capabilit. Design and abrication are also simpler

because no holes are required in the circuit board

to mount shunt diodes. In series diode switches,

insertion loss is dependent on the series resistance o

the PIN diode while isolation is primaril dependent

on the junction capacitance. These parameters are

determined b the orward bias current and reverse

bias voltage, respectivel.

The shunt-connected PIN diode confguration

optimizes high isolation and low loss across a wide

requenc range (up to several octaves), and can handle

higher power levels because the diodes are mounted

directl to the housing. The shunt switch is on when

the diode is zero or reversed biased, and o when

orward biased (the opposite o the series switch).

The insertion loss o a shunt-connected diode

at a given requenc is primaril dependent on its

junction capacitance (Cj), while the isolation provided

b the diode is dependent on its series resistance (Rs)

when the diode is orward biased. A combination

series-shunt topolog is also used and provides ver

wideband perormance, high speed, and moderate

power-handling abilit and insertion loss.

4

PIn-dIode sWItCHes

DC Return(RFC)

Series SPST Switch

Zo

Zo

Vg

DCB

RFC

Bias Supply

Shunt SPST Switch

Zo

Zo

Vg

DCB

RFC

Bias Supply

i 1 simp sPst PIn di swich



5/20

Multi-throw switches can be confgured in two

was to achieve improved perormance (Figure 2a). In

the frst method, PIN diodes series-connected to the

common junction and the diodes in the on port are

orward-biased while the remaining diodes are

reverse-biased. The result is a low-loss path or the

on port and minimal loading b the o ports.

In the second method (Figure 2b), shunt-connected

PIN diodes are placed one-quarter wavelength rom

the common junction, and the selected diodes o the

on port are reverse-biased while the o ports are

orward-biased. The result in this case is an electrical

short across each o transmission line, and the

quarter-wavelength spacing transorms the shorts to

open circuits at the junction.

These techniques are optimized through prudent

choice o transmission line impedances while

keeping stra reactance low, resulting in a switch

with acceptabl low insertion loss and VSWR, and a

3:1 bandwidth.

While it is possible to achieve isolation somewhat

greater than 40 dB with a single PIN diode (either

series or shunt-connected) at lower microwave

requencies, it is tpicall necessar at higher

requencies to increase the number o switch

elements b using additional series-mounted and

shunt-connected PIN diodes in each arm.

The isolation elements o a switch (series or shuntdiodes) are usuall spaced a quarter wavelength

apart. This results in a value o isolation 6 dB greater

than the sum o the isolation that is provided b each

pair o diodes. This structure can be repeated several

times to achieve greater than 90 dB isolation.

5

PIn-dIode sWItCHes C

Common

/4

Common

i 2 - sh di lca Qa Wav lh mCmm Jci

i 2a - si di a Cmm Jci



6/20

Insertion loss, isolation, switching speed, and power

handling abilit are tpicall the parameters used to

describe switch perormance. However, there are other

ke parameters.

Vi laka

The spurious signals at the switchs RF ports when there

is no RF signal present are collectivel called video

leakage. The switch driver produces these signals,

specifcall at the leading edge o the voltage spike

provided or high-speed switching. There can actuall

be video spikes o 10 VDC present in a sstem with a

50-ohm impedance, although 1.5 to 3.0 VDC is more

common. Most o the RF energ in the video spike is

below 200 MHz but in ver-highspeed, broadbandswitches, there can be appreciable RF energ (-60 to -50

dBm) produced as high as 1 GHz. High-pass flters can

reduce the level o low-band video leakage components,

but signals within the passband o the switch (in-band

video leakage) cannot be fltered out. In-band video

leakage can be reduced onl b using a switch with a

slower switching speed or b ver careull tailoring the

drive waveorm to suit the particular tpe o PIN diode

being used.

