AN OVERVIEW OFMICROWAVE DESIGN
CONSIDERATIONS FOR
SWEPT SOURCES
ARLEN DETHLEFSENNETWORK MEASUREMENTS DIVISION
1400 FOUNTAIN GROVE PARKWAYSANTA ROSA, CALIFORNIA 95401
Rf ~ MicrowaveMeasurementSymposiumandExhibition
Flin- HEWLETT~~ PACKARD
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C'
AN OVERVIEW OFMICROWAVE DESIGN
CONSIDERATIONS FOR
SWEPT SOURCES
MICROWAVE SWEPT SOURCEDESIGN CONSIDERATIONS
1. BLOCK DIAGRAM
2. MICROWAVE COMPONENTS
3. CONTROL AND DRIVE CIRCUITRY
O'--- -----J
1
INTRODUCTION
Microwave design and testing is highly dependent uponthe use of microwave swept sources. This paperdescribes some of the design considerations necessaryto' achieve superior performance in a microwave SweptSource used for design and production testingapplications.
Many of these concepts would apply in the general senseto any electronically tuned microwave source.
The performance of a microwave swept source is highlydependent on three major areas:
1. The block diagram concept.
2. The microwave components used in the source.
3. The control and drive circuitry.
This presentation will focus on the first two of theseareas.
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There are many block diagram concepts that can beconsidered for a swept source. We will look at some ofthe more commonly used concepts and review theadvantages of each. When considering the blockdiagrams, the performance parameters shown here have tobe kept in mind.
I;The designer has to make decisions on the relativeimportance of each of these performance perameters inchoosing the appropriate block diagram concept.
The design of the microwave components as well as thedrive an control circuitry would also have considerableimpact on these parameters.
Let's now look at some of the block diagram conceptsand determine how these various configurations wouldeffect the performance of the source.
PARAMETERS TO CONSIDERWHEN CHOOSING A BLOCK DIAGRAMFOR A SWEPT MICROWAVE SOURCE
1. FREQUENCY COVERAGE2. OUTPUT POWER3. FREQUENCY ACCURACY AND DRIFT4. HARMONIC AND SPURIOUS SIGNALS5. RESIDUAL FM6. MODULATION REQUIREMEMENTS7. RELIBILITY8. COST
The block diagrams may be placed into these four basiccategories.
Category A and B cover a single band of frequenciesusing a fundamental oscillator or an oscillator drivinga single harmonic multiplier. Category C & D are blockdiagrams for sources covering frequency ranges whichcan not be spanned by a single fundamental oscillator.
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BLOCK DIAGRAM CATEGORIESFOR MICROWAVE SWEPT SOURCES
• WOULD NORMALLY REQUIRE ONE FUNDAMENTAL OSCILLATORTO COVER THE DESIRED BAND OF fREQUENcrES.
•• WOULD NORMALLV REQUIRE TWO OR MORE FUNDAMENTALOSCILLATORS OR MULTIPLICATION BY MORE THAN ONE HARMONICNUMBER TO COVER THE DESIRED BAND OF FREOUENCIES.
SINGLE BAND' MULTI/BAND'
DEFINITIONS:
~~I B I~FREQUENCY
MULTIPLACATION
FUNDAMENTALOSCILLATORS
2
Shown here is a block diagram that fits into CategoryA.
This is the most basic and has the advantage of lowestcost and highest reliability. The output power wouldbe relatively low. Harmonics would be relatively highand FM incidental to AM would be high because theoscillator is not sufficiently isolated from theamplitude modulator.
CATEGORY A(SINGLE BAND FUNDAMENTAL OSCILLATORS)
ELECTRONICALLYCONTROLLEDOSCILLATOR
DIRECTIONALCOUPLER/DETECTOR
f----------( ~~TPUT'----.----'
"Ul--------l
This is identical to the previous diagram with theexception that an amplifier or isolator is added toisolate the amplitude modulator from the oscillator.This addition greatly reduces the Incidental FM.
The use of the amplifier has two potential advantagesover the use of the isolator:
1. Output power would be increased.
CATEGORY A(SINGLE BAND FUNDAMENTAL OSCILLATORS)
2. Harmonics from the oscillator could be improved ifthe amplifier were designed to have a negative gainslope as a function of frequency.
ELECTRONICALLYCONTROLLEDOSCILLATOR
ISOLATOR
rtl~
RFOUTPUT
DIRECTIONALCOUPLERIDETECTOR
3
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This diagram has another amplifier added after theamplitude modulator for highest output power.
