RF Circuit Design Chris Fuller /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 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

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

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.

Why is RF Not Easy? Parasitics Capacitor model for low frequency circuits Minimum Capacitor model for radio 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.)

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)

Why is RF Not Easy? Super-Sensitivity Typical cell phone: sensitive to less than 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

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

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

Example Single Chip Radio - Microsemi/Zarlink

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

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

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.