Hamic a dii

PIN diodes, like all diodes, are nonlinear in their

response characteristics, and as a result produce

harmonics and intermodulation distortion (IMD)

products. Fortunatel, these products are usuall at

ver low levels in a PIN diode switch because the diodes

themselves are either in a saturated, orward-biased

condition or are reversed-biased. The minorit carrier

lietime o the diode determines the level o IMD. A PIN

diode switchs IMD perormance is usuall described b

its third-order output intercept point (OIP). Good OIP

perormance or tpical PIN switches ranges rom +35

dBm to +45 dBm. The level o harmonics and IMD varies

widel among devices, so it is important to read the

manuacturers specifcations or these parameters or

ever model considered.

Mii Cai liim

This specifcation is ver important rom the perspective

o both diode and circuit design. Carrier lietime (TL) is a

propert o the semiconductor material, and when the

PIN diode is orward biased, injection o electrons and

holes occurs rom the N+ and P+ contacts respectivel.

These carriers have a fnite lietime, and the average

time beore the recombine is the carrier lietime.

Recombination takes place through interactionbetween the crstal lattice and impurities in the I

region and P+ and N+ regions o the diode. The carrier

lietime in a PIN diode controls the switching speed,

i.e., the time required to switch the diode rom a low-

impedance orward bias state to a high-impedance

reverse bias state. This transition is the slower o the two

transitions in a switching application since the driver

circuit is attempting to remove stored charge rom the

PIN diode.

Switching speed and minorit carrier lietime are

directl related. To visualize their interaction, it helps to

examine the relationship o minorit carrier lietime and

its orward and reverse current ratio (I/Ir) in the

ollowing equation:

t = tl (1 + I / I)

where

Trr is the diodes switching speed (commonl reerred

to as reverse recover time), and TL is the minorit

carrier lietime

This equation describes the dependence o switching

time on the minorit carrier lietime and the I/Ir ratio.

6

Key PIn-dIode sWItCHPArAMeters



7/20

Swichi sp

Rise Time And Fall Time: These parameters,

undamental to man designs, are actuall composed

o several subsets, each one defning the time

required or switching to take place between two

states in the switch response (Figure 3a). Rise time

is defned as the period between ull o and ull

on, specifcall rom 10 percent o this condition

to 90 percent o the square-law-detected RF power.

Conversel, all time is the period between 90 percent

o ull on to 10 percent o ull o. Rise time and

all time do not include driver propagation delas.

i 3a - dc r Pw, ri tim a a tim

Modulation On Time and O Time: The time lapsebetween 50 percent o ull input control signal rom

the driver to 90 percent o the square-law-detected

RF power when the device is switched rom ull o

to ull on is called the on time. The o time

begins when the 50 percent point o control signal

occurs, to the point when it achieves 10 percent o its

square-law detected RF power and the unit is

switched rom ull on to ull o. On and o

times include driver propagation delas. This is

sometimes reerred to as Modulation Time.

Cmmai (P--P swichi tim)

(Figure 3b), sometimes reerred to as Commutation

Time, is the period rom when the RF power

level at the o-going port alls to 10 percent o its

original level to the time the RF power in the on-

going port rises to 90 percent o its fnal value. In

high-speed, reective switches, commutation time is

tpicall slightl longer than on or o time. For

absorptive switches, please consult the actory.

i 3 - P--P swichi tim

All specifcations or on/o time in the Narda catalog

are or the modulation mode.

PerorMAnCe trAde-os

The design o an subsstem invariabl requires

trade-os in one or more areas o perormance.

Optimizing a design or one perormance parameter

oten occurs at the expense o another. Such is thecase with PIN diode switches.

Pw v. qc

Junction capacitance can be reduced in order to

ensure low loss at higher operating requencies. For

a given switching speed, junction capacitance can be

lowered b decreasing the area o the diode. This

increases the diodes thermal impedance, producing a

reduction in power-handling abilit.

7

Key PIn-dIode sWItCHPArAMeters C

Detected RF Power

TTL Logic 1

TTL Logic 0

10% RF

50%

Off

(Isolation)

90% RFFall

TimeOff

TimeOn (I.L.)