CATEGORY A(SINGLE BAND FUNDAMENTAL OSCILLATORS)
ELECTRONICALLYCONTROLLEDOSCILLATOR
>-----< ~~TPUT
Here a filter has been added to reduce the harmonicoutput signals. This filter can be a low pass or bandpass filter if the band of frequencies to be covered isless than an octave. If the band of frequencies isgreater than an octave. the filter could then be a YIGtuned band pass filter which is controlled by circuitryto track .the oscillator. More output power can beobtained by placing an amplifier with a filter afterthe modulator. If this is done. the filter ahead ofthe modulator could be eliminated. CATEGORY A
(SINGLE BAND FUNDAMENTAL OSCILLATORS)
4
ELECTRONICALLYCONTROLLEDOSCILLATOR
AMP
,----,-,..,....--,M( ) >--< ~~TPUT
DIRECTIONALCOUPLER/DETECTOR
-lOWPASS OR BANDPASS IF BAND IS lESS THAN AN OCTAVE.
ELECTRONICALLY TUNED FILTER IF BAND IS MORE THAN AN OCTAVE.
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BLOCK DIAGRAM CATEGORIESFOR MICROWAVE SWEPT SOURCES
SINGLE BAND' MULTI/BANIi'
Let's now look at two category B block diagrams.
This category is particularly useful when the frequencyof operation is high enough to make the multiplicationapproach more desirable from a performance/cost pointof view. It also allows for a convenient way toprovide an auxiliary output at a sub multiple of theoutput frequency. This output is useful forphase-locking the source or for using a frequencycounter.
FUNDAMENTALOSCILLATORS
FREQUENCYMULTlPLACATlON
~~I······:-:-I~:.. ]':::/j~
DEFINITIONS:
• WOULD HORMALlV REQUIRE ONE FUNDAMENTAL OSCILLATORTO COVER THE DESIRED BAND OF FREQUENCIES.
... WOULD NQRMALlV REQUIRE TWO OR MORE fUNDAMENTALOSCILLATORS OR MULTIPLICATION BY MORE THAN ONE HARMONICNUMBER TO COVER THE DESIRED BAND OF FREQUENCIES.
CATEGORY B(SINGLE BAND FREQUENCY MULTIPlER)
ELECTRONICALLYCONTROLLEDOSCILLATOR
.. FixeD BANDPASS FIL.TER IF 12 < 1, IN-~1)
TUNABLE BANDPASS FILTER IF '2> f1 (~I
f,-LOWEST Dupur fREQUENCY,
f2 ", HIGHEST OUTPUT FREQUENCY.
N .. MUL.TlPLICATlON NUMBER.
This Category B diagram has the modulator and amplifierahead of the multiplication process. The filter may bea fixed broadband band-pass filter provided f2 is lessthan f1(N+1)/N. If f2 is greater than f1(N+1)/N, thefilter would need to be a tunable bandpass filter. f1is defined as the lowest output frequency, f2 is thehighest output frequency and N is the multiplicationnumber.
It is also possible to design multipliers to balanceout the odd or even harmonics. This minimizes oreliminates the need for a bandpass filter.
RFOUTPUT
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In this diagram, you will notice that the modulator andamplifier are after the multiplication process. Theprevious diagram had the advantage of modulating andamplifying at lower microwave frequencies.
This configuration has the potential for higher outputpower and, in the case of some types of multipliers.the unwanted harmonics can be more easily controlled asthe output power is varied.
CATEGORY B(SINGLE BAND FREQUENCY MULTIPIER)
Let's now look at the block diagrams in the multi-bandarea.
ELECTRONICALLYCONTROUEDOSCILLATOR n
~ >--< ~~TPUTDIRECTIONAL
COUPLER/DETECTOR
BLOCK DIAGRAM CATEGORIESFOR MICROWAVE SWEPT SOURCES
SINGLE BAND' MULTI/BANC'
FUNDAMENTALOSCILLATORS
FREQUENCYMULTIPLACATION
6
DEFINITIONS:
• WOULD NORMALL.Y REQUIRE ONE FUNDAMENTAL OSCILLATORTO COVER THE DESIRED BAND OF FREQUENCIES.
.. WOULD NORMALLY REOUIRE TWO OR MORE FUNOAMENTAlOSCILLATORS OR MULTIPLICATION BY MORE THAN ONE HARMONICNUMBER TO COVER THE DESIRED BAND OF FREQUENCIES.
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This block diagram using fundamental oscillators hasthe advantage that the harmonics can be more easilyfiltered and there are no harmonic products below thedesired signal. The oscillators and amplifiers aredesigned to cover discrete frequency bands but themodulator and switch must operate over the lowest tothe highest frequency of interest.
