Abstract:
An electronic implantable device with a power saving circuit incorporates a radio frequency receiver with low power consumption. A high power radio receiver is normally turned off during a period of inactivity. When an analyzer detects a predetermined identification code in a received radio frequency signal, it outputs a signal to turn on the high power receiver.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention describes a method of saving power in an implantable device, such as a pacemaker, which includes RF telemetry functionality.  
         [0003]     2. Description of Related Art  
         [0004]     Since the allocation of a special frequency band for implantable medical devices using RF telemetry, the so-called MICS (Medical Implantable Communication Service) band, by FCC in the late 1990&#39;s, the development of devices including this functionality has really taken off. However, since the battery capacity in an implantable device is very limited, the introduction of a RF transceiver operating at 402-405 MHz becomes a real challenge. If the transceiver operates at 5 mA in the active mode, this might be acceptable since in the normal user scenario the on-time is only a fraction (&lt;0.01%) of the total device life time. A trickier problem is the issue of waking up the RF component from the off state to start communications in a reasonable amount of time without draining the battery.  
         [0005]     The most common method of solving this problem today is to introduce the so-called sniff mode. This means that the complete receiver RF portion of the device is turned on for a limited period of time (e.g. 10 milliseconds) during which time the device listens to see if there are any transmitters active in the vicinity wanting to make contact. By duty cycling the on (sniff) time heavily with the off time a considerable power saving can be achieved. For example having the device on for 5 ms consuming 5 mA and then off for 995 ms while consuming only leakage current of maybe 100 nA will lower the average current consumption to only about 25 μA. This is very good in most applications. However, for an implantable device consuming less than 10 μA in total this is unacceptable. Lowering the average power consumption further by decreasing the on time is difficult since a certain time is needed to start up the RF receiver and to receive a message telling the device to start transmitting a response. Increasing the off time is not preferred since the doctor who is trying to get in contact with the device expects a response within a second or two.  
         [0006]     An example of the prior art is found in U.S. Pat. No. 4,519,401 issued May 28, 1985.  
       SUMMARY OF THE INVENTION  
       [0007]     According to the present invention there is provided an implantable electronic device, comprising a first radio receiver for receiving telemetry data, said first radio receiver having a wake mode and a sleep mode, and being configured to be normally in said sleep mode; a second radio receiver with very low power consumption compared to said first radio receiver; a control unit for periodically turning on the second radio receiver during a sniff period to listen for an incoming radio frequency signal; an analyzer for verifying the properties of an incoming radio signal, said analyzer, in response to receipt of part of a valid wake-up message, prolonging the sniff period while the received part of the wake-up message remains valid until a complete wake-up message is received; and said analyzer further being operable to place said first radio receiver in the wake mode to receive incoming telemetry data in response to the detection of a complete valid wake-up message.  
         [0008]     The second radio receiver normally forms part of a transceiver for exchanging two-way telemetry data with an external device.  
         [0009]     Thus, in accordance with the invention, an implantable device with an RF telemetry transceiver has a separate low power receiver to wake-up the device to save power when an external RF unit wish communicate with the implanted device. The normal RF telemetry transceiver is turned off for most of the time except when there is an active telemetry link in operation.  
         [0010]     The low power receiver has a simplified architecture to drive down the power consumption to about 200 μA and operates as a wake-up device for the full RF transceiver. Because of the simplicity of the low power receiver it can also be turned on very quickly (less than 200 μs).  
         [0011]     For most of the time all the RF functionality in the implantable device is switched off and consumes less than 100 nA of leakage current. Every 1 second the lower power receiver is turned on by the application circuitry for about 0.5 ms to listen to see if there are any outside devices trying to get in contact. If an appropriate signal is received, the listen window is prolonged to enable the reception of a full wake-up message and trigger the turn on of the full-blown MICS transceiver, which starts to transmit and receive. The prolongation of the wake-up receiver&#39;s reception window is only sustained as long as the properties of the received message match a correct message. One example of such a property is to use so called Manchester encoding and to turn off the low power receiver as soon as any non-Manchester encoded signal is detected. This leads to additional power saving since the wake-up receiver is turned off immediately as soon as it becomes clear that the message is incorrect instead of receiving a complete wake-up message before checking if the message is correct or not  
         [0012]     With a power consumption of 200 μA from the low power receiver this will give a total average power consumption of 200 nA (100 nA leakage and 100 nA from the low power receiver).  
