Patent Publication Number: US-2004046542-A1

Title: Power measurement apparatus and method thereof

Description:
TECHNICAL FIELD  
       [0001] The present invention relates, in general, to an apparatus for and a method of measuring a power of a Radio Frequency (RF) signal or a Microwave Frequency (MF) signal using thermoelectric means.  
       BACKGROUND ART  
       [0002] The signal power of RF or MF systems ultimately determines the use and performance of the system. It is therefore necessary to accurately sense and measure the signal power of the system during its lifetime in order to monitor and assess its efficiency and performance. In order to sense and measure the signal power at radio or microwave frequencies, thermoelectric devices such as a thermistor or a thermocouple are typically employed to convert the power of an incident RF or MF signal into a measurable DC or low frequency signal.  
       [0003] Thermistor sensors are normally mounted within either coaxial or waveguide structures compatible with common transmission line systems used for RF or MF signals. A resistive element, for example, a 50 Ohm resistor contained within a thermal conducting block is used to dissipate power from an incident RF or MF signal to a thermistor device in the form of heat. The thermistor device is therefore disposed in close, thermal contact with the resistive element. In operation, the thermistor device is maintained at a constant resistance by means of a DC or low frequency AC bias. As the power of an incident RF or MF signal is dissipated to the thermistor device its resistance falls. As the resistance of the thermistor device falls, the bias supplied to the thermistor device is reduced by a corresponding amount in order to keep the resistance of the thermistor device constant. The decrease in bias supplied is therefore proportional to the power of an incident RF or MF signal and, following calibration, this relationship provides the user with a measurement of the power of the incident RF or MF signal.  
       [0004] A thermocouple sensor is a semiconductor device also comprising a resistive element, for example, a thin film resistor. The resistive element is disposed in intimate contact with a silicon chip having ‘hot’ and ‘cold’ junctions manufactured from suitable materials known in the art to form the thermocouple device. In operation, the resistive element converts the power of an incident RF or MF signal into heat which is conducted to the chip thereby forming a thermal gradient across the thermocouple device. The thermal gradient in turn gives rise to a thermoelectric potential difference. The thermoelectric potential difference provides a DC output voltage, the amplitude of which is proportional to the power of the incident RF or MF signal and following calibration, this relationship provides the user with a measurement of the power of the incident RF or MF signal.  
       [0005] As discussed above, known thermoelectric sensors comprise a resistive element to detect the RF or MF signal and dissipate the power of the RF or MF signal to a thermistor or thermocouple device as appropriate. The sensitivity and response time of such thermoelectric devices is therefore dependent upon the efficiency of the resistive element in dissipating power to the thermoelectric device. Known thermoelectric devices may suffer from ‘thermal lag’; a condition which increases response time, that is, the time taken for the power of the incident RF or MF signal to be dissipated to the thermoelectric device by way of conduction. Furthermore, as it is only power dissipated to the thermoelectric device which provides the user with a power reading, any losses incurred by inefficient conduction from the resistive element to the thermoelectric device can result in inaccurate power measurements. High frequency modern communications networks push such known thermoelectric devices to their operating limits whilst also demanding flexibility and compatibility of the thermoelectric devices with a range of electronic and communications measurement applications.  
       DISCLOSURE OF INVENTION  
       [0006] According to a first aspect of the present invention, there is provided an apparatus for the measurement of a power of an RF or MF signal, the apparatus comprising a resistive element for generating a thermal emission in response to an RF or MF signal incident thereon and an infra-red photodetector arranged for detection of the thermal emission generated by the resistive element and for generation of an output signal corresponding to the power of the incident RF or MF signal.  
       [0007] Amplification means may be coupled to the infra-red photodetector to receive, when in use, the output signal and thereby generate an amplified signal.  
       [0008] The infra-red photodetector and the resistive element may be disposed within a substantially evacuated housing. The substantially evacuated housing may comprise an internally silvered surface.  
       [0009] The resistive element may be a thin film chip resistor.  
       [0010] According to a second aspect of the present invention, there is provided a method of measuring a power of an RF or MF signal, the method comprising the steps of: generating a thermal emission by a resistive element in response to an incident RF or MF signal, and detecting the thermal emission by an infra-red photodetector to generate an output signal corresponding to the power of the incident RF or MF signal.  
       [0011] The method may further include the step of: amplifying the output signal.  
       [0012] According to a third aspect of the present invention, there is provided a use of an infra-red photodetector for a measurement of an average power of an RF or MF signal.  
       [0013] The present invention therefore provides an apparatus and a method for the detection and measurement of the power of an RF of MF signal. At least some embodiments of the present invention provide increased sensitivity and a faster response time compared to thermoelectric detection and measurement apparatus known in the art. Some embodiments of the present invention also provide greater efficiency and accuracy in addition to providing flexibility and compatibility with a range of electronic and communications measurement applications. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0014] At least one embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:  
     [0015] FIGURE  1  is a schematic diagram of one embodiment of an RF or MF power measurement and detector apparatus. 
    