On (I.L.)

RiseTime

OnTime

Insertion Loss

Port 2 Off

(Isolation)

Port 1 Off

(Isolation)

Port 2 On (I.L.) Port 2 On (I.L.)90% RF

Port-to-Port

Switching Time



8/20

Pw v. swichi sp

To optimize power-handling abilit, the diodes

junction area must be large (hence lower thermal

impedance). This increases the diodes junction

capacitance, resulting in higher insertion loss, lower

isolation (in a series switch confguration), and usuall

smaller bandwidths. To maintain low capacitance,

the diodes I region thickness must be increased to

compensate or the increase in capacitance caused

b the increased junction area. The increased length

o the I region raises the minorit carrier lietime,

which increases switching speed. An added beneft o

increasing the diodes junction area is a reduction in

its orward-biased resistance, improving isolation in a

shunt switch.

qc a bawih

For a shunt confguration, the insertion loss (in dB)

caused b the diode is given b:

10 [1+(ZCj)2] or reverse bias

As the diodes capacitance increases, the switchs

insertion loss increases dramaticall.

For a shunt confguration, the switch isolation in dB is

given b:

20 [1+Z ] or orward bias

where

Z0 is the circuits characteristic impedance

F is the RF requenc o interest

Cj is the diodes junction capacitance

Rs is the diodes orward-biased resistance

releCtIVe sWItCHes

A reective switch is one in which the incident power

at the o port is reected back to the source as a

result o the impedance mismatch presented b thePIN diode. In contrast, an absorptive switch is

designed to present a 50-ohm impedance in the o

state, and to absorb incident power.

Tpical reective switches (Figure 4) include the

previousl-described SPST series confguration, and

an all-shunt arrangement, with its inherentl higher

power-handling abilit and switching speed. The

operating bandwidth o the switch is determined b

the blocking capacitors selected, the bias circuitr,

and the diodes reverse-bias capacitance. Reducing

the diodes shunt resistance increases isolation in

this tpe o switch. This reduction is achieved either

b increasing the current or decreasing the diodes

overall resistance. In addition, b adding a ourth

shunt diode, isolation can be increased, which is

accompanied b an increase in insertion loss, but with

little impact on power handling and switching speed.

i 4 - si a sh rciv swich

8


2r

Series Switch

Bias

Shunt Switch

Bias



9/20

AntennaTX

Bias

RX

/4

Mi-thw rciv swich

Taking this design to a multi-throw confguration

(Figure 5), the low insertion loss at the on port must

be isolated rom the high insertion loss at the o

port with a series PIN diode. Isolation at the oport is a unction o requenc and diode capacitance,

and isolation will increase as the capacitance o the

series diode decreases. However, increased bandwidth

(lower capacitance) comes at the expense o reduced

power-handling abilit. The number o throws can

be extended in this tpe o switch, limited onl b

the diodes junction capacitance and the phsical size

limitations o the switch.

i 5 - Mi-thw rciv sPdt swich

Apiv swich

Multi-throw absorptive switches tpicall emplo the

series-shunt approach (Figure 6). The required 50 ohm

terminating impedance is achieved b the series

combination o the diode and terminating resistance

to ground. This tpe o termination has good highrequenc characteristics, but power-handling abilit

is limited b the abilit o the diodes and resistors to

dissipate RF power. In addition, absorptive switches

tpicall exhibit somewhat slower switching speeds.

