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RF Front End for a 20/30 GHz Briefcase Terminal
By
Josef L. Fikart
MPR Teltech Ltd.
Vancouver, B.C. Canada
Abstract
This paper describes the main features and components of the RF Front End for a briefcase terminal designed for voice and data communications via the 20/30 GHz transponders of the Kopernikus and Italsat satellites. The terminal modem has been designed for data rates of 4.8 kb/s for the uplink, spread into 128 kb/s, and for up to 128 kb/s on the downlink. The terminal as a whole is described in an accompanying paper by Joanneum Research of Graz, Austria.
The configuration, main characteristics and technology of the Front End are described and its performance data reported in this paper.
In summary, the RF Front End is physically composed of an 35 cm offset fed parabolic antenna with a detachable boom equipped with the RF head containing the horn, OMT/diplexer, SSPA and LNA. The detachable boom and the RF head can be stored in the IATA regulation type briefcase and the antenna can be folded down into it. The briefcase contains additional components of the Front End such as the up-and downconvertes to L band, and a power supply. In addition, the briefcase contains a modem with an L-band interface to the RF Front End, a Notebook PC and other supporting equipment. The whole package is powered from a small 12 V battery pack.
Electrically, the main parameters characterising this RF Front End are the measured EIRP of 36 dBW and G/T of 10 dB/deg. K. The relatively high EIRP figure is made possible by the high efficiency of the antenna and a state-of-the-art SSPA that combines high output power with a small size. The above parameters have been verified in tests via the Kopernikus satellite.
RF Front End for a 20/30 GHz Briefcase Terminal
Josef L. Fikart
MPR Teltech Ltd.,
Burnaby B.C., Canada
Phone:(604) 293 5706 Fax: 293 6158
Email: fikart@mprgate.mpr.ca
1. INTRODUCTION
This paper describes the main features and components of the RF Front End for a small briefcase terminal designed for voice and data communications in the 20/30 GHz satellite band. The terminal as a whole, called "Picoterminal", is described in an accompanying paper by Joanneum Research of Graz, Austria. The work was carried out under contract from the European Space Agency, to its specifications.
The Picoterminal was developed to be compatible with the existing CODE network architecture. Althougfh originally part of the Olympus Utilisation program (Ref. 1), the Picoterminal final design was re-directed to be able to operate over the 20/30 GHz transponders of the DFS Kopernikus and Italsat satellites. For the RF Front End, the latter requirement results in the necessity to take into account the different Tx -Rx polarisation mix on the two satellites: vertical (Tx)-horizontal (Rx) for DFS Kopernikus, and vertical - vertical on the Italsat. Thus the Kopernikus application required an OrthoMode Transducer (OMT) whereas Italsat needed a diplexer.
The Picoterminal system link budget determined the two main electrical parameters of the terminal, namely the EIRP and G/T. The physical size was dictated by the requirement of adhering to IATA regulation briefcase. This resulted in significant constraints on the size of the antenna as well as that of the RF units. The antenna size determined the required noise figure of the LNA and the output power from the SSPA. While the former did not present a particularly difficult problem, the latter, combined with the size limitations on the SSPA, was a challenge.
2. OVERALL DESCRIPTION
Physically, the RF Front End itself is composed of an 34 x 35 cm offset fed parabolic antenna with a detachable boom equipped with the RF head containing the horn and the high frequency parts of the transceiver, namely the OMT/diplexer, SSPA and LNA. The up-and downconverter and the transceiver power supply are located inside the Picoterminal briefcase and connected to the RF head by good quality EHF cables. This arrangement is reflected in the block diagram in Fig. 1 where the RF head is referred to as "EHF Unit". A photograph of the Picoterminal with the RF Front End set up for action is shown in Fig. 2.