         [0013]     In accordance with another aspect the invention provides a method of saving power in an implantable electronic device having a first radio receiver for receiving telemetry data, said first radio receiver having a wake mode and a sleep mode, and being configured to be normally in said sleep mode, said method comprising periodically listening in a sniff mode for a wake-up signal with a second radio receiver that has very low power consumption compared to said first radio receiver; in response to reception of part of a wake-up signal by said second radio receiver prolonging the sniff mode while the received part of a wake-up signal remains valid until a complete wake-up signal is received; and in response to a complete valid wake-up signal, placing said first radio receiver in the wake mode to receive incoming telemetry data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:— 
         [0015]      FIG. 1  is a block diagram of an implantable device in accordance with one embodiment of the invention; and  
         [0016]      FIG. 2  is a more detailed block diagram of an embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]     Referring to  FIG. 1 , the simplified receiver comprises an antenna  1  receiving a wake-up signal (common with the MICS band antenna or separate), an amplifier  2  which amplifies the signal, and a comparator/detector  3  that detects the amplified signal if it is above a certain power level.  
         [0018]     To further increase the security of the receiver against the device being woken up by noise, the wake-up signal comprises a predetermined coded pattern, which is analyzed in an analyzer  4  to see if it matches the wake-up pattern. Only if the pattern is correct the full transceiver  5  will be turned on by the analyzer  4 . The amplifier  2 , comparator  3  and analyzer  4  form part of a simple very low power receiver.  
         [0019]      FIG. 2  shows another embodiment where an RF signal is received by a tuned antenna  6  connected to an amplifier  7 , an optional band pass filter  8 , which in turn is connected to a rectifier  9 , connected to a comparator  10 , which is connected to an analyzer  11 . The analyzer  11  is connected to the full high power RF transceiver  12 . The amplifier  7 , the band pass filter  8 , the rectifier  9 , the comparator  10  form part of a simple low power RF receiver circuit  13 . The control block  15  controls the turning on and turning off of the low power receiver  13 .  
         [0020]     The full transceiver may use the same antenna as the low power receiver as shown in  FIG. 1  or may use a separate antenna as shown in  FIG. 2  with antenna  14  shown as a separate entity.  
         [0021]     The incoming RF signal picked up by the antenna  6  is fed into the simple receiver circuit  13 . The described solution uses a Manchester encoded On/Off Keying (OOK) modulation scheme, but other modulation schemes such as Frequency Shift Keying (FSK), Phase Shift Keying (PSK), etc. can also be envisioned for those skilled in the art.  
         [0022]     The signal picked up by the antenna is amplified by the amplifier  7 , fed into the low power band-pass filter  8  that filters the signal around the chosen wake-up frequency, in or outside the MICS band. The filtered signal is then fed into the rectifier  9  and connected to the comparator  10  as a much lower frequency signal. The comparator  10  acts as a decoder that decodes the incoming RF-signal (if any), and if the level is above the comparators threshold, which can be made programmable, starts to convert the signal into logical ones ‘1’ and zeros ‘0’. The digital signal from the comparator  10  is fed into an analyzer  11  where it checked to see if it is a Manchester encoded signal and if so is compared to a predetermined digital signal pattern and if the incoming signal matches this pattern the analyzer turns on the full RF transceiver, which starts the full RF transmission. If there is no matching Manchester encoded signal detected within the 0.5 ms window the device just goes back to sleep until the next 0.5 ms on time 995.5 ms later.  
         [0023]     For greater noise immunity it is an preferable to code the incoming signal using a more sophisticated scheme than one where the presence of signal represents a ‘1’ and absence of signal represents a ‘0’. Examples include pulse width modulation (PWM) where a long presence of a signal in a time slot represents a ‘1’ and a short presence represents a ‘0’. Alternatively the signal may be amplitude modulated using Pulse Position Modulation (PPM) or Pulse Amplitude Modulation (PAM), provided a suitable analyzer  14  is used.  
         [0024]     An additional level of security can be achieved by letting the detection of the correct signal within the first 5 ms trigger a prolongation of the low power receiver to allow a longer coding pattern to be used before turning on the full transceiver. In the invention the prolongation only continuous as long as the received pattern is Manchester code and thus matches the expected properties. However, those skilled in the art will appreciate that the said expected properties can mean any coding pattern as well as the correct pulse width, correct pulse position, correct frequency correct pulse amplitude etc. etc. This is done even before the digital wake-up message is decoded given the possibility of immediately going back to sleep as soon as the incoming message has the incorrect properties.  
         [0025]     The simplicity of the receiver  13  makes it very difficult to achieve very good receiver sensitivity. In order to attain a reasonable wake-up range it can be advantageous to use another frequency band than the MICS band, which is very limited in the allowed output power (maximum 25 μW). Examples of such frequency bands that can be used are the ISM band at 2.45 GHz, the US ISM band at 902-928 MHz, the Short Range devices band at 868 MHz in Europe. These bands all have a much higher power limit than the MICS band.  
         [0026]     The described circuitry lends itself to integration in a single chip, for example, using CMOS technology.  
         [0027]     It will be appreciated by one skilled in the art that the above description represents an exemplary embodiment, and that many variants within the scope of the appended claims are possible without departing from the scope of the invention.