CATEGORY C(MULTI-BAND FUNDAMENTAL OSCILLATORS)
ELECTRONICALLYCONTROLLEOOSCILLATOR
RFOUTPUT
DIRECTIONALCOUPLER!DETECTOR
CATEGORY D(MULTI-BAND FREQUENCY MULTIPIER)
RFOUTPUT
7
The frequency multiplier approach has the advantage ofbetter frequency accuracy and less frequency drift dueto temperature. This is because the oscillatoroperates at a lower microwave frequency where it iseasier to design a stable, linear oscillator with lowhysteresis. This will become more apparent when wereview the component designs. The relatively lowfrequency of the fundamental oscillator can easily becoupled to an auxilIary port for frequency measurementsor for phase locking to a stable reference. The onlycomponents which require designs at the highestmicrowave frequencies are the multiplier/filter and thedirectional coupler. This concept also minimizes thenumber of microwave components and drive circuitry.
Now let's focus on the various microwave componentsused in these block diagrams and how their designeffects the overall performance of the product.
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Shown here are the six major microwave components thatare used in swept sources.
Most of thedesign ofoscillatorsmost of the
effort will be spent on the selection andthe microwave oscillator since the
performance has a significant bearing onelectrical parameters of the swept source.
MAJOR MICROWAVE COMPONENTSUSED IN SWEPT SOURCES
- OSCILLATOR
- AMPLIFIER
AM MODULATORMULTIPLIERS
FILTERS
DIRECTIONALCOUPLER/DETECTOR
Here is a list of important parameters for electricallytuned microwave Oscillators. The oscillator design canbe broken down into four areas: the tuning device.active devices, circuit design and mechanical design.As you can see. the decisions made for each of theseareas impact most if not all of the performanceparameters. Let's take a brief look at each of theseareas.
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DESIGN DECISIONS THAT EFFECT ELECTRICALLY TUNEDMICROWAVE OSCILLATOR PERFORMANCE PARAMETERS
Devices and Designs that effect the paraneter
Paokage andParcrneter Tuning Active Circuit Mechanical
Device Devices Design Design
Operating frequency X X X X
Tuning Range X X X X
Output Power X X X X
Tuning Signal Linearity X X X X
frequency Accuracy X X X X
Frequency Changes X X X XYS. temperature
Tuning Sensitivity X X X
Harmonic and X X XSpurious outputs
Noise and Residual FM X X X
Magnetic Susceptibility X X X
Pulling/Pushing X X X
Weight & Size X X X X
Power ConslIJlption X X X
Total Paraneters 14 11 14 10Effected
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The tuning device chosen for an oscillator hasconsiderable impact on the oscillators performancesince it effects virtually every parameter.
We have indicated where a specific device has aninherent advantage for a given parameter.
The most commonly used electronic tuning devices formicrowave oscillators is the varactor diode and theYttrium-Iron-Garnet (YIG) sphere. Shown here is acomparison of these two devices.
In summary, the YIG tuning device is most suitable toapplications requiring high frequencies, broad tuningranges, good nOlse performance and linear change inoutput frequency as a function of the tuning signal.
The varactor tuning device is most suitable forapplications requiring fast tuning or where there is asize, cost, or power consumption constraint.
The power consumption and size of YIG tuned oscillatorsgenerally do not present a problem for most sweptsources. Sweep speeds on the order of 10 to 30milliseconds are generally accpetable for mostappl ica tions. The refore, the YIG tuned oscilla tor,with its advantages in frequency of operation, tuningrange, noise and tuning linearity, has become theoscillator that is predominantly used in sweptmicrowave sources.
on oscillator designbe limited to the YIG
The following discussionsconsiderations will thereforetuned oscillator.
Performance Parameter Dev; ce
YIG VARACTOR
Operating Frequency "'I - 40 GHz' <20 GHz
Tuning Range Multi-octave· Octave
Tuning Rate Slow Fast*
Tuning Linearity Linear* Exponential
Frequency Accuracy .Noise & Residual FM .Weight & Size .Power Consumption .Tuning Method Magnet; c Voltage
Field
COMPARISION OF THE YIG SPHERE AND VARACTOR DIODEFOR TUNING MICROWAVE OSCILLATORS
* Device has an inherent advantage on this parameter.
ACTIVE DEVICES USED IN WIDEBAND YIG TUNED OSCILLATORS
Typi cal Osci natorDevi ce Performance Characteristics
Bi-Polar Useab1e to 10 GHz
Trans; star Greater than Octave operating range can be achieved
lowest close-in Phase Noise
Good eff; ci ency
Output Power 10 ""
Field-Effect Useab1e to 26 GHz
Transistor Greater than Octave operating range can be achieved
Good efficiency
low Phase Noise
Output Power lOnw
Bulk Useable B to 40 GHz
GaAs diode Poor eff; c;ency
Power output approximately 10 to 40 11\'1
low Phase No; se
Octave Tun;"9 Range
The active devices used in broadband YIG tunedoscillators are shown in this diagram. Below 10 GHz,either the Bipolar or FET devices are generally used.Bipolar transistors presently have an advantage in thearea of close-in phase noise. so for low noiseapplications, the Bipolar devices are generally used.
