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Versatile RF Front End for Small Ka-band Ground Terminals
By
Josef L. Fikart
IMT Communications Systems Inc.
Burnaby, B.C. Canada
Abstract
This paper describes the main features and components of the RF front end for a user terminal designed for data communications via Kopernikus and Italsat and/or future Ka-band satellites with similar parameters. The outdoor part of the front end uses block conversion between 29.5-30 GHz/19.7-20.2 GHz and the standard L-band (950-1450 MHz), and can therefore interface with various indoor options such as a high-speed L-band modem, an IRD for future DVB, or standard QPSK modems at 70/140 MHz via commercially available L-band Converter Units (LCU). Its EIRP, G/T, spectral purity and instantaneous bandwidth are high enough to allow two-way communications at individual channel data rates from 9.6 kb/s to 2 Mb/s using the above satellites, with sufficient margin, or reception of signals with high data rates (such as in DVB) for further processing with an L-band IRD.
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 outdoor and indoor installation interconnected with an Interfacility Link (IFL). The outdoor installation consists of a 1.5 m offset-fed parabolic antenna and the Transceiver Outdoor Assembly (TOA). The feed unit on the antenna boom contains the horn and OMT/Diplexer integrated with the EHF part of the TOA, namely the SSPA, Upconverter (UPC) and Low-Noise Block converter (LNB). Additional auxiliary circuitry (L-band variable-gain amplifiers, LOs for the UPC and LNB, a DC/DC converter, and MAC -Monitor, Alarm and Control) is located on the back of the dish in a small enclosure to which the IFL is connected.
In a VSAT configuration, an indoor LCU is used to convert the standard L-band to/from 70 or 140 MHz. The LCU provides full frequency agility over the 500 MHz band with frequency steps as small as 50 Hz and also allows a 25 dB Tx gain and 30 dB Rx gain adjustment, both controlled from a PC. The PC also monitors and controls the TOA for Tx power, Tx alarm, Rx gain and compensation of IFL loss. The modem is typically supplied by the customer.
Electrically, the main parameters characterising this RF front end are the 950 MHz - 1450 MHz indoor/outdoor interface, Tx output power of 1.5 W and overall Rx noise temperature of 200ø K. With the 1.5 m antenna, this results in an EIRP of 53 dBW and a G/T of 24 dB/ø K. The transceiver features remote switching between co-linear and orthogonal Tx/Rx polarisations, or between equal and opposite circular Tx/Rx polarisations after inserting a dual-band circular polariser between the horn and the OMT/diplexer in the feed.
Versatile RF Front End for Small Ka-band Ground Terminals
By
Josef L. Fikart
IMT Communications Systems Inc.,
5284 Still Creek Avenue, Burnaby, B.C. Canada
(604) 320 2655 (tel.), 294 5506 (fax), fikart@imtcomsys.bc.ca
1. INTRODUCTION
This paper describes the main features and components of a Ka-band RF front end designed for the European Space Agency to be initially used for VSAT- type data communications via Kopernikus and Italsat and/or future Ka-band satellites with similar parameters. However, other applications have been considered during the development as well. The outdoor part of the front end uses block conversion between 29.5-30 GHz/19.7-20.2 GHz and the standard "low" L-band (950-1450 MHz), and, as shown in Fig. 1, can therefore interface with various indoor options such as a high-speed L-band modem, an IRD for future DVB, or standard QPSK modems at 70 or 140 MHz via commercially available L-band Converter Units (LCU). Its EIRP, G/T, spectral purity and instantaneous bandwidth are high enough to allow two-way communications with sufficient margin, using the above satellites, for single Tx channel data rates from 9.6 kb/s to 2 Mb/s with the Rx bandwidth constrained only by the LCU or L-band modem. Alternatively, Rx-only mode can be exercised e.g. for reception of signals with high data rates (such as in DVB) for further processing with an L-band IRD.
The configuration, main characteristics, technology and performance data of the front end are described below.
Fig. 1: Overall Front End Configuration
2. OVERALL DESCRIPTION
As indicated in Fig. 1, the RF Front End is composed of an outdoor and indoor installation interconnected with an Interfacility Link (IFL). The physical makeup is fairly standard and is shown in Fig. 2. The outdoor installation consists of an offset-fed parabolic antenna and a Ka/L-band transceiver split into two units, one of which is integrated with the feedhorn and the other placed on the back of the dish. The indoor installation will vary depending on the choices shown in Fig. 1. Fig. 2 shows the VSAT configuration with the LCU, 70 or 140 MHz modem and a power supply/monitor, alarm & control unit all mounted in a 19" cabinet.
