Tài liệu Microstrip bộ lọc cho các ứng dụng lò vi sóng RF (P12) pptx

For these and other requirements, a mobile base station receiver subsystem that cludes HTS microstrip filters has been developed by SUCOMS.. The HTS duplexers, preselect filters, and low

UK, and the University of Wuppertal, Germany The objective of the project was toconstruct an HTS-based transceiver for mast-mounted DCS1800 base stations

12.1 HTS SUBSYSTEMS AND RF MODULES FOR

MOBILE BASE STATIONS

The technology and system challenges of next generation mobile communicationshave stimulated considerable interest in applications of high-temperature supercon-ducting (HTS) technology [1–7] The challenges for cellular mobile base stationsvary but may focus on increasing sensitivity and selectivity:

앫 Sensitivity—The benefits of increasing sensitivity in rural areas is obvioussince the number of mobile base stations and thus the investment necessary tosecure the radio coverage of a given area will be reduced as the range of each

433

Copyright © 2001 John Wiley & Sons, Inc ISBNs: 0-471-38877-7 (Hardback); 0-471-22161-9 (Electronic)

mobile base station is increased Increasing sensitivity is also desirable in ban co-channel, interference-limited areas since it allows the mobile termi-nals to reduce the average radiated power and increase their autonomy.

ur-앫 Selectivity—The soaring demand for mobile communications place severedemands on frequency resources as the allocated bandwidth becomes increas-ingly congested Interference is a growing, pervasive threat to the mobilecommunication industry, particularly in dense urban regions Increasing se-lectivity to improve interference rejection will increase call clarity and reducethe number of dropped calls, which will lead to a general improvement in theQuality of Service (QoS)

For these and other requirements, a mobile base station receiver subsystem that cludes HTS microstrip filters has been developed by SUCOMS Figure 12.1 shows

in-a block diin-agrin-am of one typicin-al sector of the HTS subsystem For coverin-age, the bile base station is actually comprised of the three identical sectors As shown inFigure 12.1, each of them is equipped with a transmit/receive antenna and a receive-only antenna for diversity purposes The HTS duplexers, preselect filters, and low-noise amplifiers (LNAs) are tower-mounted on the top of the antenna mast of the

HTS Duplexer

HTS Bandpass Filters

HTS Bandpass Filters

Main Antenna

Diversity Antenna

Tower-mounted unit

Indoor Racks

FIGURE 12.1 Typical mobile communication base station sector using HTS subsystem

base station, whereas the transmit combiners and receive splitters are located in theshelter at the bottom of the tower of the base station This subsystem is much thesame as a conventional one, except that the duplexers and preselect bandpass filtersare made using HTS thin film microstrip components The use of the HTS compo-nents enables increases in both the sensitivity and selectivity due to extremely lowlosses in the materials (see Chapter 7 for details) The subsystem developed is for aDigital Communication System or DCS-1800 base station, but can be interfacedwith a Global System for Mobile Communication System or GSM-1800 base sta-tion It can also be modified for other mobile communication systems such as thePersonal Communication System (PCS) and the future Universal Mobile Telecom-munication System (UMTS)

The RF components shown in the tower-mounted unit of Figure 12.1 are

operat-ed at a low temperature in a vacuum cooler This is necessary for the HTS nents, but the LNAs are also cooled, which gives the system an extra reduction inoverall noise figure It has been reported [7] that the noise figure of a LNA is re-duced from a room temperature figure of <0.8 dB to <0.2 dB at 77 K Design of the

compo-RF components, the vacuum encapsulation, the compo-RF modules, and cryo-cooler are separably linked The RF components are housed in common modules This ap-proach allows the transmitter and receiver functions to be developed in parallel Itfurther allows the complete system to be progressively developed and field trailed.Figure 12.2 illustrates a dual-sided, six-channel HTS microstrip filter/LNA module,where only three channels can be seen; the other three-channels are on the otherside of the RF module The RF module for three channel HTS microstrip duplexers

in-is shown in Figure 12.3 The diameter of the RF modules in-is only about 150 mm Thedesigns and performances of the HTS microstrip filters and duplexers are fully de-scribed in the next two sections For the vacuum encapsulation, the cryogenic/RFinterconnection across the encapsulation vacuum space may be accomplished usinghigh thermal resistance RF coaxial cables In this design, a nonsuperconductor mi-crostrip feed network inside a connector ring and a novel RF/thermal link were used

to achieve a low conduction heat load The RF signals fed through K-connectorsmounted on the wall of the connector ring are propagated from the ambient temper-ature side to the cold side through short, thin bondwires