In the past, bulk GaAs diodes have been predominantlyused at frequencies above 8 GHz. However, with theadvent of 26 GHz FET devices, many new applicationsabove 8 GHz will be using the FET transistors becausethe circuit may be designed to tune over greater thanoctave frequency ranges. It also has the advantage ofreqUiring less supply power.
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Broadband microwave oscillator designs usingtransistors are normally designed using circuitconfigurations shown here. This design allows formaximum bandwidth while still achieving reasonableperformance in the areas of output power. noise. andharmonic level. Oscillators using this topology arepresently available that span 1.5 to 2 octaves. Withimprovements in devices and by using multiple tuningelements. further increases in bandwidth can beexpected in the future.
Listed below are some reference articles that deal withwideband microwave oscillator design.
MICROWAVE BROAD BAND YIG TUNEDTRANSISTOR OSCILLATOR CIRCUIT TOPOLOGIES
Oscillator Design References:
1. Ganesh R. Basawapatna and Roger B. Stancliff, "AUnified Approach to the Design of Wide-band MicrowaveSolid-state Oscillators" IEEE Trans. Microwave TheoryTech. Vol Mtt-21. No.5. pp 319 - 385, May 1979.
2. James C. Papp and Yoshiomi 1. Koyano. "An 8 - 18GHz YIG-Tuned FET Oscillator" IEEE Trans. MicrowaveTheory Tech. Vol MTT-28, No.7. pp 762.
The remaining area of the oscillator design that has tobe addressed is the magnetic structure. This is a keyelement in the design since it affects such things astuning linearity. frequency drift with temperature andtuning sensitivity.
The basic structure required to provide a magneticfield for the YIG Sphere is shown here.
BIPOLAROSCILLATOR TOPOLOGY
MESFETOSCILLATOR TOPOLOGY
It consists of asphere, a driveroscillator circuit.
magne ticcoil and
core. a gapa means to
for the YIGsupport the BASIC MAGNETIC STRUCTURE
FOR TUNING YIG OSCILLATORS
Let's take a brief look at some of the key propertiesof electromagnets and see how they effect theperformance of the YIG tuned oscillator.
MAGNETIC CORE \
lO
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o
PARAMETERSOF MAGNETIC STRUCTURES
THAT EFFECTOSCILLATOR PERFORMANCE
1. SWEEP DELAY
2. HYSTERESIS
3. LINEARITY & SATURATION
4. TUNING SENSITIVITY
SWEEP DELAY OF YIG TUNED OSCILLATORS
These are the four primary parameters of magneticstructures that affect oscillator performance.
Sweep delay is defined as the frequency lag relative tothe tuning current under continuous sweep conditions."Delay", in this context, represents frequencyinaccuracy as a function of tuning speed. As shownhere, this delay increases with increased tuningspeeds. Typical numbers for uncorrected delay would be100 MHz for an oscillator in the 8 GHz range sweepingat a 10 ms sweep rate. Choosing a magnetic materialwith high resisitivity minimizes this effect. However,in order to maintain good frequency accuracy as afunction of sweep speeds, additional corrections arenormally required in th oscillator drive circuitry.
>uzw::;)
owa:u.
TUNING CURRENT
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Hysteresis is defined as the maximum differentialfreq~ency (at a fixed-coil current) due to thehysteresis of the magnetic circuit when tuned in bothdirections through the operating range. Hysteresis canbe minimized by carefully choosing the magneticmaterial. As shown, hysteresis increases with wideroperating ranges. It also increases with increases influx density and therefore higher frequency YIG tunedoscillators have larger values of hysteresis.Hysteresis has a direct bearing on the frequencyaccuracy of the tuned oscillator since there is nosimple way of compensating for this phenomenon withexternal circuitry.
Saturation occurs when increases in coil current do notproduce further linear increases in the flux density.The saturation level depends on the properties of themagnetic material as well as the design of the magneticstructure. Unfortunately, magnetic material which ischosen for high saturation levels has properties whichincrease the hysteresis of the magnet.
The saturation level determines the maximum frequencyto which the oscillator may be tuned and also has abearing on oscillator linearity since any deviationsfrom a straight line relationship between flux densityand coil current will effect frequency accuracy as afunction of the tuning signal.
HYSTERESIS OF YIG TUNED OSCILLATORS
THYSTERESIS
>-UZ NARROWW OPERATING~
a RANGEw
1 ta:II.
COIL CURRENT
MAGNETIC SATURATION AND LINEARITY
Frequency linearity is also affected by the tuningdevice. circuit design and active devices.
Careful circuit and magnetic designs are essential inthis area to produce good performance.
>uzw~
awa:II.
