Abstract:
An electronic device with a power saving circuit incorporates a radio frequency receiver with low power consumption and including a frequency down converter. A functional device, which may be a high power radio receiver, is normally turned off during a period of inactivity. An analyzer detects a predetermined identification code in a received radio frequency signal and outputs a signal to turn on the functional device.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to communication devices, and in particular to a method of saving power in such devices and a power saving circuit for use with such devices.  
           [0003]    2. Description of Related Art  
           [0004]    More and more electronic devices are employing RF operating at higher and higher frequencies for communications. Similar devices exist using infrared and optical communication means. Such devices today often operate in the region of many hundreds of MHz to GHz. Such high frequencies imply high power consumption. Since the devices are generally battery powered, power saving becomes an important issue. Many current communication devices consume too much power to leave the communication function on all the time. This is especially true in the case of ultra low power RF devices used, for example, in medical applications. RF can also be used as a power switch to turn on and off a complete function within a device that may or may not use RF.  
           [0005]    In RF modem devices it is generally not necessary to have the RF function on at 100% level of operation constantly. Therefore the RF function often has power saving features. There are four main techniques in use today.  
           [0006]    The first technique is to let the user switch the device off when the device is not in use. This results in the most efficient power saving but is often not acceptable, e.g., in a mobile phone or a pacemaker.  
           [0007]    The second technique is to switch the RF function off for a period of time, known as the sleep mode, for example, 30 seconds, when the communication link is inactive, and then switch it on for a few seconds to decide if an RF signal is present. If so the device is restored to full functionality; if not the device goes into sleep mode again.  
           [0008]    In the third technique the RF receiver is not fully on; instead it only listens to determine whether an RF signal is present that might want to communicate with the device. If the receiver circuit in power down mode is made simple or operates on a much lower frequency than the normal carrier, a lot of current can be saved. If the device detects a signal while in power down mode, it returns to full functionality and starts detection to determine whether the detected signal is intended for the device.  
           [0009]    A fourth technique relies on a combination of the second and third techniques.  
           [0010]    The disadvantages with second technique are that in some cases the user expects to be serviced very rapidly. If the device is turned on too frequently to detect whether a signal is present, the advantages of power saving are largely lost.  
           [0011]    The disadvantage with the third technique are that if the receiver is made to have a low power consumption, it will generally be very simple and is susceptible of detecting a much wider spectrum of signals than intended. With the amount of RF signals in the air today, the low power receiver is liable to wake up the full RF functionality almost all the time. In certain environments the power saving functionality can be effectively lost.  
         SUMMARY OF THE INVENTION  
         [0012]    According to the present invention there is provided a power saving circuit for activating an electronic device with a sleep mode and a wake-up mode, comprising a radio frequency receiver with low power consumption including a down-converter for reducing the frequency of a received signal; a decoder for decoding a received radio frequency signal; and an analyzer for detecting a predetermined identification code in said received radio frequency signal and activating said electronic device in response to detection of said predetermined identification code.  
           [0013]    Down-conversion of the received signal significantly increases the power saving.  
           [0014]    The method in accordance with the invention uses a low power simplified receiver with a down conversion of the received frequency included to save power wherein an identification scheme is employed during the low energy wake-up stage. The identification code, which is unique for the particular device requested or family of devices, is present in the initial RF-transmission. The ability to detect and identify the code is already-present at the low power receiver stage. The advantage of this arrangement is twofold: First even if a very simple low power receiver is used, which detects signals all the time, the identification code associated with the wake-up signal prevents it from starting any high power RF functionality. Secondly the down conversion increases the power saving even further because the rest of the low power receiver operates on kHz. signals instead of MHz.  
           [0015]    The method can be applied in stand-alone or other low power devices that need to be in low power mode for long periods of time and then activated and deactivated with the described RF signal. These devices may or may not employ RF themselves. For example, the electronic device can be a receiver of infrared or optical signals.  
           [0016]    In another aspect the invention provides a method of saving power in a remote electronic device having a sleep mode and a wake-up mode, comprising transmitting a radio frequency signal with an identification code; receiving said radio frequency signal at said remote device in the sleep mode; down-converting said received radio frequency signal; decoding said down-converted received radio frequency signal; and in response to detection of a predetermined identification code in said received radio frequency signal, switching said remote electronic device to the wake-up mode. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The invention will now be described in more detail, by way of example only, with reference to the accompanying drawings, in which:— 
         [0018]    [0018]FIG. 1 is a block diagram of a first embodiment of a power saving circuit; and  
         [0019]    [0019]FIG. 2 is a block diagram of a second embodiment of a power saving circuit. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0020]    In the embodiments, like parts have like reference numerals.  
         [0021]    In FIG. 1, an RF signal is received by an antenna  10 . The antenna is connected to a simple low power receiver  17 , which is constantly listening to incoming signals, and an ordinary high power receiver  15 , which is normally in a sleep mode in which it is turned off or consumes minimal power.  
         [0022]    Within the low power receiver  17 , the antenna  10  is connected to a receiver chain consisting of a simple band pass filter  11 , a down converter  12 , a comparator  13 , and an analyzer  14 . In the embodiment shown, the analyzer  14  is connected to a high power RF device  16  and the ordinary high power receiver  15 . After filtering in the band pass filter  11 , the incoming signal is down-converted by the down-converter  12  and then passed through the comparator  13 , which acts as a simple decoder.  