In designing a tower-mounted transceiver based on HTS technology, it is able to make the cooler that is required to cool the HTS components and LNAs “in-visible” to the base station operator The cooler must be compact, low mass, self-contained, have minimal service utilities, and be highly reliable A commerciallyavailable Stirling cycle cooler has been used for this purpose Figure 12.4 shows ademonstration comprising the encapsulated RF modules assembled on a Stirlingcooler with dual cold finger and dual compressor The dual balanced compressorand cold head can reduce the vibration levels to a minimum The cooler provides aheat lift of >4 W at 60 K, which easily meets the cooling requirements for the HTSmicrostrip duplexers, preselect filters, and LNAs in the RF modules There are oth-

desir-er types of cooldesir-ers, such as Gifford McMahon cooldesir-ers and pulse tube cooldesir-ers whichmay also be used Figure 12.5 demonstrates a prototype of the entire tower-mountedunit with a weatherproof enclosure opened on the side The prototype consists of the

encapsulated RF modules on the cooler, temperature control loop, and power plies Lightning protection, alarms, and bypass relays are also included This mobilebase station tower-mounted unit is 650 mm ×550 mm ×300 mm with an associatedmass of 37 Kg

sup-12.2 HTS MICROSTRIP DUPLEXERS

As above discussed, the HTS microstrip duplexers and bandpass filters are essential

RF components in the RF modules for the tower-mounted transceiver developed formobile base station applications The design and performance of the HTS mi-crostrip duplexers are described first in this section The HTS-based transceiver isfor the DCS–1800 standard, which covers a receive (Rx) band from 1710 to 1785MHz, and a transmit (Tx) band from 1805 to 1880 MHz The function of each du-plexer is twofold: 1) to route the received signal from the antenna to the receiver

HTS Microstrip

Duplexer

MicrostripCircuit

FIGURE 12.3 RF module with three channels of HTS microstrip duplexers

preselect filter that covers a 15 MHz subband of the Rx band, and 2) to route thehigh-power Tx signal from the Tx channel to the antenna, which must be accom-plished without any significant amount of power from the Tx signal being incidentupon the Rx filter (good isolation between Tx and Rx) The result is that one anten-

na can be used for both Tx and Rx functions

12.2.1 Duplexer Principle

The duplexer developed consists of two –3 dB hybrids and two bandstop filters nected as in Figure 12.6 Assume that port 1 of hybrid A is the Tx port; ports 2 and 3

con-of hybrid B are then the Rx and antenna ports, respectively The operation principle

of the duplexer is described below

For our purposes, let us assume that the hybrids are ideal, represented by a tering matrix:

Consider hybrid B on the right-hand side of Figure 12.6 Its ports 1 and 4 can be minated by the two loads with reflection coefficients of ⌫1and ⌫4, which, as a mat-

ter-0–1/兹2苶

–j1/兹2苶

0

–1/兹2苶

00

–j1/兹2苶

–j1/兹2苶

00–1/兹2苶

ter of fact, are the reflection coefficients of the two bandstop filters, respectively Itcan be shown that the resultant two-port network is written as

of its low conductor loss, particularly if the circuits are to be miniaturized

Next, let us assume that the bandstop filters have no attenuation at the transmitband, so that they may be treated as two ideal phase shifters with a phase constant of

␪ If a transmitting signal represented by a1Ainputs at the transmitter port, it can beshown that

STx–Rx= ᎏb

a

2 1

B A

B A

12.2.2 Duplexer Design

As a starting point for the design, a LAO substrate (refer to Chapter 7) was chosen

in order to reduce the size of the duplexer module for encapsulation with the other

components in the tower-mounted unit of Figure 12.1 In considering the requiredoverall power handling (12 W) of the transmit channel, it was also decided to designand fabricate HTS hybrids Because there are no resonant elements at the Tx band,the power handling capability of the transmission line is more than adequate Thedesigns of the HTS hybrid and bandstop filter are described next

A Microstrip Hybrids

The simplest –3 dB hybrid would be the branch line coupler [15–17] However, itprovides very little design margin due to its limited bandwidth Therefore, it waschosen to use tandem couplers [18] A tandem arrangement of two coupled-line di-rectional couplers shown in Figure 12.7 results in a wider bandwidth, which can im-prove the overall performance of the duplexer Besides, the tandem coupler has asmaller size The tandem connection entails two line crossings, which may be real-ized using bondwire bridges The operation principle of the tandem coupler is de-

scribed as referring to Figure 12.7, where a and b represent the incident and

reflect-ed wave variables, respectively The coupling and transmission are definreflect-ed by

ᎏ = =

where c and t denotes the scaled coupling and transmission coefficients, and ␾is areference phase Note that the coupling is 90° out of phase of the transmission Forsimplicity, a reference phase ␾= 0 will be assumed, which does not affect the out-

come Let the input at port 1 be a1, then the outputs at ports 3⬘ and 4⬘ can be found

Thus, the transmissions from port 1 to ports 3⬘ and 4⬘ are given by

S3 ⬘1= ᎏb

a

3 1

⬘

ᎏ = j2ct

Notice that there is always a 90° phase difference between the two outputs less of their magnitudes The equal magnitude of the two outputs can be achievedusing the condition

