Abstract:
A radio frequency temperature sensor and a method of calibration temperature therefor are disclosed. An active radio frequency temperature sensor including a ring oscillator, a memory, a frequency counter, a radio frequency transmission interface and a micro-controller is used to calibrate and verify the efficacy of the radio frequency temperature sensor. Thereafter, a passive radio frequency temperature sensor including a regulator, a clock extractor, a ring oscillator, a memory, a frequency counter, a modulator and a state-machine is developed according to the verified results.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims the priority benefit of Taiwan application serial no.92136368, filed on Dec. 22, 2003. 
   BACKGROUND OF INVENTION 
   1. Field of the Invention 
   The present invention relates to a radio frequency measuring device, and more particularly to a radio frequency temperature sensor and a method of calibrating temperature therefor. 
   2. Description of the Related Art 
   Temperature, humidity and pressure are parameters commonly measured at home, offices and in manufacturing plants. Under special circumstances, for example, it is impossible to measure the tire pressure using a conventional measuring device when a car is moving. Accordingly, a radio frequency circuit for remotely and precisely measuring these parameters is required. In conventional art, the radio frequency circuit, the circuit is applied to the measuring devices for measuring temperature, humidity and pressure, and then the measured data is transmitted. 
   For designing the measuring devices with different applications, the radio frequency integrated circuit is designed and then the measuring devices are being verified for its accuracy or calibrated by modifying the relevant circuits for functioning accurately. However, the prior art method is time consuming and costly. Moreover, the transmission method and efficiency thereof cannot be verified whether the circuit is desired. 
   SUMMARY OF INVENTION 
   In the view of the foregoing, the present invention provides a radio frequency temperature sensor and a method of calibrating the radio frequency for verifying its accuracy and the efficacy or for improving its accuracy and efficiency. Moreover, the sensor of the present invention is applied to a passive radio frequency temperature sensor. 
   The present invention provides a radio frequency temperature sensor. The radio frequency temperature sensor comprises: a ring oscillator, a memory, a frequency counter, a radio frequency transmission interface, and a micro-controller. The ring oscillator is adapted to generate an oscillating signal frequency corresponding to a measured temperature. The memory is adapted to store an initial value. The frequency counter is coupled to the memory and the ring oscillator, and adapted to measure a temperature error in response to the measured temperature according to the initial value and the oscillating signal in a pre-set time. The radio frequency transmission interface serves as a transmission interface with a card reader. The micro-controller is coupled to the frequency counter, the memory and the radio frequency transmission interface, and adapted to transmit the initial value to the frequency counter, to control starting and ending of the pre-set time, to read the temperature error and to communicate with the card reader through the radio frequency transmission interface. 
   According to an embodiment of the present invention, the ring oscillator comprises: a thermal resistor, a capacitor, a Schmitt inverter, an inverter and a NAND gate. The thermal resistor has a first terminal and a second terminal. The capacitor has a first terminal and a second terminal, the first terminal of the capacitor coupled to the second terminal of the thermal resistor, and the second terminal of the capacitor grounded. The Schmitt inverter has an input terminal coupled to the second terminal of the thermal resistor, and an output terminal. The inverter has an input terminal coupled to the output terminal of the Schmitt inverter, and an output terminal. The NAND gate has a first input terminal, a second input terminal and an output terminal. The first input terminal of the NAND gate is adapted to receive an enable signal. The output terminal is coupled to the first terminal of the thermal resistor. The second input terminal is coupled to the output terminal of the inverter and adapted to output the oscillating signal. 
   According to an embodiment of the present invention, the memory of the radio frequency temperature sensor is a non-volatile memory. 
   In the embodiment, the frequency counter of the radio frequency temperature sensor is a down counter. 
   According to an embodiment of the present invention, the radio frequency transmission interface comprises: an antenna, a diode and a Zener diode. The antenna comprises an inductor coupled to a capacitor in parallel. An anode of the diode is coupled to a terminal of the antenna. An anode of the Zener diode is coupled to another terminal of the antenna, and a cathode of the Zener diode is coupled to a cathode of the diode. 
   According to an embodiment of the present invention, the micro-controller transforms the temperature error into the measured temperature according to a temperature reference table. 