( SATURATION LEVEL
---- -
COIL CURRENT
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Tuning sensitivity is defined as the differentialcurrent required to tune across the operating frequencyrange divided by the frequency range. The sensitivityis a function of the number of turns and the width ofthe gap.
TUNING SENSITIVITY 0<. NIS
N = # OF TURNS ON TUNING COILS = WIDTH OF THE GAP IN THE MAGNETIC STRUCTURE
In order to minimize the power necessary to tune theoscillator. it is essential that the gap be kept assmall as possible. The mechanical design of the magnetmust also be such that the gap size does not vary as afunction of temperature since this would causeinaccuracies in the frequency of the source.
•
COIL
The two magnetic structures that are normally used areshown here. The single ended design is simpler andtherefore less costly. The double ended design has theadvantage of better hysteresis and is capable of highersaturation levels since there are fewer leakage pathsfor the flux. The double ended design is also lesssusceptible to externally applied magnetic fields.
CROSS SECTION OF BASIC MAGNETIC STRUCTURESUSED FOR YIG TUNED MICROWAVE OSCILLATORS
YIGSPHERE
"'- -
-
" ~
- ....
- ~
DRIVERCOIL
CIRCUIT
r MAGNETIC MATERIAL...... --t._-,
YIGSPHERE~~=---J
SINGLE-ENDED DESIGN DOUBLE-ENDED DESIGN
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An example of a 2 to 8.4 GHz Bipolar transistoroscillator is shown here.
The magnets are made of a low hysteresis material. Thecoils are layer-wound which minimizes the size of themagnet. This structure has a saturation frequency inexcess of 12 GHz.
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}
TYPICAL PERFORMANCE OF 2-8.4 GHzTRANSISTOR YIG TUNED OSCILLATOR
The oscillator transistor is a Silicon Bipolar devicefollowed with a FET buffer amplifier. It uses a 660micron diameter sphere (26 mil) with an unloaded Q of1700.
The sphere is mounted on a sapphire rod and oriented ona temperature compensated axis. In addition. it iskept at a constant temperature with a thermostaticallycontrolled heater. This keeps the post tuning drift ofthe oscillator under 100 KHz.
The sphere and devices were specially designed atHewlett Packard for this product.
The oscillator actually operates between 1.8 and 8.6GHz and its basic performance is listed here.
OUTPUT POWER
HARMONICS
TUNING SENSITIVITY
HYSTERESIS
LINEARITY
15 mW
20 dBc
24 ma/GHz
2 MHz
16 MHz
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The phase noise characteristics aresingle side band noise is typicallycarrier at a 10 KHz offset.
shown here. The100 dB below the
2 - 8.4 GHz OSCILLATOR PHASE NOISE AS A FUNCTION OF FREQUENCY
FREQUENCY OFFSET FI()M CARRIER· Hz
Now let's briefly review some of the key designconsiderations for the other components.
The key parameters for amplifiers used in microwavesources are shown here.
The performance requirements would vary depending onthe requirements of the specific product. However. itis normally beneficial to achieve as broad a band ofoperation as the devices and circuit design will allow.
IMPORTANT AMPLIFIER PARAMETERSFOR USE IN SWEPT SOURCES AND
TYPICAL PERFORMANCE REQUIREMENTS
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PARAMETER
FREQUENCY RANGE
OUTPUT POWER
HARMONICS
INPUT & OUTPUT MATCH
TYPICALPERFORMANCE REQUIREMENTS
2:1 to 10:140 to 400 mW20 to 40 dBc2:1 V.S.W.R.
Broadband high power designs are normally best achievedby using a design approach as shown here.
The interstage matching networks are designed such thatthey provide maximum gain at the highest frequency ofoperation and reduce the gain at the lower frequency toachieve an amplifier gain that is relatively flat withfrequency.
TYPICAL BLOCK DIAGRAMFOR A
BROADBAND MICROWAVE POWER AMPLIFIER
In order to achieve sufficient power over thebroad range of frequencies. it is necessary to combinethe outputs of two or more devices. This is normallyachieved by using hybrids as shown here.
POWER COMBINING USING QUADRATURE HYBRIDS
INPUT
OUTPUT
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An example of a 2 to 7 GHz 0.5 watt MESFET amplifierfor use in swept sources is shown here. It has a gainof 18 dB @ 0.5 watt output with harmonics typically 20dB below the fundamental.
To achieve this performance. two specially designedFET's were utilized. The 1 micron x 500 micron deviceshown here was designed to have a high fmax whichsimplifies broadband amplifier designs.
For references purposes. a human hair is approximately100 microns in diameter.
1 MICRON X 500 MICRON X-BAND FET
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1.5 MICRON x 1500 MICRON LINEAR FET
This 1.5 x 1500 micron device was designed to achievehigh output power with low distortion. It can deliver300 mw @ 6 GHz.