These tpes o switches are usuall not absorptive at

their common port (in the all-o state) but can be

made absorptive or special applications.

i 6a - Apiv sPdt swich, sh tmiai

Common

Bias 1

50 50

Bias 2

tami/rciv (t/r) swich

T/R switches are used to switch a single eedline

between a transmitter and receiver and can beneft

greatl rom PIN diode switch technolog. The are

more reliable, aster, and more rugged than their

electromechanical counterparts. The basic T/R switch

consists o a PIN diode connected in series with thetransmitter and a shunt diode connected one-quarter

wavelength awa rom the antenna in the direction

o the receiver section (Figure 7). O course, quarter

wavelength spacing is not practical at low requencies,

so quarter-wavelength lumped elements can be used

instead. In T/R switches, the trade-o is between

achieving low loss or the receiver path and high

power-handling abilit or the transmitter path.

When the switch transers the eedline to the

transmitter, each diode becomes orward biased. The

series diode appears as a low impedance to the signal

approaching the antenna, and the shunt diode shorts

the receivers antenna terminals to prevent overload.

Transmitter insertion loss and receiver isolation are

dependent on the diode resistance. In the receive

condition, the diodes are zero or reverse-biased, and

present a low (shunt) capacitance which creates a low-

loss path between the antenna and receiver. The o

(transmitter) port is isolated rom this path b the

high-impedance series diode.

i 6 - Apiv sPdt swich, si tmiai

i 7 - t/r swich

9


Common

Bias 1

50

50

Bias 2



10/20

A W o divA PIN diode switch will perorm onl as

well as its driver allows. The driver must be

capable o suppling the necessar reverse

bias voltage in order to achieve the desired

diode capacitance, and must source or

sink the bias currents required to drive the

diodes to their rated orward bias resistance.

In addition, ast switching requires the

transition time between driver output levels

to be as short as possible. Relativel high

voltage spikes are also required to remove

charge rom orward-biased diodes and

speed up their switching time.

From the users perspective, the important

parameters are:

Switching speed and repetition rate

Number of switch throws

Number of control lines (i.e., one line perthrow or integral switch logic decoders)

Logic sense ( = low-loss state is typical)

Custom hybrid atpack or printed circuitboard implementation

Driver integral to switch assembly ormounted separatel. High-speed switchdriver circuits are usuall built as hbrid(chip and wire) circuits to reduce size andincrease speed, and are mounted next tothe RF section.

10




11/20

IntroduCtIon

PIN diode attenuators range rom simple series-

connected or shunt-connected diodes acting as a loss

reective switch to more complex structures that

maintain a good input match across the ull dnamic

range o the attenuator. PIN diode attenuator circuits

are used extensivel in automatic gain control (AGC)

and RF leveling applications. Although other methods

are available or providing AGC, such as varing the gain

o the RF amplifer, the PIN diode approach results in

lower power drain, less requenc pulling, and lower RF

signal distortion. Lower attenuator distortion is achieved

using diodes with thicker I regions and long carrier

lietimes. In an attenuator, the resistance characteristics

o the diode are exploited not onl at their extreme

high and low values as switches, but also at values in

between. Thus, PIN diode attenuators tend to produce

less distortion than amplifers but more than switches.

The resistance characteristic o a PIN diode when

orward biased depends on the I region thickness,

carrier lietime, and hole and electron mobilities. A

PIN diode with a thin I region will operate at lower

orward bias currents than a PIN diode with a thick I

region, but the latter diode will generate less distortion.

Careul selection o diode I laer thickness can ield a

good compromise between attenuator speed, distortion,

linearit, and power-handling abilit. In addition, it iseasier to linearize the driver or thicker diodes.

n o Aa Pmac

Understanding how the ollowing parameters aect

perormance makes it easier to choose the best tpe o

attenuator or a particular application.

Pha shi a Aai

A PIN diode attenuators phase shit varies as the

attenuation level changes. This is a result o stra

PIN diode reactance vs. bias level, or (in the case o

a switched-bit attenuator) the dierent lengths o

the transmission paths connecting the diodes that

are being switched in or out. It can never be entirel

eliminated. However, attenuators can be designed to

reduce phase shit to a ver low level, especiall over

narrow bandwidths.

