RF Circuit Design

Chris FullerChris_Fuller@IEEE.ORG952-607-850611/7/2012

Design Process

Define RequirementsDesign PrototypeDesign ReviewBuild TestAnalysesReviewIterate Design Process

Define RequirementsCommunication distance Data Rates including securityPhysical space availableAvailable battery energyCommunication media: air, metal, tissueUnit Price goalAvailable development timeCost/Availability of componentsInterference tolerance/likelihoodOperating FrequencyMany others

Overview of Radio Communications

Basic transceiver components: Antennas, Amplifiers, Mixers, Filters, Synthesizer, Baseband Processing

ComponentsAntennas: Interfaces communication media (air, body, etc.) to transceiver PA (Power Amplifier): Boosts modulated transmit signalLNA (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 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

Typical RF TestsFrequency Accuracy: Operating frequencyOutput Power: Actual versus designSensitivity: 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.)

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RF Stability

Instability = loss of controlInstability = 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.

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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

Example Single Chip Radio- Microsemi/Zarlink

Frequencies: 402 to 470 MHz, 804 to 960 MHzBandwidths: 12.5 kHz and 25 kHzPrice < $9 (one quantity)

Example Single Chip Radio- Texas Instruments CC1020

Frequencies: 135 to 650 MHzMaximum data rate: 200 kbpsPrice < $6 (one quantity)

Example Single Chip Radio- Analog Devices ADF7020-1

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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.