Integrated Front End Serves Satcom Receivers

Integrated Front End Serves Satcom Receivers

This integrated Ku-band front-end system employs a modular design approach, achieving a great deal of functionality in compact packaging for satellite communications receiver applications.

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Microwave front ends are a critical part of any high-frequency receiver architecture. They downconvert incoming signals from an antenna and pass along lower-frequency intermediate-frequency (IF) signals for further processing, typically to an analog-to-digital converter (ADC). By adopting a modular approach, an integrated front end was developed for satellite-communications (satcom) applications from 12.75 to 13.25 GHz. This front end is compact in size while also achieving outstanding electrical performance.

An effective front-end receiver design should provide good low-noise performance, as characterized by low receiver noise figure, and high gain at the frequencies of interest. For Ku-band sitcom applications, a receiver front end was designed for use from 12.75 to 13.25 GHz. This integrated front end consists of three independent function modules: a low-noise block downconverter (LNB) module, a local oscillator (LO) driver module, and a frequency downconversion module.

Fig. 1
1. The block diagram shows the components that comprise the sitcom integrated front end module.

The LNB module includes a waveguide-to-microstrip transition, three stages of low-noise and high-gain amplifiers, and an image-reject filter. The LO driver module consists of an active frequency multiplier, a bandpass filter, and a driver amplifier. The frequency downconverter module incorporates a single-balanced frequency mixer (based on a low-barrier Schottky diode) and an intermediate-frequency (IF) bandpass filter to achieve the desired bandwidth and out-of-band rejection of unwanted signals. Active circuits in these modules are biased by means of a common sequential bias circuit.

Fig. 2
2. This plot shows the frequency response of the amplifier stages.
The individual modules were fully characterized before being assembled into the satcom system housing. The active circuits were designed with the aid of an in-house nonlinear model for a commercial high-electron-mobility-transistor (HEMT) device,2 a model CFY67-08 transistor from Infineon Technologies. This is a high-reliability (hi-rel) gallium arsenide (GaAs) transistor usable from 500 MHz to beyond 20 GHz with typical noise figure of 0.7 dB at 12 GHz.

The front end incorporates novel and compact filters to achieve small size and low insertion loss. The design of the front-end modules reduces the total parts count by using a single GaAs HEMT as the active device for all of the active circuits. The circuits are implemented using microwave-integrated-circuit (MIC) technology on alumina substrates. The modules are integrated into aluminium housings with a robust mechanical design to avoid undesired coupling and cavity oscillations. Figure 1 shows a block diagram of the different function blocks in the integrated front-end design.

Active Circuitry

Fig. 3
3. This photograph shows the fabricated multiplier circuit for the integrated front end.

The LNB module includes three amplifier stages (Fig. 2): a low-noise-amplifier (LNA) stage followed by two gain stages. These amplifier stages are conjugately matched and constructed with GaAs HEMT active devices. The operating frequency range of the LNA is 12.75 to 13.25 GHz. The devices are biased at 2.0 V dc and 15 mA. Since low noise figure is a critical parameter for the LNA, in-house-extracted S-parameters are used for the GaAs HEMT device (rather than supplied by the manufacturer), along with noise data for the device to determine the actual device phase response for Γopt for optimum impedance matching for low noise figure.

Fig. 4
4. This is the response of the frequency multiplier circuit as a function of input power.

This is done since high-frequency package parasitic impedances may change the phase response of the device. By employing this design approach, it was possible to obtain a noise figure of 2.3 dB with gain of 26 dB for the LNB, with measured results closely matched to the computer simulated performance.

Fig. 5
5. This photograph shows the driver amplifier circuit used in the integrated front end.
A frequency doubler, part of the LO driver module, was designed to produce the required frequency range for the front-end design. The doubler (Fig. 3) accepts input signals across a C-band frequency range and provides output signals at Ku-band frequencies. The multiplier operates across an output frequency range of 6.225 to 6.500 GHz. The design is based on reflector network topology to achieve low conversion loss in order to provide usable output-power levels when operating with input signals at minimum input power levels.

