03 Ep603 Microwave Measurement

8/12/2019 03 Ep603 Microwave Measurement

1/20

CODE/COURSE: EP603/ MICROWAVE DEVICES PROGRAM: DEPMICROWAVE MEASUREMENT

3.1 Transmission line characteristics.

In the microwave frequency region, power is considered to be in electric and magnetic

fields that are guided from place to place by some physical structure. Any physicalstructure that will guide an electromagnetic wave place to place is called a

Transmission Line.

Transmission lines are distributed devices. R!" type models are commonly used to

appro#imate the distributed behavior of a transmission line. The single transmission line

shown below can be modeled by a networ$ consisting of a series resistance and

inductance with parallel capacitance and conductance.

RLCG Model for Single Transmission Line

The different terms included in the model describe the following physical

phenomena%

R Resistive loss of the conductor &transmission line trace'. (etermined by the

conductance of the metal, width, height, and length of the conductor.

Inductive part of the circuit resulting from the layout of the conductors.

PREPARED BY :ROHANA IBRAHIMPTSS28/07/2007

1


2/20


(etermined by the dimensions of the conductor, permeability of the metal, and

layout.

! !apacitive part of the circuit resulting from the layout of the conductors.

(etermined by the permittivity and thic$ness of the board material and the area

of the conductor.

" )hunt loss of the dielectric. (etermined by the layout of the conductors,

permittivity, loss tangent and thic$ness of the board material.

General Characteristics of Transmission Line

*ropagation delay per unit length &T0' + timedistance- psin/ 0r elocity &v0'

+distance time- inps/

!haracteristic Impedance &Z0'

*er2unit2length !apacitance &C0' pfin/

*er2unit2length Inductance &L0' nfin/

*er2unit2length &)eries' Resistance &R0' in/

*er2unit2length &*arallel' !onductance &G0' )in/

Transmission Line Equations

Propagation equation

is the attenuation &loss' factor

is the phase &velocity' factor

Characteristic Impedance equation


2

jCjGLjR +=++= ))((

)(

)(0

CjG

LjRZ

+

+=


3/20


SPECIAL CASES TO REMEMER

If the end of the transmission line is terminated in a resistor equal in value to the

characteristic impedance of the line as calculated by the formula 45&!'6.7, then the

voltage and current are compatible and no reflections occur.

hen the end of the transmission line is terminated in a short &R 5 6', the voltage at the

short must be equal to the product of the current and the resistance.


3

Vs

Zo

C :

O p

en

C irc u

it

ZsZo

Z

o

Zo

B: Short Circuit

ISOLATOR 1

0

0 =Zo

Zo0

=

ZoZo

ZoZoVsZs

Zo Zo

A: Termnated in Zo


4/20


hen the line is terminated in an open, the resistance between the open ends of the

line must be infinite. Thus the current at the open end is 8

T!E RE"LECTIO# A#$ TRA#SMISSIO# LOSSES%

hen the resistive load termination is not equal to the characteristic impedance, part of

the power is reflected bac$ and the remainder is absorbed by the load. The amount of

voltage reflected bac$ is called voltage reflection coefficient.

9 5 ri

where r5 incident voltage

i 5 reflected voltage

The reflection coefficient is also given by

9& '(L) (O*+'(L, (O*


4

Vs

Vs

Zs

A 1


5/20


-OLTAGE STA#$I#G .A-E RATIO '-S.R*%

A standing wave is formed by the addition of incident and reflected waves and has

nodal points that remain stationary with time.

Voltage Standing Wave Ratio:

)R 5 ma#min

oltage standing wave ratio e#pressed in decibels is called the Standing Wave

Ratio:

)R &d:' 5 ;6log16)R

The ma#imum impedance of the line is given by%

4ma#5 ma#Imin

The minimum impedance of the line is given by%

4min5 minIma#

or alternatively%

4min 5 4o)R

Relationship between )R and Reflection !oefficient%

)R 5 &1 < = 9 ='&1 2 = 9 ='

9 5 &)R > 1'&)R < 1'

OBJECTIVES:-

To survey the fields of microwave measurements.