Solving the equations yields

c = 0.3827 = –8.34 dB (12.9)Hence, what is needed for a –3 dB tandem coupler is to design a pair of –8.34 dBdirectional couplers For quarter-wavelength coupled microstrip line realization, thedesign equations are

Z 0e = Z0冪ᎏ1

1

莦+–

莦+

–

莦c c

ᎏ莦

where Z 0e and Z 0o are the even- and odd-mode impedance Z0is the matching pedance at the ports, and is equal to 50 ohms in our design The design of the HTShybrid on a 0.5 mm thick LAO substrate was done with the aid of a full-wave elec-

im-tromagnetic (EM) simulator [19] Shown in Figure 12.8(a) is the layout of the

hy-brid for the full-wave EM simulation, where the crosses indicate the line crossingsand all the coupled microstrip lines have a width of 0.128 mm The EM simulated

frequency response is shown in Figure 12.8(b)

B Microstrip Bandstop Filters

The specifications for the required bandstop filters are

兹2苶

0.128 mm 4.736 mm

PORT 3PORT 4

These specifications can be met with a three-pole Chebyshev bandstop filter Itslowpass prototype has the element values

g0= g4= 1.0

g2= 1.1474For microstrip realization, we used three open-loop resonators that are coupled to a

50 ohm microstrip line as Figure 12.9(a) shows The separation between the onators as denoted by L is approximately a quarter-wavelength This is a narrow-

res-band res-bandstop filter, and the design of this type of filter has been detailed in

Chap-ter 6 Figure 12.9(b) plots the measured and full-wave simulated performances of the bandstop filter for L = 11 mm, showing a good agreement except for some fre-

quency shift The filter was measured without any tuning at a temperature of 80 K

in an open bath liquid nitrogen cooler The return loss of the filter shows an metric frequency response Although a more symmetric response could be achieved

asym-by slightly increasing the interresonator separation L [20], the asymmetric one

would be more desirable in regard of a lower return loss at the transmit band

12.2.3 Duplexer Fabrication and Test

Shown in Figure 12.10 is a photograph of the fabricated HTS duplexer in a testhousing The HTS microstrip hybrids and bandstop filters were fabricated usingdouble-sided YBCO thin films on 0.5 mm thick LAO substrates As can be seen, thetwo hybrids and the two bandstop filters are on separated substrates Each of the hy-brids had a circuit size of 15.5 × 22.5 mm and the size of each bandstop filter was

38 × 13 mm They were mounted on gold-plated titanium carriers using conductivesilver epoxy, and then the carriers were fixed into the test housing by screws Bond-wires were used for interconnections

The test of the HTS duplexer in the test housing was carried out in an open bathliquid nitrogen cooler Figure 12.11 shows the measured performance of the HTSduplexer at 80 K As can be seen, an excellent performance was achieved The in-sertion loss from the antenna port to the Rx port was less than 0.3 dB over the de-sired Rx subband (15 MHz) The insertion loss from the Tx port to antenna port wasalso less than 0.3 dB across whole the Tx band (75 MHz) The isolation between the

Tx and Rx ports was larger than 35dB over the Tx band A very good match wasalso obtained at all ports A summary of the measurement performance is given inTable 12.1 Three similarly fabricated HTS duplexers have been assembled in the

RF module, as shown in Figure 12.3

Microstrip bandstop filter

FIGURE 12.10 Fabricated HTS microstrip duplexer in a test housing

FIGURE 12.11 Measured performance of the HTS duplexer in the test housing at T = 80K

(a)

(b)

TABLE 12.1 Summary of measured performance of the duplexer in the test housing (T = 80K)

Frequency 1770–1785 MHz (Rx band) 1805–1880 MHz (Tx band) Antenna receiver loss < 0.3 dB

Transmitter receiver isolation > 43 dB > 35 dB

Antenna port return loss > 27 dB > 26 dB

Receiver port return loss > 25 dB

12.3 PRESELECT HTS MICROSTRIP BANDPASS FILTERS

12.3.1 Design Considerations

As has been described early in this book, there are different types of filter istics (e.g., Chebyshev, Butterworth, etc.), and a bandpass filter may be designed tohave any one of them Therefore, it is important to assess the performance of differ-ent types of bandpass filters against the preliminary specification for the preselectbandpass filters to be integrated in the subsystem of Figure 12.1 This helps to iden-tify the optimum type and the degree of filter to meet the specification For this pur-pose, we have studied three types of filters, namely, Chebyshev; quasielliptic, andelliptic function filters against these simplified specifications:

앫 Transmit band rejection (1805–1880 MHz) ⱖ66 dB

Figure 12.12 shows typical transmission characteristics of the three types of filters

As can be seen, the distinguishing differences among them are the locations oftransmission zeros Whereas the Chebyshev filter has all transmission zeros at dcand infinite frequencies, the elliptic function filter has transmission zeros at finitefrequencies and exhibits an equal ripple at the stopband The quasielliptic function

FIGURE 12.12 Typical transmission characteristics of three types of eight-pole bandpass filters for a passband from 1770 MHz to 1785 MHz