Fig. 1 Picoterminal Transceiver Configuration
Fig. 2 Picoterminal with deployed RF Front End
The "EHF Unit" has been designed to be outwardly physically identical for both the co-polar (Italsat) and cross-polar (Kopernikus) use. In the complete Picoterminal, this unit and the detachable boom can be stored in the briefcase (35 x 50 x 25 cm) and the antenna can be folded down into it. The briefcase contains additional components such as the modem with an L-band interface to the RF Front End, a Notebook PC and other supporting equipment as shown in Fig. 2. The whole package is powered from a small 12 V battery pack.
Electrically, the main parameters characterising the RF Front End are the specified EIRP of 35.6 dBW at 1 dB compression point and a G/T of 10 dB/ø K. Given the size of the antenna and good efficiency, these requirements translate into Tx antenna gain of about 38.6 dB (including the horn and OMT) and corresponding SSPA power of 27 dBm (0.5 W). On the Rx side, antenna gain of 35.2 dB was expected, resulting in a noise figure of 2.9 dB maximum, after allowing for some degradation to the overall noise temperature caused by sky and atmospheric noise.
3. BLOCK DIAGRAM
Electrically, the functional block diagram of the RF Front End is shown in Fig. 3 The interface with the modem of the terminal is at an IF of 888 MHz +/- 10 MHz.. However, the IF portions of both the upconverter and downconverter circuitry have been designed to accept up to 1450 MHz ( to also cover the standard 950 - 1450 MHz band for future considerations). The Front End also receives its Tx and Rx LO reference signals from the modem in a narrow band at approx. 1 GHz. With appropriate multiplication, the LO chain delivers the frequency range necessary to tune the RF Front End over the 29.5 - 30 GHz (Tx) and 19.7 - 20.2 GHz (Rx) bandwidth. However, the Tx LO filtering requirements result in two 500 MHz options for the transmitter.
The combiner is realised in two options; for the crosspolar applications, the OMT itself provides sufficient suppression of the Tx signal penetration into the receiver. Hence no filter is used. In the co-polar case, there is no isolation other than that due to the possible resonant character of the probes or slots used in the splitter/combiner itself. Therefore, an additional filter for Tx signal suppression must be used, thus increasing the Rx noise figure by the filter loss (0.3 dB or so). In both cases, no Tx filter is used to minimise power loss; it is assumed that the SSPA harmonics will be below the specified level (-20 dBc) at 1 dB compression which is supposed to be the highest operating point.
Fig. 3 RF Front End Block Diagram
One thing to note in Fig. 3 is the two different transceiver "boundaries" with the antenna. Physically, the combiner (i.e. OMT or diplexer with filter) and the horn are part of the EHF Unit of the transceiver; electrically however, the transceiver and antenna subsystems performance is best measured and appropriately allocated by including the OMT/diplexer in the antenna subsystem. In particular, any degradation in the antenna cross-polarisation performance that the OMT itself may cause would not otherwise be detected. The results reported in Section 5 reflect the electrical boundary defined in Fig. 3.
4. IMPLEMENTATION AND TECHNOLOGY
4.1 Antenna
The antenna subsystem has been designed and manufactured for MPR by IMT Inc. of Winnipeg, Manitoba. The reflector and horn have been designed to meet the above-mentioned gain limits (Tx gain > 38.6 dB and Rx gain >35.2 dB) as well as to satisfy ETSI requirements on sidelobe levels (S = 29 - 25 log j ) and cross-polarisation performance (< - 27 dBc). Mechanically, the antenna had to fit the briefcase dimensions, weigh less than 7.5 kilograms and have the capability of elevation adjustment of 0 to 90 degrees with 0.5 degree resolution and polarisation adjustment of +/- 90 degrees, with 1 degree resolution. The azimuth adjustment would be taken care of by Joanneum Research in conjunction with the mounting of the antenna assembly into the briefcase.
The antenna incorporates an offset reflector configuration to avoid feed aperture blockage. To fit the briefcase and obtain the gain required, a 34 cm x 35 cm dish has been chosen. It has a focal length of 35.6 cm positioned at an offset angle of 30 degrees and half apex angle of 25 degrees. The shape of the reflector is the intersection of a paraboloid and an elliptic cylinder. The feed assembly is a dual-band corrugated conical horn that provides symmetric radiation patterns with low cross-polarization characteristics.