To provide insight into the concepts behind the measurements.

The measurement of microwave involves measuring of :-

F!"#!$%&

W'(!)!$*+,

VSWR

P.!

A++!$#'+$

I1!'$%!

N!


5


6/20


Test equipments required for microwave measurements may be divided into :-

TYPES OF MEASUREMENT E4UIPMENTS

FRE4UENCY-DOMAIN Wavemeter s (absorption, transmission or reaction). lotted lines.

pectrum analy!er, frequency sweepers and frequency counters.

DISP5AY OF TIME-DOMAIN ampling oscilloscope.

"scilloscope.

VSWR lotted lines ( direct method or double minimum method)

POWER

#owermeters.

$etectors with oscilloscopes.

pectrum analy!ers.

WAVE5ENGTH %oa&ial and waveguide slotted lines

NOISE 'oise meters.

'etwor analy!er multifunctional test equipment.

*icrowave measurement required a set of test-des which comprise of + types of test-des i.e :-

GRED

re measurement used to obtain *erupaan penguuran untu mendapatan nilai-nilai yang tepat. alat

daipada penguuran ini urang daripada /.

GRED 2

*erupaan penguuran gelombang miro secara umum dan buan bertu0uan untu mendapatan data-data

yang tepat tetapi hanya sebagai pemerhatian terhadap perlauan gelombang miro saha0a.

FIG : B5OC DIAGRAM OF MICROWAVE TESTDES 5AYOUT :


6

MICROWAVESOURCE

ATTENUATOR

WAVEMETER

DIRECTIONAL

COUPLER

SLOTTED LINE TUNER TERMINATOR

POWER METER VSWRINDICATOR


7/20


FUNCTION

MICROWAVE SOURCE

1enerates microwave source in 2-band (3 4+ 15!)6 e.g lystron, magnetron or TWT

ISO5ATOR /CIRCU5ATOR

llow wave to travel through in one direction while being attenuated in the other direction or it is use to eliminate the unwanted generator frequency pulling (changing the frequency of the generator) due to system

mismatch or discontinuity. (to prevent reflected energy from reaching the source)

ATTENUATOR

%ontrol the amount of power level in a fi&ed amount, variable amount or in a series of fi&ed steps fromthe

from the microwave source to the wavemeter.

WAVEMETER

7sed to select 8 measure resonant cavity frequencies by having a plunger move in and out of the cavity thus causes the the cavity to resonate at different frequencies.

DIRECTIONA5 COUP5ER

amples part of the power travelling through the main waveguide and allows part of its energy to feedto a secondary output port. 9deally it is used to separate the incident and reflected wave in a transmission line.

S5OTTED 5INE

7sed to determine the field strength through the use of a detector probe that slides along the top of thewaveguide.

VSWR INDICATOR

$enotes the value of W measured by the slotted line.

TUNER

llows only the desired frequency to appear at the output. ny harmonic frequencies that appear at theoutput are reduced to an acceptable level.

TERMINATOR

*ay range from a simple resistive termination to some sort of deep-space antenna array, active repeater orsimilar devices. ; special cases of transmission line i.e short circuit, open circuit, match impedance.

FRE4UENCY MEASUREMENT


7


8/20


9t is important to select a particular resonance mode( normally that which maes the effective cavity si!e smallesti.e that for which the srew is srewed farthest inward) since there are several possible modes of resonances.

The frquency meter used has a cavity which is coupled to the waveguide by a small coupling hole which is usedto absorb only a tiny fraction of energy passing along the waveguide.

d0usting the micrometer of the


9/20


> W+

*ethods used depends on the value of W whether it is high or low. 9f the load is not e&actly matched to theline, standing wave pattern is produced.

eflections can be measured in terms of voltage, current or power. *easurement using voltage is prefferedbecause it is simplicity.

When reflection occured, the incident and the reflected waves will reinforce each other in some places, and inothers they will tend to cancel each other out.

DOUB5E MINIMUM METHOD MEASUREMENT 9 VSWR = 0

?$ouble *inimum@ method is usually employed for W values greater than about 4=.

0

+'$%! ')$* +,! )$!