IMd a Hamic

Ever PIN-diode-based device generates some level

o harmonics and intermodulation products because

diodes are non-linear devices. In this regard, switched-

bit attenuators outperorm analog voltage variable

attenuators (VVAs) because switched-bit attenuators

are basicall just PIN diode switches. That is, their

diodes are biased either ull on or ull o.

Pw-Hai Aii

An attenuators power-handling abilit is dictated b its

design, bias conditions, and switching speed. Generall

speaking, aster VVAs handle less power, especiall at

low requencies. An attenuators maximum operating

power level is defned as the amount o power

required to cause 1 dB attenuation compression. Ator near the 1 dB compression point, the attenuator

will produce its highest levels o IMD and harmonics.

Generall, the aster diodes will handle less power at

lower requencies because o the compression points

dependence on I laer thickness. The attenuators

survival rating is dictated b the diodes survival rating.

As might be expected, attenuator power-handling

specifcations var considerabl and can be tailored to

the needs o a specifc application.

11

PIn-dIode AttenuAtors



12/20

Mici

This is a required attribute o an tpe o attenuator,

regardless o the application. Without a monotonic

attenuation relationship to the analog or digital

control commands, the attenuators accurac and

other characteristics can never be optimal. Non-

monotonic behavior can be exhibited b switched-bit

attenuators as a result o uncompensated internal

VSWR interaction, and in digitall-controlled analog

attenuators with errors in digital calibration toward

the band edges.

Ma Aai

This parameter is the average o maximum and

minimum values o attenuation over a given

requenc range or a given control signal. It is o

particular importance in wideband analog VVAs,

as the tpicall have a parabolic attenuation vs.

requenc response, and the minimum-to-maximum

attenuation vs. requenc at higher levels can be as

large as 5 dB in multi-octave designs.

Aai a

The attenuation variation rom the mean

attenuation over a given requenc range or a given

attenuation value is called attenuation atness, and

is expressed in dB.

Aai Accac

This parameter is the maximum deviation o the

mean attenuation rom the nominal value o the

programmed attenuation, expressed in dB.

CoMPArIson o AttenuAtor CHArACterIstICs

Paam swich-bi diia-liaiz Aa

Switching Speed Ver high (20 ns) Moderate (>100 ns)

Attenuation Accurac High Highest

Attenuation Flatnesswith Frequenc Best Moderate

Power Handling High Moderate

Operating Frequenc Broad Moderate

Bandwidth (two to three octaves) (1 octave)

Resolution High (1 dB) Highest (0.25 dB)

Calibration Fixed Selectable within unit

Cost High Moderate

Survival and

Compression Power High Moderate

12

PIn-dIode AttenuAtorsC



13/20

Bias

Hybrids

In Out

Bias

Attenuator Pad

Bias Control Thru Path

dIgItAlly-lIneArIZed AnAlogAttenuAtors v sWItCHed-bItAttenuAtorsThere are dozens o possible attenuator

confgurations, each one with its own unique

characteristics that make it better suited or one

application over another.

diia-liaiz Aa Aa

Other than switched-bit tpes, all attenuators are

essentiall analog devices. There are as man analog

attenuator confgurations as there are sstem

applications that require them. This guide covers onl

digitall-linearized analog attenuators, shunt-mounted

diode arras, and switched-bit attenuators, because

the are the most common and versatile tpes.

Tpical VVAs contain rom one to our shunt-

mounted diodes (Figure 8a). Adjusting the bias

current changes the resistance o the PIN diodes,

reecting more o the RF signal, which produces

the desired attenuation. This approach is similar to a

reective switch because it presents a poor match at the

input and output ports. Most VVAs o this tpe are built

in pairs and mounted between 3-dB hbrids (Figure 8b).