The elevation and polarisation adjustments are effected as follows:
a) Elevation
The dish is mounted on a support arm which in turn is attached to an elevation adjustment mechanism (1/100 ratio gear box) allowing a very fine adjustment of elevation with 0.5 degree resolution. The elevation indicator indicates -15 degrees when the antenna is in a horizontal position to compensate for the reflector tilt angle. The assembly is shown in Fig. 4. The adjustment is done by using the wormdrive concept in the gear box . The elevation adjustment knob or crank, hidden in Fig. 4 but visible in Fig. 2 (below the base of the boom), is attached from the front and through the support arm to the worm which drives the gear, which in turn rotates the entire box and the dish support arm with it.
Fig. 4: Antenna Assembly
Fig. 5: RF Front End Transceiver Units
Fig. 6: OMT/Diplexer
b) Polarisation
The usual method of polarisation adjustment, i.e. turning the feed assembly around the horn axis, has been replaced by turning the entire dish together with the boom containing the feed assembly (EHF Unit with horn) about an axis perpendicular to the antenna plane. In this way, the mutual position of the feedhorn on the boom, and the dish remains constant. This was necessary in order to meet the stringent cross-polarisation specs at all polarisation settings. A worm gear assembly with a 1/100 ratio is used. This is clearly visible in Fig. 4, also showing the adjustment knob on the worm. The angle scale is marked from -90ø to + 90ø in 0.5 degree increments.
The mechanical design had to address three main problems, namely demanding electrical performance specifications, light weight and limitations due to briefcase size. This has produced several challenges to the design of the antenna steering mechanism.
The antenna model was designed in a SDRC solid modelling system and then moved to a unigraphics CAD/CAM system to generate the CNC machine codes.
The relatively small dish diameter and the required performance necessitated the use of very accurate manufacturing processes. The reflector is machined from one slab of metal (aluminum for light weight) on a CNC machine, to meet the required surface smoothness and dimensional accuracy.
The OMT/Diplexer, although electrically considered part of the antenna, is physically integrated with the transceiver and is described below.
4.2 Transceiver
The transceiver, designed and made at MPR, is divided into the "EHF Unit", mounted on the boom, and the UPC, DNC and Power Supply units mounted inside the briefcase. The EHF Unit contains the SSPA and LNA mounted on a box containing the OMT (or diplexer) to which the horn is attached. The complete set of these units is shown in Fig. 5. The UPC and DNC are joined together to form one mechanical assembly. The EHF Unit has the SSPA mounted on top of the OMT/Diplexer box and the LNA is on its side. (For size comparisons, the horn attached to the OMT/Diplexer box is approx. 60 mm in diameter).
Fig. 6 shows the interior of the OMT/Diplexer box. This is the view obtained when the left side cover of the box as shown in Fig. 5 is removed. The cylinder mounted on the front wall to which the horn would be attached is an OMT which can also function as a co-polar combiner/splitter depending on its orientation with respect to the attached rectangular waveguides. The unit shown is the cross-polar type. The OMT is fairly conventional in that it uses a circular waveguide with two different diameters to allow for the proper propagation of the 20 GHz and 30 GHz signals which are applied to the waveguide in an orthogonal fashion. One slight difference here is that instead of the usual connection of the Tx rectangular waveguide on the side as with the Rx waveguide, a "taper" transition is used and attached to the back of cylinder where the smaller circular waveguide exits. At that point the Tx waveguide (WR 28) is connected which at the other end attaches to the SSPA (top right and through the sidewall of the box). The Rx waveguide (WR42) flange attaches to one side of the OMT, coupled through a slot to the circular waveguide inside. The other end attaches to the LNA (top left and through the bottom wall of the box). The middle of the Rx waveguide run contains a removable waveguide section to be replaced by a filter for the co-polar option.