Fig. 2a: Outdoor Installation (VSAT)
Fig. 2b: Indoor Installation (VSAT)
The feed unit on the antenna boom (EHFU in Fig. 2) contains the horn and OMT/Diplexer integrated with only the EHF part of the transceiver, namely the SSPA, upconverter and Low-Noise Block converter (LNB). Additional auxiliary circuitry such as L-band variable-gain amplifiers for controlling Tx and Rx gain, demultiplexers for separating DC, L-band and reference signals from the IFL, local oscillators for the upconverter and LNB, a DC/DC converter, and Monitor, Alarm and Control (MAC) circuitry are placed into the Power Supply and Interface Unit (PSIU in Fig. 2) which is physically located on the back of the dish. This mechanical division has been done to have a reasonable size feed unit while still allowing for a direct, no-loss connection between the SSPA and the OMT (eliminating the flexible waveguide typical in installations with the SSPA and other Tx parts separated from the feedhorn and mounted lower down on the boom). The interconnecting RF and DC cables between the EHFU and PSIU run inside the boom. The Interfacility Link (IFL), connected to the PSIU, consists of the Tx and Rx L-band coaxial cables, a DC power cable and a twisted pair link for alarm/control. A monitor port is provided on the PSIU to assist with the antenna adjustment.
In case of linear polarisation, the initial adjustment for a given Tx/Rx polarisation is done by physically rotating the EHFU in its mount. Afterwards, the Rx polarisation can be remotely changed. With circular polarisation, no initial adjustment is required.
The outdoor transceiver can then be used with different antennas and indoor options as follows:
a) In the VSAT configuration a relatively large antenna (0.9 -1.5 m) is likely to be used. Fig. 3 shows the 1.5m antenna specifically designed for the ESA VSAT project. The photograph shows the back of the antenna with the PSIU mounted and the feed arrangement with the EHFU. As can be seen, the inside of the boom can be conveniently used for the cables between the PSIU to the EHFU, when mounted.
In the VSAT case, the PSIU will be equipped with all the units mentioned above. Indoors, an LCU is used to convert the standard L-band to/from 70 or 140 MHz. Depending on the make of LCU, frequency agility over the 500 MHz band is achieved with frequency steps of 5 MHz, 1.25 MHz or as small as 50 Hz . In addition to frequency settings, the LCU will also allow sufficient Tx and Rx gain adjustment that could be used to compensate for the IFL loss. However, using gain in the Rx path after, instead of before, high IFL loss may sometimes significantly affect the noise figure of the entire front end. Therefore it is better to utilise the above mentioned variable gain capability in the outdoor transceiver for IFL loss compensation. Hence the LCU variable gain in conjunction with that of the modem is only used to set up the correct levels. The LCU typically has its own display and keypad for frequency and level settings.
Fig. 3: VSAT Dish with PSIU, and EHFU in the feed
b) In TV reception (Rx-only) mode, a smaller antenna similar to the Ku-band DTH assemblies would typically be used. The SSPA/UPC is easily removed from the EHFU assembly leaving only the horn, OMT and LNB - a relatively light assembly. The LNB in this case would be a DRO-equipped option, without the need for Rx LO PLL circuitry. In fact, in this mode the PSIU is not needed. Indoors, the IRD could be very much like the ones now used in Ku-band, with polarisation selection done by the standard 13V/18V switch of DC power for the LNB. Since circular polarisation is expected in the future (almost all proposed satellite systems feature circular polarisation), the EHFU has been designed to be easily modified from linear to circular by inserting a dual-band circular polariser between the OMT and the horn. This is visible in Fig.4 showing this option, with a 60 cm antenna.
Fig. 4: 60 cm Rx-only Outdoor Assembly
Fig: 5: 60 cm Rx/Tx Outdoor Assembly
c) In future residential terminals for multimedia applications, a smaller antenna as in b) above is also envisioned. Both the EHFU and PSIU would be used similar to the VSAT mode but with some simplifications such as the DRO-equipped LNB and "bare-bones" PSIU (no Rx LO, no Rx variable gain, demultiplexer etc.) for cost-reduction. Also, the L-band modem indoors is expected to provide sufficient power on its L-band Tx port for the outdoor transmitter. Due to the relatively short length of the IFL in a typical home, an inexpensive multi-purpose TVRO cable would be used to replace all the coaxial, DC and comlink cables needed in the VSAT application. Fig. 5 shows this option of the outdoor assembly, again with a 60 cm antenna and circular polariser but now with the transmitter included.
