System Hardware

Hardware Design Considerations in Transponder System
For circuits in transponder system operating in W band and V band, normally their designs are based on MMICs (Monolithic Microwave Integrated Circuits). MMICs are fabricated by Gallium Arsenide (GaAs), Indium Phosphide (InP), or some other III-V compound semiconductor materials. Compared to Silicon technology which is mostly used in digital and low-frequency RF applications, these materials own higher cutoff frequency and lower noise. So MMICs are more suitable for satellite communication systems. MMICs are also well-suited for large arrays, which can facilitate our design if we want to realize large phased-array transceiver systems in the future.
There are mainly 5 important building blocks in a transponder system, including Low-Noise Amplifier (LNA), Local Oscillator (LO), Mixer, Power Amplifier (PA) and Filter. Since our team will design a thermal control system on the satellite, we don’t need to worry about the low-temperature impact on our transponder circuits, so we can re-use all the circuit techniques on the ground station to our satellite.
System Architecture

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Low Noise Amplifier
Two important requirements for LNA in our satellite communication system are to provide high gain and low Noise Figure (NF). Linearity is not an issue if we use QPSK or M-PSK modulation because all the transmitted and received signals have the same amplitude. Normally we should use multi-stage structure to achieve high gain. A design example in [1] is shown here to demonstrate the feasibility. In this example, the LNA was fabricated on 70nm GaAs technology. There are 4 stages to provide around 25 dB gain, while the NF is 2.7dB. Also the bandwidth of this LNA is 70-105 GHz, which covers the operation V & W band in our proposal.

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The following table concludes several MMIC LNA designs in V & W band. According to this table, above 20 dB gain and below 3 dB NF can be achievable for our satellite transponder circuits.

3

Local Oscillator
There are two major ways for LO design in W & V band system.

    YIG oscillator has wide tuning range and fine resolution. YIG oscillator is tuned by varying an external magnetic field. Normally, YIG oscillator operates below 40 GHz, so we may need a frequency doubler or tripler to generate LO signal in desired band.
    The multiplier circuits can be achieved by Schottky Barrier Diode (SBD) or by active circuits.
      A Gunn diode is a two terminal device, typically made from Gallium Arsenide (GaAs). Just like a normal diode, they exhibit non-linear I/V behavior, but have a negative resistance characteristic, thus, can be used as an oscillator. This negative resistance region occurs due to the energy band structure of GaAs. [2]

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      The center frequency of the oscillator can be tuned by changing the bias voltage. The following figure is the tuning range of the oscillator with respect to bias voltage in a W band receiver. [2]

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      Normally, the fundamental frequency of Gunn oscillator is between 30GHz and 70GHz. So in order to generate LO signal in V band & W band, we need a doubler or extract the second order harmonic of the circuit as the following graph shows.

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      Gunn Oscillator can be phase locked to a microwave source. The following graph is the Phase Locked Loop (PLL) structure diagram of our system.

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      Mixer
      Through literature review, there are three common ways to design a mixer for a satellite communication system.

        A schottky barrier diode (SBD) is a small area barrier between a metal and a semiconductor. SBD Mixers can operate at room temperature, and they are easy to match to a waveguide or planar antenna due to small barrier capacitance. Also, it is quite stable. However, since SBD only has conversion loss, so it requires high-level input power and LO power. Compared with other devices, it owns less nonlinearity.
        Due to its characteristics, SBD Mixer is suitable for up-conversion and down-conversion mixer on the satellite. The following figure is an example of GaAs SBD Down-conversion Mixer in [3], which achieves 7 dB conversion loss and better than 40 dB LO-RF isolation in W band. The DSB noise temperature is 650K.

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          When cooled below the critical temperature of the superconductors constituting the junctions, a SIS junction exhibits an extremely strong nonlinearity, which is very helpful for mixer design.