   According to an embodiment of the present invention, the micro-controller communicates with the card reader through a general input/output port thereof. 
   The present invention also provides a method of calibrating temperature suitable for a radio frequency temperature sensor comprising a ring oscillator, a memory and a frequency counter. The method comprises: generating an oscillating signal related to a standard measuring temperature, such as 40° C., by the ring oscillator; measuring a frequency count value of the frequency counter according to the oscillating signal in a pre-set time; and storing the frequency count value in the memory as a initial value of the frequency counter. 
   The present invention provides a radio frequency temperature sensor according to another embodiment, which comprises a regulator, a frequency extractor, a ring oscillator, a memory, a frequency counter, a modulator and a state machine. The regulator is adapted to receive and transform an oscillating current of an antenna into a working voltage of the radio frequency temperature sensor. The frequency extractor is adapted to generate a clock signal of the radio frequency temperature sensor according to the signal received from the antenna. The ring oscillator is adapted to generate an oscillating signal frequency corresponding to a measured temperature. The memory is adapted to store an initial value. The frequency counter is coupled to the memory and the ring oscillator and adapted to measure a temperature error in response to the measured temperature according to the initial value and the oscillating signal in a pre-set time. The modulator serves as a transmission interface with a card reader. The state machine is coupled to the frequency counter, the memory and the modulator, and adapted to transmit the initial value to the frequency counter, to control starting and ending of the pre-set time, to read the temperature error and to communicate with the card reader through the modulator. 
   According to an embodiment of the present invention, the ring oscillator comprises: a thermal resistor, a capacitor, a Schmitt inverter, an inverter and a NAND gate. The thermal resistor has a first terminal and a second terminal. The capacitor has a first terminal and a second terminal. The first terminal of the capacitor is coupled to the second terminal of the thermal resistor, and the second terminal of the capacitor is grounded. The Schmitt inverter has an input terminal coupled to the second terminal of the thermal resistor, and an output terminal. The inverter has an input terminal coupled to the output terminal of the Schmitt inverter, and an output terminal. The NAND gate has a first input terminal, a second input terminal and an output terminal. The first input terminal of the NAND gate is adapted to receive an enable signal. The output terminal is coupled to the first terminal of the thermal resistor. The second input terminal is coupled to the output terminal of the inverter and adapted to output the oscillating signal. 
   According to an embodiment of the present invention, the memory of the radio frequency temperature sensor is a non-volatile memory. 
   According to an embodiment of the present invention, the frequency counter of the radio frequency temperature sensor is a down counter. 
   In order to make the aforementioned and other objects, features and advantages of the present invention understandable, a preferred embodiment accompanied with figures is described in detail below. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a block diagram showing an active radio frequency temperature sensor according to an embodiment of the present invention. 
       FIG. 2  is a drawing showing a ring oscillator according to an embodiment of the present invention. 
       FIG. 3  is an operation waveform of the radio frequency transmission interface  140  of the present invention when the card reader is reading data. 
       FIG. 4  is an operation waveform of the radio frequency transmission interface  140  of the present invention when the card reader is writing data. 
       FIG. 5  is a block diagram showing a passive radio frequency temperature sensor according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a block diagram showing an active radio frequency temperature sensor according to an embodiment of the present invention. Referring to  FIG. 1 , the radio frequency temperature sensor  100  comprises a ring oscillator  110 , a memory  120 , such as a non-volatile memory, and a frequency counter  130  for measuring temperature. The measuring principle is described below. 
   First, the ring oscillator  110  is adapted to generate an oscillating signal Clock_in frequency corresponding a measured temperature. The oscillating signal serves as the clock pulse of the frequency counter  130 .  FIG. 2  is a drawing showing a ring oscillator according to an embodiment of the present invention. Referring to  FIG. 2 , the ring oscillator  110  comprises a thermal resistor  210 , a capacitor  220 , a Schmitt inverter  230 , an inverter  240  and a NAND gate  250 . 