Electrically, the main parameters characterising this RF front end are the 29.5-30 GHz Tx and 19.7-20.2 GHz Rx frequency ranges, 950 MHz - 1450 MHz indoor/outdoor interface, Tx output power of 2 W and overall Rx noise temperature (at the OMT) of 215 ø K. In the VSAT case, with a 1.5 m antenna this results in an EIRP of 53 dBW and a G/T of 24 dB/ø K. The transceiver features remote switching between co-linear and orthogonal Rx polarisations for a set Tx polarisation, or between equal and opposite circular Rx polarisations for a set Tx polarisation. The latter is an option achieved by inserting a dual-band circular polariser between the horn and the OMT/diplexer in the feed.
3. BLOCK DIAGRAM
A basic block diagram of the transceiver is shown in Fig. 6.
Fig. 6: Basic Front End Block Diagram (VSAT indoor option)
The PSIU is conceptually designed with versatility in mind while taking advantage of certain standard features available in indoor units. A DC voltage is typically multiplexed onto the L-band Rx port in indoor units to power the outdoor LNB via the Rx coaxial cable, and the PSIU utilises that. LCUs will also typically put DC power on the Tx cable, usually in the range of 24V - 36V. However, in most cases the available current is often insufficient for a 2W, 30 GHz transmitter; therefore we have also provided an input on the PSIU for a separate cable from an indoor power supply. Similarly, the LCU and an L-band modem will multiplex a reference (usually 10 MHz or 100 MHz ) onto their L-band ports. Some will apply the reference to both ports, some others only to the Tx port. The PSIU has been designed to be able to utilise a frequency reference anywhere from 10 MHz to 100 MHz and it extracts this reference in its Tx demultiplexer and applies it to the Tx and Rx LOs. In case of an IRD for TV reception, a reference is usually not provided as the drift of the typical free-running LO associated with the LNB is expected to be acceptable. Therefore the Rx LO in the PSIU is designed accordingly.
On the Tx side in the PSIU, the L-band signal from the IFL is passed through a variable gain amplifier adjusted to offset the IFL loss. Another function of this amplifier is in fine compensation of Tx gain vs. temperature (see below). The signal then passes through a switch (not shown) automatically thrown open in case of a Tx LO alarm to prevent transmitting at an arbitrary frequency.
On the Rx side in the PSIU, the Rx signal from the EHFU is passed through a variable gain amplifier also adjusted to offset the IFL loss. A demultiplexer in the path is used to re-route the DC voltage on the IFL (which in the case of an LCU indoors is constant) and switch it between 13V or 18V. This is then passed onto the LNB in the EHFU. A Rx level monitor is also equipped to assist with antenna adjustments.
The MAC unit within the PSIU is used for several purposes: Tx and Rx LO alarms, Tx power alarm and Tx power shutoff, control of fine compensation of Tx gain dependence on temperature, control of compensation of both Tx and Rx IFL loss as well as extra Rx gain adjustment, and finally selection of Rx polarisation setting. These monitoring, alarm and control functions are remotely implemented via the RS485 interface either from a keypad and display included in the indoor power supply unit or from a PC.
In the EHFU, the L-band input signal from the PSIU is brought to the EHF upconverter (EHFUPC) which contains a subharmonic mixer the output of which is amplified and applied to the OMT/Diplexer. An output level detector and temperature sensor are used for reporting and for fine gain compensation of the Tx gain through the MAC and variable gain amplifier in the PSIU. In the LNB, its dual input selects either a verticaly or horizontally polarised signal from the OMT. This is done by using a switchable DC voltage on the Rx coaxial cable to the LNB (13V/18V) as is customary with Ku-band LNBs. In case of VSATs, the switching is initiated via the comlink and actually carried out in the Rx demultiplexer in the PSIU as mentioned above; in case of TV Rx-only, the DC multiplexed on the Rx cable for the LNB is switched directly from the IRD since the PSIU is not equipped with the Rx part in this case.
