RF System Design Considerations

The electromagnetic environment of Mars is substantially different from that in which terrestrial radios such as cell phones operate - leading to unique tradeoffs in the design space.  Some of the important considerations taken into account in this project include:

  • UHF spectrum (390 - 450 MHz) is available and is the preferred frequency range.  This frequency is standardized within the Prox-1 protocols, but also provides a good compromise between receive sensitivity and antenna size (antenna effective aperture is larger at lower frequencies for omni-antenna patterns).
  • The interference environment and the thermal noise floor are significantly lower than on earth, shifting the emphasis away from large-signal handling specifications like intercept point and more toward minimizing noise figure and power.
  • Ambient surface temperatures range from less than -100 C to about +20 C, making careful attention to parameter drift in circuits essential.  Temperature drift in filters and oscillators are especially significant, but the wide temperature range requires that all analog circuits be temperature compensated.  The extreme temperature range also creates some important reliability issues that must be addressed.
  • Atmospheric shielding from ionizing radiation is minimal, making the use of radiation-hardened electronic circuits essential.


IC Process Selection

To meet the goals of a highly miniaturized, low-mass, low-power design within these constraints, a custom IC chipset is being developed.  The selected IC process is a Silicon-on-Sapphire technology from Peregrine Semiconductor that includes the following features, uniquely suited to the project:

  • Significantly higher quality-factor inductors than those in standard CMOS foundries due to the use of an insulating substrate and thick metal.  This is essential for RF circuit design of fully-integrated LNAs, low-power oscillators, and high-efficiency power amplifiers.
  • Higher levels of integration than offered by GaAs - permitting the integration of frequency synthesizer and high-density digital circuits on the same die as the RF circuits.
  • Inherent radiation-hardness from the Silicon-on-Insulator structure.
  • Space-rated history of applications.




Top-Level Block Diagram

The Mars Proximity Microtransceiver consists of a small set of custom ICs, together with a minimum of off-chip components. While it is possible to implement the entire transceiver (minus a crystal reference) into a single die using the selected IC process, a multi-chip development approach was adopted to reduce development time, cost, and risk.  One chip, the "RFIC", implements a robust superhet receiver architecture together with a transmitter employing a direct-upconversion I/Q modulator and 10mW / 100 mW power amplifiers. Both receiver and transmitter share an on-chip frequency synthesizer which is locked using a Commercial-Off-the-Shelf (COTs) crystal oscillator (TCXO).  The remaining off-chip component shown in the diagram is a 10.7 MHz IF bandpass filter (BPF) for channel-preselection prior to analog-to-digital conversion.  For applications requiring a full 1 Watt RF transmitter output, a third, optional IC is used.  This IC, like the main RFIC, implements its function in fully integrated form - without the need for external passives.

The companion digital modem and control IC provides precision IF filtering and I/Q demodulation of BPSK signals during receive, together packet assembly, scrambling, and channel coding on transmit and control and housekeeping functions needed by the RFIC.  On receive, the digital IC must compensate for both doppler uncertainty in the downlink frequency from the orbiter and for variations in the synthesized LO created from the TCXO reference frequency.  Unique digital-signal-processing (DSP) algorithms provide a pull-in range of up to +/- 50 kHz at data rates of 8 and 1 kbps.  Transmit-side data rates of up to 256 ksps or higher can be created, although surface-to-orbiter links will typically operate between 2 and 32 kbps depending on the transceiver power selected for the application and on the orbital pass profile.


Detailed Block Diagrams

Detailed block diagrams of the RFIC and digital IC are shown below.  For in-depth discussion of these diagrams and the underlying circuits and algorithms, visit the associated pages by clicking the diagram or by returning to the main page of this site.


RFIC Block Diagram


Digital Modem IC Block Diagram


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