   Referring to  FIG. 2 , the thermal resistor  210  comprises a first terminal and a second terminal. The capacitor  220  comprises a first terminal and a second terminal, the first terminal of the capacitor  220  coupled to the second terminal of the thermal resistor  210 , and the second terminal of the capacitor  220  grounded. The Schmitt inverter  230  has an input terminal coupled to the second terminal of the thermal resistor  210 , and an output terminal. The inverter  240  has an input terminal coupled to the output terminal of the Schmitt inverter  230 , and an output terminal. The NAND gate  250  has a first input terminal, a second input terminal and an output terminal. The first input terminal of the NAND gate  250  is adapted to receive an enable signal En. The output terminal is coupled to the first terminal of the thermal resistor  210  forming a ring circuit. The second input terminal is coupled to the output terminal of the inverter  240  and adapted to output the oscillating signal Clock_In frequency corresponding to the measured temperature. The variation of the frequency of the oscillating signal Clock_In results from RC delay of the circuit by the variation of the resistance of the thermal resistor  210  in response to temperature. 
   Referring to  FIG. 1 , the thermal resistor of the ring oscillator  110  has an error of thermal constant (B-constant) less than 1%, and an absolute error about 5%. In addition, the capacitor of the ring oscillator  110  has a capacitance error about 5%. In the worst case, the error of the ring oscillator  110  is about 10%. Without adequate calibration, the error of the measured temperature will be not acceptable. The memory  120  serves to store the initial value of the frequency counter  130  as to calibrate the error resulting from the thermal resistor and the capacitor of the ring oscillator  110 . 
   During calibration, the ring oscillator  110  is adapted to generate the oscillating signal Clock_In according to the standard measuring temperature, such as 40° C. By the oscillating signal Clock_in, a frequency count value, such as 2,800, of the frequency counter  130  in a pre-set time is measured. The frequency count value, 2,800, is stored in the memory serves as the initial value of the frequency counter  130 . 
   When the actual temperature is measured, the initial value stored in the memory  120  is transmitted to the frequency counter  130 , which can be, for example, a down counter. The initial value is regularly reduced during the pre-set time for obtaining the temperature error with standard temperature 40° C. In other words, when the measured temperature is 40° C., the count value of the frequency counter  130  is 0; when the measured temperature is lower than 40° C., the count value of the frequency counter  130  is less than 0 because of the higher frequency of the oscillating signal Clock_In of the ring oscillator  110 ; when the measured temperature is higher than 40° C., the count value of the frequency counter  130  is more than 0 because of the lower frequency of the oscillating signal Clock_In of the ring oscillator  110 . The temperature error can be transformed into the measured temperature according to a temperature reference table stored in the memory  120  or a card reader  200 . When the temperature range to be measured is large, a plurality of frequency count values can be measured as initial value of the frequency counter  130  to obtain a precise result. For example, when the temperature range is from 0–70° C., the initial values of the frequency counter  130  can be measured at 0, 35 and 70° C. for measuring the other temperatures. 
   Accordingly, the active radio frequency temperature sensor of the present invention is used to calibrate and verify the efficiency thereof. The present invention uses a radio frequency transmission interface  140  and a micro-controller  150  for transmitting the initial value to the frequency counter  130 , for controlling the starting and ending of the pre-set time, for reading the temperature error, and for communicating with the card reader  200  thereby. The power of the radio frequency temperature sensor  100  is supplied by an external circuit (not shown). The micro-controller  150  has a general input/output port (GPIO) for communicating with the card reader  200 . Following are the descriptions of the communication theory. 
   Referring to  FIG. 1 , the radio frequency transmission interface  140  comprises: an antenna, comprising an inductor  141  coupled to a capacitor  142  in parallel, a diode  143  and a Zener diode  144 . An anode of the diode  143  is coupled to a terminal of the antenna  141 . An anode of the Zener diode  144  is coupled to another terminal of the antenna, and a cathode of the Zener diode  144  is coupled to a cathode of the diode  143 . 
   The resonance frequency of the inductor  141  and the capacitor  142  and the frequency of a carrier signal should be set with same value. When resonance occurs, another terminal of the antenna generates a sinusoid signal. When the sinusoid signal is in negative bias and the GPIO of the micro-controller  150  is coupled to the terminal of the antenna, current is extracted from GPIO of the micro-controller  150  as to result in latch-up effect. Therefore, the GPIO of the micro-controller  150  is coupled to the terminal of the antenna through the diode  143  for filtering the negative bias. 