4. IMPLEMENTATION AND TECHNOLOGY DETAILS
4.1 Antenna
For the VSAT appplication, a 1.5 m antenna has been designed and manufactured for IMT Comsys by Precision Antennas of Stratford-upon-Avon, England . The reflector and horn have been designed to meet a Tx gain > 51 dB and Rx gain > 48 dB and to satisfy ETSI specifications for sidelobe levels (S = 29-25 log j ), cross-polarisation performance (< - 25 dBc) as well as wind load, safety and other ETSI requirements. Mechanically, the antenna had to have the usual capability of elevation adjustment of 0 to 90ø degrees and 0-360ø azimuth adjustment with a pointing accuracy of better than 0.1ø along the geostationary orbit.
The antenna incorporates an offset reflector configuration to avoid feed aperture blockage and obtain good sidelobe performance. With a 1.5 m dish, this particular antenna has a f/d ratio of 0.8, offset angle of 35 degrees and half apex angle of 32 degrees. The feed assembly is a dual-band scalar rings horn that provides symmetric radiation patterns with low cross-polarization characteristics. This type of horn is suitable for casting and has therefore potential for low cost.
Obviously, other antennas can be used for the VSAT front end as long as the horn is designed for the particular geometry as well as for the transceiver OMT interface. For the residential application, a smaller Ka-band antenna would be typical as previously shown in Figs. 4 and 5. However, it would also appear advantageous to examine some of the DTH TV Ku-band antennas on the market for their suitability for 20/30 GHz.
4.2 Outdoor Transceiver
As stated before, the transceiver (designed and made at IMT Comsys) is divided into the "EHF Unit" (EHFU), mounted on the boom, and the PSIU mounted on the back of the reflector. The EHFU contains the SSPA, EHFUPC and LNB integrated with the OMT and horn. A picture of the EHFU is shown in Fig. 7 and a drawing for descriptive purposes in Fig. 8. (For size comparisons, the horn is approx. 51 mm in diameter).
Fig. 7: Photograph of EHFU
Fig. 8: Drawing of EHFU
The OMT/Diplexer uses a circular waveguide with two different diameters to allow for separation of the 20 GHz and 30 GHz signals. For the 30 GHz Tx signal, a circular-to-rectangular transition is used and attached to the back of the OMT cylinder. At that point the Tx waveguide (WR 28) is connected which at the other end attaches to the SSPA. Two Rx waveguide (WR42) flanges attach to the side of the OMT at right angles to each other, coupled through a slot to the circular waveguide inside. The slots are resonant at 20 GHz to increase the Tx to Rx isolation particularly for the case of co-linear Tx/Rx polarisation. In addition, special thin FSS (Frequency-Selective Surface) filters are incorporated in the flanges to enhance the above isolation as well as to make sure that a short circuit at 30 GHz is created at the inner surface of the circular waveguide. This is important to prevent "skewing" of the Tx polarisation plane by the co-linear Rx slot. The LNB is mounted onto the OMT with one of its input flanges directly on one OMT flange and the other connected with a short section of a WR42 waveguide.
As for the electronics of the transceiver, the following are some of the characteristic aspects of the design approach taken and of the technology used:
a) In both the Tx and Rx LOs, step-recovery diode (SRD) multipliers are used in the LO chain. The SRD approach still appears the simplest and least expensive of all the available options. If designed correctly and not pushed for extreme efficiency, the SRD multiplier will be stable and reliable.
b) In the EHFUPC, a subharmonic mixer is used. This limits the output frequency of the multiplier chain to approx. 14.5 GHz which avoids the problem with SRDs when multiplying to 28 GHz (too high).
c) The SSPA which has a gain of ¯ 53 dB uses mostly internally matched MMICs, both for the low-level drivers and the final stage. The required output power of 33 dBm (2W) is obtained from power combining four MMIC chips, each of which already combines two devices on the chip. The chip can deliver ¯ 28 dBm (at 1 dB. C.P.) For lowest loss, the combining is done by waveguide magic tees. No Tx filter is used to minimise power loss; it was assumed (and confirmed) that the SSPA harmonics would be below the specified level (-20 dBc) at 1 dB compression which is supposed to be the highest operating point. An electronically controlled variable attenuator is used in conjunction with a temperature sensing circuit to compensate for amplitude variations of the Tx chain caused by temperature changes.
d) The LNB has a gain of approx. 55 dB and uses 4 stages of amplification in the LNA, a mixer with about 10 dB loss and 2 IF MMIC stages. The LNA has a dual input stage with two transistors switched ON and OFF for polarisation setting.