The detector output (proportional to field strength squared) is plotted against position. The probe is moved aling

the line to find the minimum value of signal.

9t is then moved either side to determine + positions at which twice as much detector signal is obtained. Thedistance d between these two positions then gives the W according to the formula :


9

VSWR ; 9 I'


10/20


POWER MEASUREMENT

*ethods of measurement of power depend on the frequency of operation, levels of power and whether thepower is continuous or pulsed.

#ower measurements can be divided into ; power ranges:-

Aow power ( B 4mW C =dDm)

*edium power ( antara 4 mW - 4= W C = E= dDm)

5igh power ( F 4= W C E= dDm)

#ower is detected at microwave frequencies by the following devices. Dridge circuit is used to measure absolutepower of 4= mW whereby one of the resistance is used as part of the circiutry :-

Dolometers (therrmistors dan baretters). Thermocouples

*icrowave crystal

BO5OMETERS

re power detectors that operate on thermal principles. esistance of the bolometer changes in proportionto its temperature. ince the temperature of the resistance is dependent on the signal power powerabsorbed, the resistance must also be in proportion to the signal power.

+ inds of bolometers are available, baretters and thermistors. Doth are sensitive power detectors and is

used to indicate microwatts of power. They are used with bridge circuits to convert resistance to powerusing a meter or other indicating devices.

BARETTERS

re usually thin pieces of wire such as platinum. They are mounted as terminating devices in a section oftransmission line. The section of transmission line with the mounting structure is called a detector mount.

The increase of temperature of the baretter due to the power absorbed from the signal in the line causes thetemperature of the device to increase.

The temperature coefficient of the device causes the resistance to change in value in proportion to the change intemperature of the device (positive temperature coefficient i.e the resistance increases with increasingtemperature6 t).

THERMISTOR


10


11/20


re beads of semiconductor material that are mounted across the line. They have a negative temperaturecoefficient i.e the resistance decreases with increasing temperature6 48 t.

The impedance of baretters and thermistors must match that of the transmission so that all power is absorbed bythe device.

ariations in resistance due to thermal-sensing devices must be converted to a reading on an indicating devicesuch as a meter. This can be done accurately using a balanced bridge arramgement as shown below:-

With no power to the detector that contains the sensor element, the sensor-line 4 is ad0usted to !ero readingthrough the meter *4 and the bridge circuit is balanced.

When signal is applied to the sensor element, causing its temperature to change, the sensor resistancechanges, causing the bridge to become unbalanced.

esistor 4 is ad0usted to balance meter *4. The change in the reading of meter *+ in the sensor element legis a direct measure of the microwave power.

THERMOCOUP5ES

re used as power monitors in the low-to-medium power regions and are very sensitve.

9s a thin wire made of two disimilar metals. 5ence there will be two 0unctions (hot G cold).

When the temperature at two 0unctions are different, a voltage is developed across the thermocouple (i.e acrossboth 0unctions). This developed voltage is proportional to the difference between the two 0unction temperatures.

When the temperature at both 0unctions are the same, the difference in voltage > =.

MICROWAVE CRYSTA5S

re non-linear detectors that provide current in proportion to the power. 9t is limited to maing low-powermeasurements.

The current is proportional to the power due to the square-law characteristic of the crystal. This square-lawcharacteristic only occurs for small signal levels.


11

DC

VOLTA

GE

R

R

1DETECT

ORS


12/20


13/20


SOL1TIO#S TO MICRO.A-E PROLEMS 1SI#G SMIT! C!ART

The types of problems for which )mith charts are used include the following%

1. *lotting a comple# impedance on a )mith chart

;. ?inding )R for a given load

3. ?inding the admittance for a given impedance

@. ?inding the input impedance of a transmission line terminated in a short or

open.

7. ?inding the input impedance at any distance from a load 4.

. ocating the first ma#imum and minimum from any load

B. Catching a transmission line to a load with a single series stub.

D. Catching a transmission line with a single parallel stub

E. Catching a transmission line to a load with two parallel stubs.

PLOTTI#G A COMPLE2 IMPE$A#CE O# A SMIT! C!ART

To locate a comple# impedance, 4 5 R


14/20


(raw a circle with a radius equal to the distance between the 1.6 point and the

location of the normali8ed load and the center of the )mith chart as the center.

