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RF Circuit Design

RF Circuit Design. Chris Fuller Chris_Fuller@IEEE.ORG 952-607-8506 11/7/2012. Design Process. Define Requirements Design Prototype Design Review Build Test Analyses Review Iterate Design Process. Define Requirements. Communication distance Data Rates including security

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RF Circuit Design

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  1. RF Circuit Design Chris Fuller Chris_Fuller@IEEE.ORG 952-607-8506 11/7/2012

  2. Design Process • Define Requirements • Design Prototype • Design Review • Build • Test • Analyses • Review • Iterate Design Process

  3. Define Requirements • Communication distance • Data Rates including security • Physical space available • Available battery energy • Communication media: air, metal, tissue • Unit Price goal • Available development time • Cost/Availability of components • Interference tolerance/likelihood • Operating Frequency • Many others

  4. Overview of Radio Communications • Basic transceiver components: Antennas, Amplifiers, Mixers, Filters, Synthesizer, Baseband Processing

  5. Components • Antennas: Interfaces communication media (air, body, etc.) to transceiver • PA (Power Amplifier): Boosts modulated transmit signal • LNA (Low-Noise Amplifier): Boosts signal sensed at antenna while adding little noise to the desired signals. • RF Filters: Passes desired RF modulated signals & blocks undesired signals. • IF Filters: Blocks undesired signals from received signals. • Synthesizer: Reference RF frequency used to convert from baseband to RF or from RF to baseband. • Usually very accurate frequency & low-noise • Mixers: Converts baseband signal into a representation of the baseband signal at an RF frequency (and vice versa). • Based on trigonometric identity: • Baseband: source and destination for data.

  6. Why is RF Not Easy? Parasitics Minimum Capacitor model for radio frequency circuits Capacitor model for low frequency circuits • Capacitor values and their parasitics change in complex ways as they age and with varying voltages, temperatures, humidity, vibration levels, etc. • Slight changes in capacitor values and parasitics can cause great changes in circuit performance. • Other types of component types are similarly affected (e.g. transistors, inductors, resistors, etc.)

  7. Why is RF Not Easy? Component size ≈ λ λ/4 Long Circuit Board Traces with Open and Short Terminations Open Circuit becomes a short & Short Circuit becomes open • Effects of component size ≈ λ • Circuit layout more important • Components using circuit traces (e.g. Wilkinson Power Divider)

  8. Why is RF Not Easy? Super-Sensitivity Typical cell phone: sensitive to less than 10-12 Watts! Example self-generated noise interference: • Factors critical for good sensitivity performance: • Very low impedance ground • Isolation/protection from power supply • Isolation/protection from noisy (e.g. digital) circuits • Shielding of circuitry from external fields I=J*E formula integral form

  9. Typical RF Tests • Frequency Accuracy: Operating frequency • Output Power: Actual versus design • Sensitivity: Input signal where receiver begins to no longer detect the received signal. • Noise Figure: How much noise is added to the received signal. • Selectivity: Ability to only detect desired signal over undesired signal. • Dynamic Range: Signal level over which the output signal is a good replica of the input signal. • Low sensitivity end of range: Thermal and self-generated noise floor and environmental. • High sensitivity end of range: Non-linearities (amplifiers, mixer, etc.)

  10. RF Stability • Feedback from: • Circuit components • Circuit board & traces • Impurities • Step 4: Output increases until: • Device destruction • Power supply limits • Uncontrolled oscillation FEEDBACK • Instability = loss of control • Instability = unpredictable affects • May prevent other circuits from behaving properly Step 3: Input and feedback overlap and add together maximally Step 2: Part of amplified signal is fed back to input of the amplification device. + OUTPUT INPUT AMP Step 1: Input signal is amplified

  11. Stability tests • Monte Carlo simulation of circuit • Verify stable vs. production tolerances • Load pull instability tests • Vary circuit impedances to detect instabilities • Opas sweep tests • Large and small signal stimulate circuit to verify stable • On-board stability tests • Measure small signal reflections to verify stability • S-parameter stability tests • Measure circuit characteristics to verify stable

  12. Example Single Chip Radio - Microsemi/Zarlink

  13. Example Single Chip Radio - Microsemi/Zarlink

  14. Example Single Chip Radio - Texas Instruments CC1020 • Frequencies: 402 to 470 MHz, 804 to 960 MHz • Bandwidths: 12.5 kHz and 25 kHz • Price < $9 (one quantity)

  15. Example Single Chip Radio - Analog Devices ADF7020-1 • Frequencies: 135 to 650 MHz • Maximum data rate: 200 kbps • Price < $6 (one quantity)

  16. Conclusions • Design process for RF products similar to other products. • Components used in RF design implement relatively simple functions. • RF design is complex (in part) because of complex parasitics and wavelength effects. • Radio level tests required to ensure specifications and regulations being met. • Some examples of highly integrated, low-cost single chip radios described. RF DESIGN IS COMPLEX, BUT LESS SO IN RECENT YEARS THANKS TO LOW-COST SINGLE-CHIP RADIOS.

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