The technology used in the realisation of microwave circuits for all units except the SSPA is packaged and mostly discrete components on softboard. An example of this is shown in Fig. 9. This technique is routinely used by various Ku-band LNB manufacturers and has been proven to be very cost-effective. Our partners in this project, GMTL, have successfully used this approach in other products with good performance up to about 24 GHz. We have extended this to the 1/30 GHz upconverter. In the SSPA, the classic "chip and wire" approach is used with alumina substrates and semiconductor chips (at 30 GHz, the 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). The substrates and chips 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 SSPC box.
Fig.9: Inside View of the LNB Unit
5. PERFORMANCE
The overall RF Front End parameters were determined by separate measurements on the antenna and the transceiver. EIRP and G/T were obtained by adding the corresponding antenna and transceiver parameters. Tests via satellite are planned for the near future. A summary of all relevant Front End parameters and the measured results is given in Table I.
Table I: RF Front End Performance (at L-band Interface)
Item
Parameter
Performance
Comment
1.
Tx Frequency Range:
29.5-30.0 GHz
2.
Rx Frequency Range
19.7-20.2 GHz
3.
Tx Polarisation
Vertical
4.
Rx Polarisation
Hor./Vert.
Remotely switchable
5.
Antenna Tx /Rx gain @ 29.5 GHz
51/48 dB
Including OMT
6.
Sidelobes
< ETSI Spec
7.
Cross-polarization at 1 dB contour
<-27 dBc
Including OMT
8.
EIRP @ 1 dB C.P., 25 ø C,
nominal IF input
53 dBW
With 1.5 m antenna
9.
EIRP Flatness @ 1 dB C.P. over any 72 MHz band
1 dB p-p
10.
SSPA power at 1 dB C.P., 25ø C
32 dBm
11.
Power Stability with Temperature
3 dB p-p
12.
Transmitter IF
950 - 1450 MHz
13
Linear Tx Gain (IF in to SSPA out)
38-58 dB
14.
Minimum Rx G/T
24 dB/ø K
With 1.5 m antenna
15.
Receiver Noise Figure (LNA input) @ 25ø C
2.2 dB max.
Entire Rx chain!
16.
Receiver IF
950 - 1450 MHz
17.
Rx gain (LNA input to IF out)
35-65 dB
18.
Rx gain flatness over any 72 MHz
1 dB p-p
19.
Rx gain variation with temperature
3 dB p-p.
20.
Size of main reflector
1.5 m
For VSAT
21.
Power consumption
90 W
6. SUMMARY
A 20/30 GHz RF Front End with a versatile outdoor transceiver has been designed and prototyped, and its performance verified in bench and antenna range testing. The transceiver is intended to be used with offset-fed parabolic antennas 0.6 m to 1.5 m in diameter and for terminals ranging from TV Rx-only through a multimedia Tx/Rx to a VSAT.
The technology used in the SSPC is "classical" thin film mounted on metal carriers, with MMICs utilising on-chip combining of PHEMTs in the SSPA power stage. For all the other circuitry, SMT on softboard is used (this includes the 20/1 GHz LNB and 1/30 GHz upconverter).
The RF Front End is characterised by a Tx power of 2W and noise figure of the entire Rx chain (including indoor unit) of 2.2 dB. With a 1.5m antenna, this translates into an EIRP of 53 dBW and G/T of 24 dB/K. The transceiver has an L-band IF interface with the indoor units. The DC power consumption of the outdoor transceiver is 90 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, Mr. J. Horle and Mr. F. Feliciani of ESA, and Mr. A. Bastikar 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 PML and GMTL in England is gratefully acknowledged.
Finally the author wishes to thank his colleagues on the IMT Comsys management team for their encouragement and to IMT?s members of the technical staff involved in this project for their extraordinary efforts in this very challenging assignment.
REFERENCES
1. ETSI Draft pr. 300 xxx-May 1993, "Satellite Earth Stations (SES); Transmit/Receive Very Small Aperture Terminals (VSATs) used for data communications operating in the Fixed Satellite Service (FSS) 20/30 GHz frequency band". |