Fig. 6
6. This is the driver amplifier’s output power at 1-dB compression as a function of input power.

The design was simulated and developed by means of nonlinear analysis using harmonic-balance computer-aided-engineering (CAE) simulation software (Fig. 4). The GaAs HEMT active device in this doubler is operating in its pinchoff region to help achieve the aforementioned low conversion loss. This loss is 3 dB from C-band to Ku-band when operating with optimum input power of +2 dBm.

Fig. 7
7. This simple circuit was used for the frequency mixer in the integrated front end.
The driver amplifier (Fig. 5), which is also part of the LO driver module, was designed to provide small-signal gain to the output of the frequency doubler output in order to feed the frequency mixer’s LO port  at an optimum power level. The operating frequency of the driver amplifier is 12.45 GHz with a 5% bandwidth. The driver amplifier is operated at its 1-dB compression point (P1dB) to provide constant power at the mixer’s LO port. Nonlinear analysis by means of harmonic-balance CAE simulation was also performed on the driver amplifier to help optimize the design (Fig. 6).

To achieve P1dB at low input power, the output load analysis of the driver amplifier was based on optimizing the small-signal gain, where the output load is maintained at a high value (based on load line analysis of the active device). This approach makes it possible to obtain higher gain compression at lower input signal level. The driver amplifier is biased at +2 V dc and 15 mA. It delivers 9-dB gain and provides +4.0 dBm output power (P1dB).

Fig. 8
8. This plot shows the response of the mixer when driven by the LO.

A single-balanced diode-based frequency mixer, which is part of the frequency downconversion module, was designed for operation from 12.2 to 13.2 GHz. The mixer incorporates low-barrier Schottky diodes operating at Ku-band frequencies at low LO drive levels. The mixer is designed with a 180-deg. hybrid for high RF-to-LO isolation and suppression of even-order harmonics generated by the LO.

Fig. 9
9. The photo shows the basic structure of a resonator-type filter.
Quarter-wavelength traps were incorporated at the IF port to suppress undesired RF and LO harmonics. As with the frequency doubler and driver amplifier, nonlinear analysis was also performed using harmonic-balance CAE simulation software. The computer simulations helped to achieve conversion loss of 7 dB with a drive level of 0 dBm. Figure 7 shows the assembled mixer, with simulated response in Fig. 8.

The image-reject filter, which is part of the amplifier module, was implemented as a resonator-type filter (Fig. 9). The function of the filter is to reject noise and signals at image frequencies. The filter, which measures 10 × 5 mm, has a bandwidth of 200 MHz with insertion loss of 2.5 dB, with better than 45-dB image rejection.    

Fig. 10
10. The photograph shows the fabricated IF filter.
The LO bandpass filter, with a structure and design similar to those of the image-reject filter, is a part of the LO driver module. It selects the desired second harmonic from the doubler and eliminates fundamental-frequency signals and unwanted harmonics produced by the multiplier. The compact LO bandpass filter was designed for a bandwidth of 200 MHz at a center frequency of 12.45 GHz. It exhibits insertion loss of 3.0 dB across the passband, with better than 40-dB rejection of unwanted, out-of-band signals.

The IF filter, which is part of the frequency downconversion module, is based on microstrip transmission-line technology using a compact open-loop resonator.3-7 The filter is designed for a center frequency of 549 MHz and a bandwidth of 20 MHz with insertion loss of 4.0 dB across the passband. It provides maximum notch rejection of 40 dB and second-IF rejection of better than 25 dB.

Fig. 11
11. The modular front end measures 150 × 180 × 20 mm and weighs 750 g.

The filter measures 30 × 30 mm, which is compact for a component operating at UHF. Figure 10 shows a photograph of the fabricated IF filter. All three filters were designed with the aid of the LINMIC simulation software, now available from Computer Simulation Technology (CST). The circuit designs were verified using electromagnetic (EM) software simulation tools.