The intersection of the right2hand side of the circle with the hori8ontal resistance line

locates the value of the )R.

"I#$I#G T!E I#P1T IMPE$A#CE AT A#3 $ISTA#CE "ROM T!E LOA$

The load impedance is first normali8ed and is located on the )mith chart.

The )R circle is drawn for the load.

A line is drawn from the 1.6 point through the load to the outer wavelength scale.

To locate the input impedance on a )mith chart of the transmission line at any given

distance from the load, advance in cloc$wise direction from the located point, a

distance in wavelength equal to the distance to the new location on the transmission

line.

PO.ER LOSS

Return *ower oss% hen an electromagnetic wave travels down a transmission line

and encounters a mismatched load or a discontinuity in the line, part of the incident

power is reflected bac$ down the line. The return loss is defined as%

*return5 16 log16*i*r

*return5 ;6 log161 9

Cismatch *ower oss% The term mismatch loss is used to describe the loss

caused by the reflection due to a mismatched line. It is defined as

*mismatch 5 16 log16 *i&*i2 *r'


14


15/20


WITH REFERENCE TO SMITH CHART :

t the very bottom of the chart the word CENTER printed beneath a small straight vertical line.

This line will intersects the hori!ontal line (which cuts the circular chart into half) at a point mared 0this is the start of the chart i.e the center of the circle.

&is of normali!ed resistance begins at the left : =, =.4, =.+. ......4.= (at the center ...... +=, = and infinity 8 atthe right). These numbers represent normali!ed resistance values as given by :

N')>! R!+'$%!? ; R / @0 where6 > actual resistive part of comple& load

H=> characteristic impedance


15

Arcs of + jx

Arcs of - jx

Axis of

Normalized

Center

Of Chart

CENTER


16/20


There are also + sets of arcs on the chart. "ne set curves upward from its common point of tangency andrepresents normali!ed values of inductive reactance as given by :

N')>! I$#%+(! R!'%+'$%!? < ; / @0

The other set of arcs curves downward from a common point of tangency and represents normali!ed values ofcapacitive reactance as given by :

N')>! C'1'%+(! R!'%+'$%!? < ; - / @0

ll the complete circles intersect all the arcs. ny given point of intersection is a normali!ed load impedance

value as given by :

N')>! 5' I1!'$%!? > 5 ; @5 / @0 ; R ' ; ' () @0

The #+!+ circular scale represents +'$%! $ $')>! .'(!)!$*+,. The !%+$ +.'the

)' on the % is always %#$+!%)%.!. The !%+$ +.' +,! *!$!'+ on the % is always

%)%.!O$! !()#+$on the chart represents $!-,') .'(!)!$*+, 9!/2of a tansmission line. % can be used with any characteristic impedance of the transmission line or any frequency of operation.

USAGE OF SMITH CHART

USE TO SO5VE A VARIETY OF MICROWAVE PROB5EMS :-

i ) #lotting real, imaginary and comple& loads.

ii )


17/20


9f a transmission lines has infinite length (I) or it is terminated in its characteristic impedance, all the power

applied (incident wave) to the line by the generator is absorbed by the load at the othe end.

"n the other hand, if a finite piece of line is terminated, HAH=some of the power (incident wave) will be

absorbed by the termination. The remaining power will be reflected. Thus there are two waves on the line i.e,incident wave and reflected wave.

The resultant wave at any part of the line is the algebraic sum of the two waves and it produces a standing wave

on the line.

REF5ECTION COEFFICIENT

REF5ECTIONCOEFFICIENT? *

5OAD? @5 VSWR? + REMAR

> -4 short circuit, HA > =6 > =

$ue to phase reversal i.e change of phase thus

the incident and reflected wave will be cancelled.

> 4 open circuit , HA > I > ITotal refelection occurs because the + waves arein phase.

> = *atching load, HA > H= > 4 'o reflection occurs only have incident wave.

