Quartz crystals for remote patient monitoring systems
26 July 2018
Remote patient monitoring is no longer something from the realms of science fiction, but a medical reality enabled by advanced electronic systems. This piece explains how ultra-miniature quartz crystals designed for use in medical-implantable wireless RF transceiver applications are helping enable these remote patient monitoring systems.
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In the early days, the technology used was an inductive pick up communications link. But using conventional inductive link methods of providing communication between a programmer/controller (outside the body) and the implanted device has severe limitations. For instance, the maximum separation between the two inductive coils, one inside and the other outside the body, must not exceed 60mm – meaning that the external coil must be kept close to the body.
The other major disadvantage is that the rate of data transfer was incredibly slow: you could liken it to a dial-up internet connection, compared with today’s high-speed broadband. Millions of people suffer from chronic diseases, such as heart conditions, pain, diabetes or hypertension.
Early diagnosis is vital so that doctors can provide the necessary medical intervention for patients. Active implant devices with wireless capabilities can be used to diagnose and raise alerts to support human life.
There is a myriad of devices in modern medicine suitable for implanting, such as implanted cardiac defibrillators (ICD), pacemakers, neuro-stimulators, drug pumps and so on. Having control external to the patient is necessary for different purposes, including device parameter adjustment, transmission of stored information and real-time transmission of vital monitoring information for short periods.
Having control external to the patient is necessary for different purposes, including device parameter adjustment, transmission of stored information and real-time transmission of vital monitoring information for short periods.
This is where medical implant communication systems (MICS) come into their own – and in 1999, international protocols were agreed and a frequency band of 402-405MHz was set aside. This frequency band is ideal for implant applications, as it coincides with the signal propagation characteristics of the human body.
Protocols were agreed to minimise interference and ensure safe coexistence of multiple MICS devices; hence the band is broken into 300kHz wide channels. These protocols specify that devices must ‘listen’ for other devices before transmitting; if interference is encountered, the radio switches channels and listens again.
This is termed as adaptive frequency agility, and allows for safe data transmission, free from interference.
The quartz crystals required for MICS must offer extreme reliability, be ultra-miniature in size and optimised for wireless RF transceiver operation using Bluetooth Low Energy (BLE) or Medical Implant Communication Service specifications.
It is important that the crystals exhibit very good aging characteristics, as significant frequency changes or current consumption increases could have serious consequences for the system integrity. The crystals ideally should be micro-miniature, with dimensions such as 2.0 x 1.0mm or 1.6 x 1.0mm to keep transmitter size to a minimum.
The crystal manufacturer must have the capability for particle impact noise detection (PIND) testing, acoustic interference testing and X-ray capability. Furthermore, the screening required for crystals used in implants is quite onerous, as the potential for litigation in the event of failure could be catastrophic for equipment manufacturers.
Clearly any remote monitoring system needs to be accurate and reliable, therefore signal integrity is critical. Factors such as phase noise and phase jitter become a major issue: if these are poor, then the signal integrity could be compromised.
Phase noise can be defined as the noise arising from the short-term phase fluctuations that occur in a signal. The fluctuations manifest themselves as sidebands, which appear as a noise spectrum spreading out either side of the signal.
Phase jitter can be defined as the deviations in a signal output transition from their ideal positions, and this deviation can either be leading or lagging the ideal position.
Phase noise can be generated within a circuit; these noise sources may be:
a) the noise profile of active components, such as transistors, ICs, voltage regulators and so on
b) thermal noise in passive components
c) flicker noise in active components, sometimes called pink noise
d) poor system ‘Q’ that may also contribute to the problem
Jitter measurements can generally be classified into three categories:
- Cycle-to-cycle jitter is the change in a signal output transition from its corresponding position in the previous cycle
- Period jitter is the maximum change in a signal output transition from its ideal position
- Long-term jitter is an aging characteristic of a system, where jitter elements develop with time
As the deployment of MICS increases, the cost of the transmitters will reduce and will provide more routine monitoring of many health conditions remotely. This frees up scarce hospital resources and brings further cost benefits to health services. The benefits to patients are better targeted access to healthcare, an improvement in the quality of care, daily peace of mind and reassurance.
This rapidly growing area of technology continues to develop apace, with more component and system improvements expected to be developed and released in a very short period.
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