Safety Issues with Medtronic Long Range Telemetry Implants

RF Circuit DesignChris FullerChris_Fuller@IEEE.ORG952-607-850611/7/2012

Design ProcessDefine 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 CommunicationsBasic 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-noiseMixers: 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 circuitsMinimum Capacitor model for radio frequency circuitsCapacitor 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 /4 Long Circuit Board Traces with Open and Short TerminationsOpen Circuit becomes a short & Short Circuit becomes openEffects of component size Circuit layout more importantComponents using circuit traces (e.g. Wilkinson Power Divider)

Why is RF Not Easy? Super-SensitivityTypical cell phone: sensitive to less than 10-12 Watts!Example self-generated noise interference:Factors critical for good sensitivity performance:Very low impedance groundIsolation/protection from power supplyIsolation/protection from noisy (e.g. digital) circuitsShielding of circuitry from external fieldsI=J*E formula integral formTypical 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.)10RF StabilityInstability = loss of controlInstability = unpredictable affects May prevent other circuits from behaving properlyAMPFEEDBACK+INPUTStep 3: Input and feedback overlap and add together maximallyOUTPUTStep 4: Output increases until:Device destructionPower supply limitsUncontrolled oscillationFeedback from:Circuit componentsCircuit board & tracesImpuritiesStep 1: Input signal is amplifiedStep 2: Part of amplified signal is fed back to input of the amplification device.11Stability testsMonte Carlo simulation of circuitVerify stable vs. production tolerancesLoad pull instability testsVary circuit impedances to detect instabilitiesOpas sweep testsLarge and small signal stimulate circuit to verify stableOn-board stability testsMeasure small signal reflections to verify stabilityS-parameter stability testsMeasure circuit characteristics to verify stableExample 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 CC1020Frequencies: 135 to 650 MHzMaximum data rate: 200 kbpsPrice < $6 (one quantity)Example Single Chip Radio- Analog Devices ADF7020-1

16ConclusionsDesign 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.