The reected RF power is absorbed b the termination

at the hbrids isolated port, presenting a good match at

the VVAs input and output ports.

i 8a - rciv

13


i 9 - swich bi Aa

i 8 - n-rciv

An analog driver/linearizer or a digital driver (D/A

converter with EPROM) can then be used to calibrate and

linearize the VVAs attenuation vs. control signal response.

diia-C swich-bi Aa

When broadband, ultra-ast-switching perormance isneeded, the digitall-controlled switched-bit attenuator

is the onl solution. It excels in attenuation accurac and

atness over broad requenc ranges, and its switching

speed is equivalent to a high-speed PIN diode switch (25

ns or better). Its onl disadvantages are higher insertion

loss and higher cost.

The digitall-controlled attenuators topolog is based

on switching fxed attenuator pads in or out o the RF

path using PIN diode SPDT switches. It uses one control

bit per attenuator pad, and attenuation step size is

determined b the lowest attenuator pad value. The totalattenuation range is the sum o all the attenuator pads.

Bias



14/20

14


As stated earlier, attenuators are designed to match

the requirements o specifc applications. When the

application requires ast switching speed combined with

high power-handling abilit (as in electronic warare

sstems, or example), the switched-bit attenuator is the

optimum choice (Figure 9). It emplos one or more pairs

o SP2T switches, with a low-loss connection between

one pair o outputs, and a fxed attenuator between the

other outputs. The diodes are switched between their

orward-biased and reverse-biased states, which gives

the attenuator higher switching speed.

The switched-bit attenuator achieves low, consistent

VSWR perormance throughout its dnamic range, and its

power-handling abilit (i.e., compression point and IMD)

is also higher than that o an analog VVA because it uses

PIN diode switches. O course, like all attenuator tpes,

the switched-bit attenuator has some disadvantages. Its

smallest attenuation step size at microwave requencies

is limited to about 0.5 dB because o VSWR interaction

among the various high-loss and low-loss transmission

paths and their associated bias circuits. This interaction

also causes attenuation ripple, which can cause slight

degradations in monotonicit. These errors are usuall

less than about 0.5 dB.

Finall, the switched-bit attenuator is a comparativel

complex RF circuit with more components, and is

usuall more expensive. These considerations aside,

the high speed and power handling abilities o the

switched-bit attenuator make it appealing or

demanding applications.



15/20

PIn-dIode lIMIters

This limiting behavior is defned or tpicall three

operating regions (Figure 16). ln the linear region

(low incident power), the incident signal is passed

with relativel little power loss. lnsertion loss and

VSWR are defned in this region. As the incident

power level increases to the 1 -dB compression point,

the limiter enters the compression region, where the

RF power causes the PlN diodes to sel-bias.

15

PlN diode limiters are designed to protect power-

sensitive microwave components such as mixers,

detectors, and amplifers against damage rom

high-power CW and pulsed microwave signals.

Their specifcations are alwas achieved through a

compromise between operating requenc, input

power, and leakage.

Basic PlN-diode limiters utilize shunt-mounted PlN

diodes with relativel thin l regions, resulting in low

insertion loss at low power levels. Power levels below

the limiters threshold pass through the PlN diodes

unattenuated. lnsertion loss is a unction o junction

capacitance and parallel resistance in shunt with

the transmission line, both o which are unctions o

diode geometr. As the input power increases above

threshold, the diode starts to recti the input power.

Charge is injected into the diode, and i a DC return

path is present, a DC current will be generated.

This current decreases the orward resistance

o the diode, resulting in a progressive increase in

attenuation, while output power remains relativel

constant as input power increases. At saturation, the

diodes series resistance does not change with the

rectifed current, and output power will increase until

the diode reaches its burnout temperature.

i 10 - limi opai Chaaciic

CompressionRegion

(Burnout)Low Level

Insertion Loss

1 dBCompressionPoint

P out

Pout

=PIN

P in

LinearRegion

Hard LimitingRegion

Peak Power

Pulse Width

Spike Leakage

Flat Leakage

(To 1dB)

(To 3dB)

