rf circuit design chris fuller [email protected] 952-607-8506 11/7/2012
TRANSCRIPT
RF Circuit Design
Chris [email protected]/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.