As for the electronics of the SSPA, LNA, UPC and DNC, the following are some of the characteristic aspects of the design approach taken and of the technology used:
a) In both the UPC and DNC, Step-recovery diode (SRD) multipliers are used in the LO chain. Despite the current pre-occupation with using MESFETs, HEMTs etc. for everything including multipliers, the SRD approach is still the simplest and least expensive. If designed correctly and not pushed for extreme efficiency, the SRD multiplier will be stable and reliable.
b) In the UPC, a subharmonic mixer is used. This limits the output frequency of the multiplier chain to approx. 14.5 GHz.
c) The SSPA which has a gain of approx. 34 dB uses mostly internally matched MMICs, both for the low-level drivers and the final stage. The required output power of 27 dBm (0.5 W) is obtained from one MMIC chip which combines two devices on the chip. Therefore no off-chip combining was necessary in this application. The chip can deliver approx. 28 dBm (at 1 dB. C.P.) in on-wafer tests but when imbedded and followed by an iso-adapter for protection and conversion into a waveguide medium, a loss of almost 1 dB is incurred.
An electronically controlled variable attenuator was used in conjunction with an output power detector and a temperature sensing circuit to compensate for amplitude variations of the Tx chain caused by temperature changes.
d) The LNA has a gain of approx. 36 dB and uses 6 stages of amplification. Although it also has a temperature control circuit, this turned out to be unnecessary in terms of meeting the given specifications. Otherwise, there are no features of particular interest in the LNA design.
e) The technology used in the realisation of the above circuits and units is the classic thin-film on alumina substrates; however, for the parts up to and including 20 GHz, MPR?s MHMIC version (Miniature Hybrid Microwave Integrated Circuits) of this technology has been used. This technique is a refinement of the standard thin-film, making possible very small features (down to approx. 14 micron wide lines and gaps) and other improvements which in turn facilitate the use of spiral inductors, interdigitated and MIM type of capacitors, blind via holes etc., with good electrical properties up to 20 GHz or even higher. The resulting circuits/substrates are much smaller than "regular" MIC substrates and they resemble MMICs; the only parts to add are the semiconductors, and the size is only about 2x or 3x bigger than that of an electrically equivalent MMIC. An example of an MHMIC substrate is shown in Fig. 7.
Fig. 7: MHMIC Chip
The substrates are mounted on U-shaped metal carriers for good thermal properties and for prevention of waveguide modes. The carriers are then cascaded and attached to the bottom plate of the unit box itself. An example of this type of construction is shown in Fig. 8.
It appears that the use of softboards with SMTs would have also been possible up to 20 GHz. While cheaper, it would have resulted in larger size. For the SSPA however, i.e. at 30 GHz, the classic thin-film approach is still very difficult to replace with the softboard alternative mainly because of packaging problems with the semiconductors. While 30 GHz packages are beginning to appear on the market, a lot of work has to be yet done to make this approach work satisfactorily.
5. PERFORMANCE
The RF Front End parameters were first measured on the bench and then the EIRP and G/T were confirmed in tests via the Kopernikus satellite In Graz, Austria, using a hub station located there. Hub-to-Pico, Pico-to-hub, and Pico-to-Pico tests were succesfully conducted in the fall of 1995. Since then, the RF Front End has been integrated into the Picoterminal briefcase and overall tests via both Kopernikus and Italsat are planned for this fall.
A summary of all relevant Front End parameters and the measured results is given in Table I and, specifically, the antenna Tx pattern is shown in Fig. 9 to illustrate its excellent performance at 30 GHz.
Fig. 8: Inside View of the DNC Unit
Table I: RF Front End Performance
Item
Parameter
Measured
Comment
1.
Tx Frequency Range:
29.5-30.0 GHz
2.
Rx Frequency Range
19.7-20.2 GHz
1.
Tx Polarisation
Vertical
2.
Rx Polarisation
Hor./Vert.
EHFU Option
3.
Antenna Tx /Rx gain @ 29.5 GHz
39.9/35.2 dB
Including OMT
5.
Sidelobes
< ETSI Spec
See Tx pattern, Fig. 9
9.