   The closer the antenna to the card reader  200 , the larger the voltage sensed thereby. If the sensed voltage is not restricted, the device may be damaged when the sensed voltage is higher than the breakdown voltage of the devices of the micro-controller  150 . Accordingly, the Zener diode  144  is used to clamp the sensed voltage for protecting the devices. 
     FIG. 3  is an operation waveform of the radio frequency transmission interface  140  of the present invention when the card reader is reading data. Generally, the GPIO of the micro-controller  150  can be set as floating, pull-low-resistor mode, or output low mode by setting the register thereof. In these states, the radio frequency transmission interface  140  has the highest equivalent resistance in floating, the middle resistance in pull-low-resistor mode and the lowest resistance in output low mode. By the variation of the resistance, the amplitude of the carrier signal is changed for transmitting data to the card reader  200 . 
   Referring to  FIG. 1 , when the card reader  200  transmits the carrier signal, and the radio frequency temperature sensor  100  is close to the card reader  200 , the antenna of the radio frequency transmission interface  140  senses the signal and the resonance occurs. Accordingly, the sinusoid signal shows up at the terminal of the antenna. If the signals to be transmitted by the radio frequency temperature sensor  100  are 101001, the GPIO of the micro-controller  150  is set as floating, pull-low-resistor mode, floating, pull-low-resistor mode, pull-low-resistor mode and floating. Therefore, the waveforms at the antenna terminal of the radio frequency transmission interface  140 , the GPIO of the micro-controller  150 , and the logic level of the signal are shown in  FIGS. 3(   a ), ( b ) and ( c ), respectively. Accordingly, the data can be transmitted to the card reader. Although the operation above is performed according to floating and pull-low-resistor mode, it is not limited thereto. One of ordinary skilled in the art can understand that any two of the three states can be applied to perform the operation. 
     FIG. 4 , illustrate an operation waveform of the radio frequency transmission interface  140  of the present invention when the card reader is writing data. When the card reader  200  is in writing mode, the GPIO of the micro-controller  150  is set as pull-low-resistor mode. Then, the card reader  200  transmits the waveform shown in  FIG. 4(   a ). In other words, the sinusoid signal shows up periodically for representing 1 and 0. The radio frequency temperature sensor  100  receives the signal by the antenna, which is filtered by the diode  143  as to generate the waveform of  FIG. 4(   b ). If the parasitic capacitor of the GPIO of the micro-controller  150  is huge enough, the waveform generated at the GIPO of the micro-controller  150  is shown as  FIG. 4(   c ). Therefore, the waveform shown in  FIG. 4(   d ) can be obtained by sampling the voltage of the GIPO of the micro-controller  150  to recover the data transmitted from the card reader  200 . Accordingly, the data can be written into the radio frequency temperature sensor  100 . 
   The descriptions above are about the active radio frequency temperature sensor, which uses an external power supply. After the verification, the circuit is transformed into a passive radio frequency integrated circuit without the external circuit. 
     FIG. 5  is a block diagram showing a preferred passive radio frequency temperature sensor of the present invention. Referring to  FIG. 5 , the radio frequency temperature sensor  500  comprises: a regulator  560 , a frequency extractor  570 , a ring oscillator  510 , a memory  520 , a frequency counter  530 , a modulator  540  and a state machine  550 . Without the external power supply, the regulator  560  is used to extract the current from the antenna  580 , transferring the current into the working voltage of the radio frequency temperature sensor  500 . In addition, the frequency extractor  570  generates the clock signal of the radio frequency temperature sensor  500  according to the signal received from the antenna  580 . The modulator  540  serves as the transmission interface communicated with the card reader  200 . The state machine  550  transmits the initial value stored in the memory  520  to the frequency counter  530 , controls the starting and ending of the pre-set time, reads the temperature error and communicates with the card reader  200  through the modulator  540  for replacing the micro-controller  150  in  FIG. 1 . The detail descriptions of the ring oscillator  510 , the memory  520  and the frequency counter  530  are similar to those of  FIG. 1 , and therefore detailed description thereof are not repeated hereinafter. 
   Although the present invention has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be constructed broadly to include other variants and embodiments of the invention which may be made by those skilled in the field of this art without departing from the scope and range of equivalents of the invention.