         [0023]    The antenna  10  is also connected to an ordinary high power receiver  15 , which is used for receiving data when in wake-up mode. The ordinary high power receiver  15  may or may not be used to communicate with the high power device  16 .  
         [0024]    The simple receiver  17  can be made totally separate, complete with its own antenna, or it can use part of the main RF communication system. The external RF signal to wake up the device may or may not use the same frequency as is used for normal data communication.  
         [0025]    The incoming RF signal picked up by the antenna is fed into the simple receiver circuit  17 . The modulation can, for example, be Frequency Shift Keying (FSK), which is significantly lowered in frequency by the down converter. This reduction in frequency enables power saving in both the comparator  13  and the analyzer  14 . The down-converter can reduce the frequency of an incoming signal from the MHz to KHz range.  
         [0026]    The analyzer  14  includes digital logic to separate out logic ones in the FSK signal. The timing of the frequency shifts and the embedded digital code are used to prevent the receiver  17  from turning on the ordinary high power RF receiver  15 . Alternatively, the signal may be amplitude modulated using Pulse Position Modulation (PPM), Pulse Width Modulation (PWM) or Pulse Amplitude Modulation (PAM), provided a suitable analyzer  14  is used.  
         [0027]    The frequency band used to wake-up the device can be different from the one used for ordinary communication, thus enabling higher power levels in the wake-up signal than otherwise allowed (many frequency bands used in various data applications have an upper power limit).  
         [0028]    It will be appreciated that modulation schemes other than the above mentioned can be employed as will be understood by one skilled in the art.  
         [0029]    In this exemplary embodiment, the antenna  10  receiving the RF signal is a wide band antenna enabling the frequency of the wake-up signal to be different from the ordinary frequency without using a separate antenna. However, in an alternative embodiment, a separate antenna can be employed.  
         [0030]    The signal picked up by the antenna is fed into the low power band-pass filter  11 , which filters the incoming signal around the chosen wake-up frequency, e.g., 27 MHz. The filtered signal is then fed into the comparator  13  that after the down conversion detects an incoming signal with an amplitude higher than a preset level and outputs a digital signal to the analyzer. The comparator  13  thus acts as a decoder that decodes the incoming RF-signal (if any), and if the frequency is correct starts to convert the signal into logical ones ‘1’ and zeros ‘0’. The analyzer  14  then compares the digital bit stream with a preset wake up code stored in memory, and if found correct turns on the ordinary high power RF receiver  15  or another high power device  16 . The receiver  15  then receives the incoming signal in its wake-up mode.  
         [0031]    The receiver  15  and high power device  16  are programmed to return to the sleep mode after a predetermined period of inactivity.  
         [0032]    To further improve the power saving function, use can be made of a so-called RF tag as shown in the embodiment illustrated in FIG. 2. A tag is used in circuits that do not have a battery. Such circuits include ID cards for door opening systems. The tag has a pick-up coil  20  and a rectifier  21  that half wave rectifies the incoming signal and feeds it into two branches. One branch has a low pass filter  23  with a very long time constant (τ) that turns the rectified signal into a DC voltage that powers the electronic circuitry of the device. The other branch feeds the signal into another low pass filter  22  with a much shorter time constant (τ). Also present in the incoming RF carrier signal is communication information carrying data. The data can, for instance, be a simple pulse-pause scheme which is easily detected by the comparator  13  as two different voltage levels. Thus the incoming RF signal can contain an address of the device for which this particular communication is intended. The address can be made unique for each individual device or for the same type of devices depending on the application.  
         [0033]    If the device reads the correct incoming code it turns on the full RF functionality. Otherwise it just continues to be ready to detect the correct signal. The tag part of the device does not consume any power and thus can be ready a 100% of the time without wasting any power. In this case the incoming frequency is lowered into the 100 Hz to 1 kHz range with blocks  21  and  23 . This is desirable since otherwise can it be difficult to get enough power to supply the remainder of the circuitry.  
         [0034]    In FIG. 2, the coil  20  picks up the RF signal, at e.g. 70 kHz. The energy is then converted in the rectifier  21  and filter  22  to a DC voltage that supplies power to blocks  23 ,  13 ,  25 , and  14 , which together form the RF receiver. The low pass filter  23  filters the rectified signal and feeds it to the comparator  13 , where it is converted a square wave. The square wave passes into the decoder  25 , where the two different voltage levels are converted into ones and zeros. The stream of bits is then compared to a register in the analyzer  26 , and if the decoded signal matches the stored code, the ordinary high power RF receiver  15  is turned on. The receiver then starts to receive the ordinary RF signal from the antenna  10 .  
         [0035]    A correct detected code by the analyzer  26  can also turn on a separate high power block/chip  16  within the device.  
         [0036]    Alternatively the coil  20  may be replaced by an antenna suitable for the frequency to be used. The proposed solution can also be used without converting the RF signal to the supply voltage for blocks  23 ,  13 ,  25  and  14 .  
         [0037]    The described circuitry lends itself to integration in a single chip, for example, using CMOS technology.  
         [0038]    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.