The three modules, along with an integrated bias card, were assembled into a front-end housing. The modules were characterized independently to determine performance levels prior to integration in the front-end housing. They were characterized over wide temperature extremes to ensure that minimum performance levels would be met even at operating temperature extremes. The various tables provide summaries of key performance parameters. Table 1 provides details on the amplifier module while Table 2 offers information on the performance of the LO driver module.

Table 1

Table 3 presents details on the performance of the frequency downconversion module. It was characterized for a constant IF of 549 MHz and LO power of +4 dBm. Table 4 shows the performance attributes of the integrated microwave front-end assembly. The front-end subsystem is designed to operate with + 5 and -15 V dc supplies with DC power consumption of 0.7 W. The performance of the front-end system has been verified over an operating temperature range of -30 to +60°C. Variations in gain and noise figure as a function of temperature are controlled to better than ±1.0 and ±0.5 dB, respectively.

Table 2

All three modules were integrated into a common modular package (Fig. 11). The circuits within these modules were implemented using MIC technology on alumina substrates, mounted on Kovar plates and interconnected by means of ribbon bond wires. The length and width of the ribbon bonds were optimized to less than 15 mils for an impedance match to 50 Ω.6 Tuning stubs near the ribbon bond wires were used to fine-tune any impedance mismatches in the interconnections.

Table 3

Great care was taken in routing DC wire connections from the active stages (using short wire lengths to avoid radiating loops), such as the amplifier module and the LO driver module, to the integrated bias card to avoid undesired low-frequency oscillations. Ferrite beads were incorporated with feedthrough capacitors to form a lowpass filter, further rejecting spurious energy and avoiding in-band oscillations within the active circuits.

Table 4

The housing was fabricated from 6061 aluminium alloy. It is designed to avoid mutual coupling between the gain stages and also avoid direct coupling between the LO drive module and the three-stage amplifier module. Cavity oscillations are minimized in these two modules by providing ground planes (conducting walls) and microwave absorbers at the electric field maxima regions. The modular front-end subsystem measures 150 × 180 × 20 mm and weighs 750 g. The compact front end was designed for integration into a command and ranging receiver for future spacecraft applications.

Acknowledgment

The authors would like to recognize the support and guidance from the senior colleagues of the Receiver Division of the Communication System Group (CSG) of the ISRO Satellite Centre (ISAC). Also, the authors are grateful for support provided to the Deputy Director of the ISRO  Communication and Power area. The authors would also like to thank the Director of the ISRO Satellite Centre for encouragement during this work.

Avjit Roy Choudhury, Engineer

Deepankar Roy, Engineer

Alapati Venkata Sucharita, Engineer

Yateendra Mehta, Engineer

Communication System Group, ISRO Satellite Centre (ISAC), PB No. 1795 Vimanapura Post Bangalore - 560 017, Bangalore, India

R. Ramsubramanian, Engineer

Satellite Navigation Program, ISRO Satellite Centre (ISAC), PB No. 1795 Vimanapura Post Bangalore - 560 017, Bangalore, India

References

1. Jia-Sheng Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave Applications, Wiley, New York, 2004.

2. Avjit Roy Choudhury and R. Ramsubramanian, “HEMT Modeling and

Validation for Low Power Nonlinear Circuits, 2013 ICMARS Proceedings.

3. Deepankar Roy, “Miniature Filter Screens VHF Band,” Microwaves & RF, July 2009.

4. Deepankar Roy and R. Ramasubramanian, “Planar Filter Provide High Image Rejection,” Microwaves and RF, October 2007.

5. LINMIC 6.2 +/N User Guide, Computer Simulation Technology.

6. Inder Bahl, Lumped Elements for RF and Microwave Circuits, Artech House, Norwood, MA.

7. Joseph S. Wong, “Microstrip Tapped-Line Filter Design,” IEEE Transactions on Microwave Theory & Techniques, Vol. MTT-27, No. 1, 1979, pp. 44-50.

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