The 500 micron device is used to drive the 1500 microndevice as shown here.
Two 1500 devices re combined with a quadrature hybridto achieve the 0.5 watt output.
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Frequency multilpliers can be categorized as shown inthis slide. Passive multiplers have no gain mechanismwhile an active multiplier has the ability to providemore output RF power at the multiplied frequency thanis provided at the input of the multiplier. Passivemultiplication is normally achieved by using rectifiertype diodes or step recovery diodes. Activemultiplication is achieved using field effecttransistors.
FREQUENCY MULTIPLIER CATEGORIES
1. PASSIVE
(a) RECTIFIER DIODE
(b) STEP RECOVERY DIODE
2. ACTIVE
(a) FET
Typical passive multipliers are shown here. Thepassive doubler is essentially a full wave rectifierwhich is rich in even order harmonics. The passivetripler is a diode limiter which is rich in odd orderharmonics.
The comb multipler using a step recovery diode has anoutput wave shape that is essentially an impulse andtherefore generates a comb of frequencies of both oddand even order.
PASSIVE MULTIPLIERS
DOUBLER
IVV\TRIPLER
JUlJl310
.toSTEPRECOVERYDIODE
COMB
TrTTto
IVV\
~~ fVVVV\ to LPF
fo 2'0
~ RECTIFIER TYPE DIODES~
20
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device which is biased toSince the device has gain,
larger in magnitude than the
The active doubler is a FETrectify the input signal.the output signal can beinput signal.
BALANCED ACTIVE DOUBLER
RF IN10
DUALGATEFET'S
/RF OUT
210
An example of a single band active doubler is shown inthis slide. The input power of the doubler is +13 dBmas is its output.
This design has the modulator following the multiplierand also utilizes an 18 to 26.5 GHz post amplifier.The amplification compensates for all circuit losses.
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The devicesFET's and aamplifier.here.
used in this doubler are two dual gate0.5 x 350 micron gate device is used in the
The pattern of the dual gate FET is shown
The 0.5 x 350 micron FET used for the amplifier has anfmax of 60 GHz and is capable of delivering 40 mw @ 26GHz.
These two devices are also special HP designs.
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1 MICRON X 400 MICRON DUAL GATE FET
0.5 MICRON X 350 MICRON K BAND FET
An example of a frequency multiplier using a steprecovery diode is shown here. The input frequency is 2to 7 GHz and the YIG filtered output frequency is 2 to26.5 GHz using multiplication numbers of 2, 3, and 4.
The magnetic structure was designed using two differentmagnetic materials.
The center body and pole tips are made of a lowsaturation material while the end pieces are made of alow hysteresis material. The shape of the pole andpackage was optimized to minimize flux leakage paths.
Thermal shorts were designed to carry heat away fromthe pole tips. The magnet saturates at frequencies inexcess of 30 GHz.
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The HP designed step recovery diode has a transitiontime less than 30 ps.
This particular multiplier also has provisions for amultiplexed 10 MHz 2.4 GHz signal so that theassembly can deliver a 10 MHz to 26.5 GHz swept signalfrom a single port.
The 680 micron (27 mil) YIG sphere is mounted in a 254micron (10 mil) thick sapphire substrate. The YIGsphere is kept at a constant temperature by athermostatically controlled heater.
on the order of 10 dB areat 26 GHz. Fractional and
35 dB below the desiredtypically 50 dB below the
Typical conversion lossesachieved to 20 GHz and 15 dBsubharmonics are typicallysignal and harmonics aredesired desired signal.
The functions of the Amplitude Modulator are shownhere.
Items and 2 are virtually essential for all modernswept sources. Items 3 and 4 are normally designed tomeet the performance objectives of the source.
FUNCTIONS PROVIDED BY AM MODULATOR
1. RF LOSS CONTROL MECHANISM FOR AUTOMATICLEVEL CONTROL AS A FUNCTION OF FREQUENCY.
2. SETS THE LEVEL OF RF OUTPUT POWER.
3. BLANKS RF OUTPUT ON RETRACE OFSWEEP OSCILLATOR.
4. PROVIDES MEANS OF AMPLITUDE MODULATINGTHE RF SIGNAL.
(a) SINUSOIDAL AND SQUARE WAVE
(b) PULSE
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Most microwave modulators today utilize the PIN diodein either a series or shunt configuration or acombination of the two to provide the desiredperformance. The series and shunt versions arecompletely reflective while the combination circuit canbe designed to have reasonable input and output matchspecifications.