STUB MATCHING

When a line is ?matched@ the reflection coefficient > = and so the standing wave ratio, > 4. *ost system aretherefore designed to wor with as near to 4 as possible.

value of F 4, represent mismatched and end to loss of power at the receiving end. 9n other cases it maycaused a voltage breadown as in high power radar system or distortion in tv.

9t it therefore necessary to match a line. *atching in the case of two wire lines, may be done by using one ormore stub and is called ?stub matching@ or by the use of quarter wave transformer.

The use of stub in matching a comple& load to the line is to achieve a complete power transfer (W > 4.=).The stub used has to be placed in parallel with the line and load, thus has to deal with admittance, notimpedance.

EAMP5E

1iven : HA > = J 0 = , H=> = . Calculate (i) norali!e i"e#ance (ii) #ra$ t%e &' circle

(iii) &' (i*) re+lection coe++icient (*) an,le o+ re+lection (*i) inan# a- (*) tu/ len,t% (*i) tu/

#itance.


17


18/20


(i) 'ormali!e impedance, !A > HA 8 H= > = J 0 = =

> ,. ("oint )

(ii) lot !Aon %

(iii) %onstruct W circle with center % passing through !A.

s > 26

(iv) $raw a radial line passing through !Afrom center %, K> =.4L+ K D8W

(v) D 8 W > =.4L+ K J =.+ K> =.E4+ K (i.e towards generator)

(vi) t point D, admittance y > 48 !A > 0 0 ,

re+erence $a*elen,t% KD > 03

(vii) t point %, admittance y > 4 < > ,

$a*elen,t% K% > 062

Therefore, stub distance, d > =. K - KD- K% > =. K - =.E4; K J =.4L+ K

> 02

(viii) t point $, subsceptance > - < ; - $a*elen,t% K$ > =.;M K

the stub will have a short circuit at its far end, admittance at this point t%e $a*elen,t% at t%i "oint i 0.25 K

therefore, stub length, l > K$ - =.+ K > =.;M K - 0.25 K

> 02

in r in - 0 a- r a- - 0 re+lection coe++icient 0-.# 0.38 - 50 2.6 - 50 an,le o+ re+lection 12 , 10 ,

EAMP5E 2


18


19/20


comple& load of HA> ; J 0 L N terminating a = N transmission line. 7sing a % (i) plot the point of the load

normali!ed impedance and find the value of (iii) W (ii) coefficient reflection (v) angle of reflection (vi) minand

ma& (v) stub length and the distance required to obtain a matched line-load

4. 'ormali!ed load impedance, !A > HA 8 H= > ; J 0 L N=

> 07 3 (point )

2. lot !A on mith %hart.

;. %onstruct the W circle ( at point % > 4.= passing through !A)

W, s > 3

e+lection coe++icient 0-# an,le o+ re+lection 3-

in r in - 0 a- r a- - 0

0.23 - 50 4.3 - 50

11-3 , #13 ,

E. t #oint D, admittance y > 48 !A > 032 - 06 ,

e+erence 'a*elen,t% KD > 00

. t #oint %, admittance y > 4 < > 6 ,

'a*elen,t% K% > 07

therefore, stub distance, d > =. K - KD- K% > =. K - =.E=O K J =.4MO K

> 027

&tu/ len,t% l

L. t #oint $, subsceptance > - < ; - 6

'a*elen,t% K$ > =.;;O K


20/20


CONC5USION

short circuitted-stub =.=3O wavelength long located on the line =.+M away from a comple& load of H A> ; J 0 L N

will reduce the W on the line to 4.== (no reflection) and a ma&imum transfer of power from source to load will be

achieved.

EERCISES :

%onstruct the W circle for the given comple& load :

4. HA> +3 - 0 L= N , H= > = +. HA> M= - 0 N , H= > =

;. ;4.+ J 0 4= N G H= > = (b) HA> E4.+ - 0 ++. N G H= > M

E. 1iven : > E N, % > +L.p =.4+ 15!, H=> ;= N.

03 Ep603 Microwave Measurement

Documents

Transcript of 03 Ep603 Microwave Measurement