    
     DETAILED DESCRIPTION  
     [0016] Referring to FIGURE  1 , an RF or MF power measurement and detection apparatus  10  comprises an input port  12  such as a coaxial input connector coupled to a first terminal of a resistive element such as a 50 Ohm thin film chip load resistor  14  for generating a thermal emission, a second terminal of which is coupled to ground. A photodetector such as an infra-red photodiode  16  is disposed adjacent the load resistor  14  for detecting the thermal emission generated by the load resistor  14  when in operation. Both the load resistor  14  and the infra-red photodiode  16  are encapsulated within an evacuated housing  18 . The evacuated housing  18  is constructed with an internally silvered surface (not shown) so as to minimise any loss of energy dissipated in the load resistor  14 .  
     [0017] The evacuated housing  18  can be constructed using techniques typical to that of the construction of a filament lamp and the evacuated housing  18  is dimensioned to comprise both the load resistor  14  and the infra-red photodiode  16  therein.  
     [0018] An anode terminal of the infra-red photodiode  16  is coupled to ground and a cathode terminal of the infra-red photodiode  16  is coupled to a first input terminal  20  of an amplifier  22  such as a current feedback operational amplifier AD8014 as manufactured by Analog Devices. A second input terminal  24  of the amplifier  22  is coupled to a first terminal of a first resistor  26 , a second terminal of which is coupled to ground. The cathode terminal of the infra-red photodiode  16  is also coupled to a first terminal of a feedback resistor  28  and a first terminal of a first capacitor  30 . A second terminal of the first capacitor  30  and a second terminal of the feedback resistor  28  are coupled to an output terminal  32  of the amplifier  22 . The first capacitor  30  and the feedback resistor  28  are disposed in parallel circuit configuration with respect to each other. The output terminal  32  of the amplifier  22  is coupled to an output port  34  such as a coaxial style connector.  
     [0019] In operation, an RF or MF signal  40  is incident upon the load resistor  14  and, in response to the power of the incident signal  40 , the load resistor  14  dissipates power by way of thermal emissions  42 . The thermal emissions  42  generated by the load resistor  14  are detected by the infra-red photodiode  16  so as to generate an output signal corresponding to the power of the incident signal  40  in the form of, for example, a photocurrent. The output signal generated by the infra-red photodiode  16  is received by the amplifier  22  in order to linearise the response of the infra-red photodiode  16  and provide an amplified signal having a wide dynamic range. The amplified signal corresponding to the power of the incident RF or MF signal is applied to the output port  34  and provides the user with a measurement of the power of the incident signal  40 .  
     [0020] As the infra-red photodiode  16  and the load resistor  14  are disposed within an evacuated housing  18 , external sources of electromagnetic radiation, convection and conduction are obviated or at least mitigated from affecting measurements as any such external sources are substantially prevented from modifying the “effective power” in the load resistor  14 , that is, the power of the signal, incident upon the load resistor  14 . Furthermore, as the infra-red photodiode  16  measures energy radiated by the load resistor  14 , thermal lag is obviated or at least mitigated as thermal conduction from the resistive element to a thermoelectric device is not required.  
     [0021] It should be appreciated that, whilst the above description refers to a “power”, the term “average power” is commonly used in the art for specifying the power of RF or MF signals. Average power is the power of the signal integrated over time, that is, integrated over many periods of the signal. The integration time (averaging time) for average power detection and measurement is typically from several hundredths of a second to several seconds. The term “power” is therefore intended to include reference to average power and, in a second embodiment of the present invention therefore, there is provided an apparatus and a method for the detection and measurement of the average power of an RF or MF signal.