RF Telemetry for Medical Implants - MEPTEC.ORG - Medtronic - Snodgrass.pdf · RF Telemetry for...
Transcript of RF Telemetry for Medical Implants - MEPTEC.ORG - Medtronic - Snodgrass.pdf · RF Telemetry for...
RF Telemetry for Medical Implants
Kirk Snodgrass
Presented at
MEPTEC/SMTA Medical Electronics Symposium Sept 27 & 28, 2011
MICS MEDS MICS=Medical Implant
Communications Service (402 – 405 MHz)
Implant to Instrument Tx/Rx 25 uW (-16 dBm) EIRP Max 300 kHz per channel Listen Before Talk, Clear
Chanel Assessment
MEDS=Medical Data Services (401 – 402 MHz and 405 – 406 MHz)
Implant to Implant, Implant to Instrument, or Instrument to Instrument
Allows Tx-only at 250 nw (-36 dBm) EIRP or 25 uW if LBT, CCA
Max 100 kHz per channel Facilitates WBANs &
sensors
MICS and MEDS
MEDS
404.85
404.55
404.25
403.95
403.65
403.35
403.05
402.75
402.45
402.15
401-402 MHz
MICS (402-405 MHz)
405-406 MHz
MEDS
MICS= Medical Implant Communication Service (Implant to Instrument)
MEDS= Medical Data Service (WBAN)
Frequency Considerations ISM bands offer many frequency choices, but
are heavily used. Interference is an issue. Lower frequencies are less attenuated in the
body. Losses are proportional to frequency. Higher frequencies allow smaller antenna size.
(Dk of muscle and water are approx 64—shortens antenna 8x from air)
MICS/MEDS band has low interference (Shared services are Wx balloons and satellites) and employs Clear Channel Assessment and Listen Before Talk.
Signal Characteristics
Bandwidth requirements for telemetry applications. Control functions are slow, and only need a few kHz. Real Time monitor functions can take up to 100 kHz. Factory test, code maintenance, etc. Need all the
speed they can get! Hundreds of kHz are common. Typical Modulation types are FSK, BPSK,
QPSK, etc. Constant amplitude conserves bandwidth and eases transmitter linearity requirements.
Forward Error Correction Corrects randomly distributed errors. Greatly reduces error rate with low overhead. Reed-Solomon is used on CD’s, DVD’s, satellite
communications, deep space probes.....
Data Security
AES (Advanced Encryption Standard) is a robust block cipher found in common applications such as Zigbee, 802.11, etc.
Direct Conversion Receiver
Direct conversion to base band— results in a Zero IF which simplifies filtering
Uses Local Oscillator operating at the receive RF frequency
Requires careful management of the LO leakage as well as DC offsets
SAW Filter Piezoelectric device that launches transverse Surface
Acoustic Waves across a substrate (Lithium Tantalate or Lithium Niobate are common)
Interdigital combs determine the frequency response. Special structures suppress spurious responses and
minimize insertion loss. The SAW filters for 400 MHz operation are as small as 2
square mm!
Range and Power Range needs to be arm’s length to room size.
This requires an implanted transmitter output of 0.1 mW to 1 mW (400MHz).
Battery energy is limited and battery impedances are high. Low current and low voltage circuits are a must!
Duty Cycling is needed to further reduce current drain. Wake-up strategy syncs the implant with the programmer/instrument.
Crosstalk
RF testing of telemetry can require shielding or screen room isolation to avoid crosstalk between testers. This is especially important for receiver sensitivity measurements.
Telemetry Standards IEEE 802.15.6 Wireless Body Area Networks ETSI TR 102 343 (V1.1.1) ETSI EN 301 839-1 "Electromagnetic compatibility and Radio
spectrum Matters (ERM);Radio equipment in the frequency range 402 MHz to 405 MHz for Ultra Low Power Active Medical Implants and Accessories
FCC Rules and Regulations, “MICS Band Plan”, Part 95, Jan. 2003 47 CFR 95.601-95.673 Subpart E, Federal Communications
Commission, 1999 ITU-R Recommendation SA.1346 (1998) (revised to RS.1346)
(MICS)