RF InputPulse

RF OutputPulse

0dB

Limiter RelativeAttenuation

Turn-OnTime

Continued increases in RF power level produce

corresponding increases in attenuation until the

diode reaches its saturation point. The limiter then

provides almost constant, relativel high attenuation,

and the output power will begin to increase in

proportion to the input power. This is defned as the

hard limiting region. Eventuall, thermal stresses

on the PlN diodes lead to burnout. With pulsed

input signals, additional parameters are needed to

ull describe the operation o PlN diode limiters

(Figure 17). O particular interest is the relationship

o the input pulse parameters to the output pulse

parameters and the dnamic nature o a pulsed input.

i 11 - tpica limi P rp



16/20

16

PIn-dIode lIMIters (C)

i 19 - tpica limi Pmac v PIn di I la thick

Power output remains at this level until the pulse

ends or burnout occurs, depending on the amplitudeo the input power. Ater the pulse is removed, it takes

time or the limiter to return to its low-loss condition

because o the intrinsic recover time o PIN diodes.

The limiters series resistance at saturation determines

the maximum isolation o the limiter diode.

lncreasing the I region thickness o the diode

results in an increase in threshold and leakage power.

This is illustrated in Figure 19 or diodes with base

widths o 2, 4, and 15 m. Above the threshold level,

the limiter diode is ver reective. Some o the RF

signal is reected and some is absorbed when thediode is limiting. The relationship between input

power and dissipated power is:

Pi = Pi 200r(50 + 2rs)

2

where

Rs is the resistance of the diode in self-biased limiting mode.

Limiting capabilit can be improved b adding a second

limiter diode (Figure 20). ln the basic two-diode limiter,

For a short period o time ater pulsed high power

is applied to the limiter, it will pass signifcantl more

power than when it is totall saturated. The increasein power is a spike o energ on the leading edge o

the leakage pulse (Figure 18). The rise time o the

pulse and the turn-on time o the diode determine

the amplitude o the spike, and the are difcult to

measure because the depend on both the rise time

o the incident pulse and the characteristics o the

PlN diodes. The usual procedure is to speci spike

leakage (measured in ergs), and calculate it as ollows:

spik aka () = x P x 107

where

ts is the spike width at the half-power point (in seconds),

and Ps is the maximum spike amplitude in watts.

Measurements o spike leakage are usuall

subjective unless attention is given to controlling the

rise time o the incident pulse and the linearit o the

detection sstem. Ater the limiter has ull turned

on, the output pulse reaches a constant level, which is

defned as the at leakage level.

i 18 - spik laka

3dBRecovery

Spike Widthat Half Power

Point

Low Level Signal

Power

Flat Leakage

Spike Leakage

2 Micron

4 Micron

15 Micron

Peak Power Input (dBm)

PowerOut(dBm

)

+66+60+50+40+30+20+100

+50

+40

+30

+20

+10

0



17/20

17

PIn-dIode lIMIters (C)

For higher peak and CW power-handling abilit,

dual high-power PlN diodes (preerabl abricated

rom the same chip) are used, ollowed b medium-

power and clean-up limiter diodes.

AddtonAl lMter ConsderAtons

ln addition to the specifcations just described, there

are others that must be considered when speciing a

limiter or a particular application.

rcv tim

This represents the transition time rom the high-loss

to the low-loss state ollowing the removal o a high-

power input. lt is defned as the time rom the end o

the high-power pulse to the time when insertion loss

has returned to within 3 dB o the quiescent (low-power) state.

Pw-Hai Aii

When speciing a limiter, two important considerations

are its peak pulsed power-handling abilit and its

source VSWR. For narrow pulses, peak pulsed power

equates to an equivalent CW power b multipling the

peak power the dut ccle. When the pulse is longer

than 10 s, the peak power is considered CW.

The limiter is a short circuit across the transmission

line when it is ull turned on and up to 90 percento the incident power is reected back towards the

source. An mismatch at the source will reect power

back to the limiter, causing standing waves on the line.

l the limiter-source phase relationship is correct, a

maximum current point will occur at the input diode in

the limiter, causing the diode to dissipate much more

power than the incident power level would indicate.