MICROWAVE AMPLITUDE MODULATOR TOPOLOGIES
SHUNT
RFIN~RFOUT
t JPIN DIODE
MOD BIAS
SERIES
PIN DIODE
nIT"'"'MOD BIAS
COMBINATION SERIES/SHUNTPIN DIODES
RF IN )
}., n 14
1<RF OUT
BIAS MOD BIAS MOD
AM MODULATOR WITH INPUT/OUTPUT BUFFERS
In order to mlnlmize problems associated with modulatorinput and output match changes as a function offrequency and RF output level, it is good design toinclude an input and output amplifier or isolator asshown here.
RFINPUT
MOD BIAS
AMPLIFIER VERSION
RFINPUT
MOD BIAS
ISOLATOR VERSION
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The directionalfunctions:
coupler/detector has two primary
COUPLERINPUT
FOWARDSIGNAL
)-
1. to provide a DC output that is proportional to theRF output power.
2. to improve the source output match.
The output is amplified and fed back to the amplitudemodulator to achieve leveled output power as a functionof frequency.
FUNCTIONS PROVIDEDBY DIRECTIONAL COUPLER AND DETECTOR
1. PROVIDE A DC OUTPUT SIGNAL
THAT IS PROPORTIONAL TO THE
RF OUTPUT POWER.
2. IMPROVE OUTPUT SOURCE MATCH.
To achieve good levelling, the combination of couplingloss and detector response together need to provide aDC output that does not vary as a function of frequencyfor a given output power level.
Good source match is achieved when the output connectorhas a good VSWR and the coupler has high directivity.
LEVELLING AND SOU~CE MATCH DEGRADATIONCAUSED BY OUTPUT COUPLER PARAMETERS
COUPLEROUTPUT
>< j<C ' =~;:~TEDDIODE \
DETECTOR "'- ERROR DUEOUTPUT TO COUPLER
L- --' OUTPUT MATCH
ERROR DUETO COUPLERDIRECTIVITY
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TYP leAL PERFORMANCEOF THE
HP 86260A SWEPT SOURCE02.4-18.0 Gft:r.)
!LOCl DIACRAM CATEGORY A
osc. TUNING DEVICE YIO
BULKOSC. ACTIVE DEVICE GaAS DIODE
OSC. MAGNETIC STRUCT. DOUHE ENDED
TYPE OF HARMONIC FILTERING NONE
AUX. OUTPUT FORCOUNTEI OR PHASE-LOC¥. NO
OUTPUT POWER (.,,) 12
FREQUENCY ACCURACY 30(KHz)
HYSTERESIS (MHz) 10
RES IDUAL FM (I.H:r. PEAl I'IN 10 I.H:r. BANDWIDTH)
HARMONICS (dB BELOW 30FUNDAMENTAL)
INTERNAL LEVELED + -0.5POWER VARIATION (dB)
TYPICAL PEItFORHANCEOF THE
HP 83545A SWEPT sou aCE(5.9 TO 12.4 GH:r.)
SLOCl DIAGRAM CATEGORY A
OSC. TUNING DEVICE YlO
OSC. ACTIVE DEVICE FET
OSC. MAGNETIC STlUCT. SUCLE END
FIXEDTYPE OF HARMONIC FILTERING LOW-PASS
AUX. OUTPUT FOR NOCOUNTER OR PHASE-LOCI.
OUTPUT POWER (m,,) 60
FREQUENCY ACCURACY I'(MHz)
HYSTERESIS (Hh) 20
RES IDUAL FM (1Hz PEAl. 10IN 101Hz BANDWIDTH)
HARMONICS (dB BELOW >40FUNDAMENTAL) (7 -1 2GHz)
INTERNAL LEVELED +-0.4POWER VARIATION (dB)
In concluding, we will identify, by block diagramcategory type, some current Hewlett-Packard designs ofswept sources to determine what performance isachievable using the concepts presented in this paperand state-of-the-art microwave devices and designs.
For Category A, the HP 86260A is a single band sweptsource using a bulk GaAs diode. Output power is 12 mw.Frequency accuracy is 30 MHz with a hysteresis of 10MHz.
Another Category A unit, the HP 83545A, is a singleband unit designed for high output power. It uses aFET transistor oscillator and typically provides 60 mwleveled output between 5.9 and 12.4 GHz.
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Still another example of Category A is the HP 83540B, aproduct designed for high power, good harmonics andfrequency accuracy. It utilized a double-endedoscillator structure with a tracking YIG filter toachieve very low output harmonics over itsdouble-octave frequency range.
For Category B, the HP 83570A is an 18 - 26.5 GHzdoubler type source with 11 mwoutput power. Theoutput of the fundamental oscillator, 9.0 - 13.25 GHz,is made available as an auxiliary output. This signalcan be used as the RF sample for phase-locking or canbe applied to a microwave counter.