Cross-polarization at 1 dB contour
<-27 dBc
Including OMT
10.
Polarization adjustment/resolution
Full turn/1ø
11.
Elevation adjustment/resolution
> 0-90ø /0.5ø
3.
EIRP @ 1 dB C.P., 25 ø C,
nominal IF input (0 dBm )
35.6 dBW min.
Confirmed in tests
over satellite
5.
EIRP Flatness @ 1 dB C.P. over any 90 MHz band
+/- 0.4 dB
6.
SSPA power at 1 dB C.P., 25ø C
26 dBm min.
Power Stability with Temperature
+0/-2 dB
low @ low temp.1
Transmitter IF
878 - 898 MHz
Linear Tx gain (IF in to SSPA out)
28 dB
10.
Minimum Rx G/T
10.2 dB/ø K
Sky temp. 50ø K
Receiver Noise Figure (LNA input)
2.8 dB max.
At 25ø C
Receiver IF
878 - 898 MHz
Rx gain (LNA input to IF out)
86 dB min.
7.
Rx gain flatness over any 90 MHz
1.1 dB p-p
8.
Rx gain variation with temperature
3.6 dB p-p.
16.
Size of main reflector
34 x 35 cm
Fits briefcase
17.
Other Front End dimensions
Fit IATA briefcase (50 x 35 x 25 cm)
18.
Antenna weight (incl. adjustment mechanism, boom & horn)
4.7 kg
18.
Total Front End weight
8.2 kg
Antenna only: 4.7 kg
17.
Front End Power consumption
36.0 W
Fig. 9: Antenna Transmit Pattern and Sidelobe Limits
6. SUMMARY
A 20/30 GHz RF Front End (antenna and transceiver) for a portable briefcase terminal has been designed and prototyped, and its performance verified in bench testing as well as in tests via the Kopernikus satellite. With its 35 cm antenna and small transceiver, the RF Front End fits the IATA regulation briefcase which also houses all the other parts of the complete terminal for low bit rate data and voice communications. The antenna subsystem design allows storage in the briefcase by detaching the boom with the RF head and folding the dish into the brieface.
The technology used is "classical" thin film and MHMICs mounted on metal carriers, with HEMTs in the LNA and MMICs utilising on-chip combining of FETs in the SSPA power stage. For the LO chains, Step-Recovery Diode multipliers are employed in conjunction with a subharmonic mixer in the transmitter and a "regular" mixer in the receiver.
The RF Front End is characterised by an EIRP of 36 dBW and G/T of 10 dB/ø K and by a 900 MHz IF interface with the modem which also provides the Tx and Rx LO reference signals at approx. 1 GHz. The DC power consumption is 36 W.
7. ACKNOWLEDGEMENTS
The author wishes to thank the European Space Agency (ESA) and Canadian Space Agency (CSA) for the financial and technical support of this project, and particularly Mr. C. Hughes, Ms. P. Glover, Mr. J. Horle and Mr. F. Feliciani of ESA, and Mr. A. Bastikar and Mr. P. Price of CSA, for their contributions to the design process in their roles as project managers and/or scientific authorities. Furthermore, the cooperation of our colleagues at Joanneum Research in Graz, and of Dr. W. Bauer of Deutsche Telecom, responsible for the provision of satellite capacity on DFS Kopernikus, is gratefully acknowledged.
Thanks are also due to Prof. L. Shafai of the University of Manitoba and Dr. M. Barakat of IMT Inc. also of Winnipeg, Manitoba, who were responsible for the design and manufacture of the antenna subsystem for MPR, for their contributions and their cooperation with the author during the project.
Finally the author wishes to thank the management of MPR?s Wireless Division for their encouragement and to MPR?s members of the technical staff involved in this project for their extraordinary efforts in this very challenging assignment.
REFERENCES
1. C. Netzberger et al, "The Picoterminal Design Concept of Code", The Results of the Olympus Utilisation Programme Conference (Abstracts p. 48), Seville, Spain, 1993. |