B multipling the source VSWR b the incident power,

the maximum eective power can be obtained or a

source VSWR o to 2:1.

DCBlock

High-PowerLimiterDiode

FastLimiterDiode

DCBlock

DC Return

OutputInput

/4

DCBlock

FastLimiterDiode

2 High PowerLimiter Diodes

OutputInput

/4

i 20 - baic tw-di limi

the frst diode is chosen or its abilit to handle the

expected input RF power because it is subjected to all

o the pulse energ. This diode is ollowed b the

clean-up diode, which has a lower breakdownvoltage. The low-breakdown diode gets turned on

b the spike leakage, which helps turn on the slower

diode. The clean-up diode also provides attenuation o

the RF leakage rom the frst diode. Output power

(leakage power) o this two-stage scheme is

determined b the selection o the clean-up diode.

ln order to reduce leakage power below +10 dBm,

a PlN-Schottk limiter is oten used (Figure 21). The

Schottk diode is turned on frst since the orward

voltage is about 300 mV. This voltage biases the PlN

diode. This scheme produces a limiter with a lower

leakage power but a longer recover time becausealthough the Schottk diodes turn-on and turn-o

times are ver short, there is no DC return path or

the PlN diode to discharge.

i 21 - limi wih schk bai di lih Cp li



18/20

Apiv dvic A device in which the

specifed VSWR is maintained and all power is

absorbed in the device during the high-loss state.

Accac/liai In voltage-variable

attenuators, the variation o the mean attenuation

rom the best straight line o attenuation vs. control

signal transer unction.

Aa Aa A unit in which attenuation

level is controlled either b an applied current in a

driverless unit or b a voltage in a unit with a driver.

Attenuation level is continuousl variable.

Aai Accac The deviation o meanattenuation rom the nominal attenuation value at

a specifed temperature (usuall room temperature).

bia The control voltage or current signals

supplied to a unit that provide proper operation or

devices without integral drivers.

Cai sppi The minimum ratio o carrier

output power to the translated carrier output power

in a phase shiter operated as a requenc translator.

Cmmai With all other ports set to

isolation, one port is switched rom insertion loss

to isolation, while another port is switched rom

isolation to insertion loss. This specifcation applies

onl to multi-throw switches.

diia-C Va-Vaia

Aa (VVA) An analog attenuator with

an integral driver in which control inputs are logic

bits. Attenuation is not continuousl adjusted,

but is selected in steps. The steps are defned b

the number o bits emploed b the device, the

maximum attenuation o the unit, and the logic

levels applied to it.

div The circuit used to convert analog or logic

command signals to the bias conditions needed to

execute control o active devices.

a tim A measure o switching speed

represented b the time between the 90 percent

and 10 percent points o the detected RF power,

when the unit is switched rom insertion loss (on) to

isolation (o).

Ii l The dierence, measured in dB,

between input power level and output power level

when the unit is in a low-loss condition.

Iai The dierence, measured in dB,between input power level and output power level

when a unit is in a high-loss condition.

Ma Aai The average attenuation over

an attenuators range o operating requencies.

Mai With all other ports set to isolation,

one port is repeatedl switched on and o.

Mai bawih The maximum repetition

rate at which a device can be switched.

Mici As control input level is increased,

attenuation level continuousl increases. At no point

in the range does an increase in control input cause

a decrease in attenuation at an requenc or value

o attenuation.

o tim A measure o switching speed

represented b the time between the 50 percent

point o input control pulse to the 10 percent point

o detected RF power, when the unit is switched

rom insertion loss (on) to isolation (o).

18

glossAry



19/20


20/20

435 Moreland Road, Hauppauge, Ny 11788

Tel: 631.231.1700 Fax: 631.231.1711

e-mail: [email protected]

www.nardamicrowave.com

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