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tYP leAL PERFORMANCEOF THE
HP 835408 SWEPT SOURCE(2 TO 8.4 CRt)
BLOCK DIAGIAM CATEGORY A
osc. TUNING DEV ICE YlC
OSC. ACtIVE DEY ICE B I-POLAR
ose. MAGNETIC STRueT. DOUBLE ENDED
TYPE OF KARMone FILTERING YIC TUNED
AUX. OUTPut FOR NOCOUNTEI. 01. PHASE-LOCt:;
OUTPut POWER (aw) 30
FREQUENCY ACCURACY '-,(KHid
HYSTERESIS (MHz) 1.2
IES IOUAL FH (lUz PEAl: ,IH 10 KHz IIAMDW lDTH)
HARMONICS (d. BELOW '0FUNDAMENTAL)
INTERNAL LEVELED +-0.8POWER VARIATION (dB)
TYPICAL PERFORHANCEOF THE
HP 83.570" SWEPT SOUICE(18 TO 26.5 CHz)
IILU\.I: DIACUM CATEGORY •OSC. TUNING DEVICE YlC
osc. ACTIVE DEVICE FET
ose. MACNETIC STlueT. SINGLE ENDED
FIXEDTYPE OF HARMONIC rlLTElING HICH-PASS
AUX. OUtput FOR YESCOUNTEI OR PHASE-Lon
OUTPUT POWER (mv) II
FREQUENCY ACCURACY 20(MHd
HYSTERESIS (MHz) 12
RES IOOAL FM (1Hz PEAl 20IN 10 J:H:r. BANDWIDTH)
HARMONICS (dB BELOW 30FUNDAMENTAL)
INTERNAL LEVELED "'-1.2POwER. VAiIATIOM (d 8)
f
TYPICAL PERFORMANCEOF THE
HP 8359211 AND HP 835951.SWEPT SOURCES
8359211 83595,\
BLOCK DIAGRAM CA TEeORY D D
osc. TUNING DEVICE VIG VIG
osc. ACtIVE DEVICE 81-POLAI. Ill-POLAR
osc. HAGNETIC sTluer. DOUIlLE ENDED DOUBLE ENDED
TYPE OF HARHONIC FILTERING VIG TUNED VIG TUNED
FR.EQUENCY RANGE (GHz) .01 TO 20 .01 TO 26.5
OUTPUT POWER. (.w) 25 o. 25 TO 20 GR1, TO 26.5 GR1
FREQUENCY ACCURACY , 5(MHz)
HYSTERESIS (MHz.) 1.2 1.2
IES IDVAL rM (1Hz PEAl: 3 @ 6 GR1 3 @ 6 GK<IN 10 K" BANDW 10TH) 10 @ 20 GR1 12@ 26. 5 GK<
HARKON les (dB BELOW 25 BELOW 2.' GN1 25 BELOW 2. , GM1FUNDAMENTAL) 50 ABOVE 2. , GR1 50 ABOVE 2. , GR1
HARMONICALLY RELATED 35 35(d. BELOW YU HOAKEN!At)
lNTEINAL LEVELED +-0.7 +-0.7POWER VARIATION (dll)
For Category D, the HP 83592B and 83595A are multi-bandsources designed to span the .01 to 20GHz and .01 to26.5GHz bands with good power and excellent frequencyaccuracy and residual FM. Output power is typically 25mw at 20 GHz with frequency accuracy of 4 MHz. Inorder to achieve this type of accuracy you will noticethat the hysteresis in all bands is typically 1.2 MHz.This is achieved because the oscillator operates overnarrower ranges as the source is tuned to higherfrequencies.
The residual FM performance of this product at highfrequencies is superior to many of the single bandunits because the residual FM of the 2 - 8.4 GHzoscillator is only 3 KHz at 6 GHz. This noisemultiplied by four yields a residual FM performance of12 KHz at 26.5 GHz.
The aUXiliary output from this unit's fundamentaloscillator covers 2 to 6.7 GHz, yet it can be countedor phase-locked as if it were a 26 GHz signal.
In summary then, we have reviewed several designcriteria necessary to achieve superior performance inswept sources.
SUMMARY
Drive & Control Circuity Microwave Circuitry& Interface Circuitry
Most of the focus has been on the microwave blockdiagrams and microwave components. However, in allsystem designs it is essential that the drive andcontrol circuitry is carefully designed so that it doesnot degrade the inherent performance of the microwavecomponents.
As in all system designs, many compromises have to beconsidered in order to have a cost effective product.These compromises require good judgment and properevaluation of the important parameters. However, thesedecisions should not jeopardize the reliability of theproduct.
In order to achieve a reliable prOduct, it is essentialthat the basic building blocks have been designed andchosen with reliability in mind and that design marginsare considered in each area of design.
.
- IMPORTANCE OF DRIVE ANDCONTROL CIRCUITRY
- RELIABILITY
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MAY 1982
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PRINTED IN U.S.A.