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          SIS Mixers own the best sensitivity in millimeter and submillimeter wave bands, so they are widely used in radio astronomy. The only disadvantage is that it must be operated in very low temperature (several tens K), so considering the cost, we just want to implement it in the ground station.
          The following example [4] shows a SIS Mixer design which can fulfill the ALMA science requirements. The bandwidth of the mixer is 84-116 GHz, and the DSB noise temperature is 16-20K.

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            This kind of mixer has a wider bandwidth and requires less LO power compared with other mixers. However, the electrical stability is rather poor, and it is easy to saturate because of wide bandwidth. So it is not suitable for our satellite link.
            Power Amplifier

              In a klystron tube, an electron beam is formed by accelerating electrons emitted from a heated cathode through a positive potential difference. The electrons enter a series of cavities, typically five in number, which are tuned around the operating frequency and are connected by cylindrical "drift tubes". It has the highest output power compared with other kinds of PAs, though the bandwidth is narrow. Since it owns very high output power and it is quite heavy, normally it is suitable for PA in the ground station.

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              In [5], people made a KPA with 2.0KW peak output power at 94.05 GHz. The efficiency is around 32%.

                TWTAs are built using foot-long vacuum tubes to do their amplifying. Power is applied to the tube generating temperatures in the plasma ranges. A radio frequency is then emitted into the tube and the radio wave is amplified as it passes through the tube's heated interior. Since TWTA is more expensive than the other PAs, and we don’t need very wide bandwidth in our satellite link, so it is not suitable to implement TWTA in our transponder system.
                  Just as we mentioned before, we can implement PA based on MMICs. Since MMIC is much smaller in size, we can do power combining on chip by using parallel modules. SSPA is very suitable for the PA on satellite due to its small size, less power consumption and medium output power.

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                  In paper [6], people reported a GaN W band PA. The highest measured output power from 4-way power combining is 3W with 10.2% PAE. According to our link budget, this kind of PA is definitely doable on the satellite.

                    Since we are going to use QPSK or M-PSK modulation, linearity and back-off efficiency are not big problems in our transponder system. One other problem is the stability issue. Since all these PAs have a relatively high gain, if the PA cannot be unconditionally stable in all frequencies, we need add some tricks in our circuits to decrease the gain in low frequency to ensure it is stable.
                    Filter
                    In our system, we need several microwave band-pass filters. In general, most microwave filters are made up of one or more coupled resonators, so any technology that can be used to make resonators can also be used to make filters. The unloaded quality factor (Q) of the resonators will generally set the selectivity for the filter. The lumped-element filter works well at low frequencies, but in high frequencies people mostly use transmission lines and couplers to design filters, mainly because of distributed effect.
                    There are several ways to design microwave band-pass filters. [7]
                      A number of coupled lines will admit to an equivalent circuit of alternating series and parallel resonant circuits, and the design parameters of the prototype filter can be imposed onto the structure of parallel coupled lines.

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                        Because the lines are shorted at opposite ends, the structure takes the form of interlaced fingers, and is called an interdigital filter. This topology is straightforward to implement in planar technologies.

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                        Reference
                        [1] W. Ciccognani, “Full W-Band High-Gain LNA in mHEMT MMIC Technology”, Proceedings of the 3rd European Microwave Integrated Circuits Conference
                        [2] http://gunn.winterwolf.co.uk/reports/interim
                        [3] S. Raman, “A High-Performance W-Band Uniplanar Subharmonic Mixer”, IEEE Trans. Microwave Theory Tech, Vol. 45, No. 6, June 1997
                        [4] S.-K. Pan, “A Fixed-Tuned SIS Mixer with Ultra-Wide-Band IF and Quantum-Limited Sensitivity for ALMA Band 3 (84-116 GHz) Receivers”,15                        th International Symposium on Space Terahertz Technology
                        [5] A. Roitman, “State-of-the-Art W-Band Extended Interaction Klystron for the CloudSat Program”, IEEE Tans. Electron Devices, Vol. 52, No.5, May 2005
                        [6] A.Fung, “Power Combined Gallium Nitride Amplifier with 3 Watt Output Power at 87 GHz”, 2011
                        [7] David Pozar, “Microwave Engineering”, third edition, 2005