True average wide dynamic range power sensor

Abstract: The present invention provides a true average wide dynamic range (TA-WDR) power sensor that can be used to make accurate power measurements from -70 dBm to + 20 dBm or more (wide dynamic range), regardless of the format of the signal (true average). In one preferred embodiment, the present invention provides a true average wide dynamic range power sensor comprising means for receiving RF signals having wide dynamic power ranges; a first RF path including a low power diode sensor for measuring RF signals having low power ranges; a second RF path including an attenuator high power sensor for measuring RF signals having high power ranges; and means for isolating the first RF path when the high power RF signals exceeds the square law region of the diode sensor.

Description: BACKGROUND OF THE INVENTION The present invention relates to a true average wide dynamic range power sensor (TA-WDR) that can be used to make accurate power measurements from -70 dBm to + 20 dBm or more (wide dynamic range), regardless of the format of the signal (true average). True average sensors have been in use for many decades in the -70 dBm to -20 dBm (based on diode operating in its "square-law" region), and -30 dBm to +20 dBm (based on thermocouple, or thermistor, or diode sensor with an attenuator at its input) separately. There is not a single sensor that covers the entire dynamic range stated above. Recently, diode based wide dynamic range sensors have been introduced that cover the entire -70 dBm to + 20 dBm range by pushing a diode sensor way beyond its limit (-20 dBm) for measuring the power. Because the diode is beyond its limit for true average power measurement in the -20 dBm to + 20 dBm, these sensors can only be used for measuring power accurately of a single-frequency (continuous wave signal or CW signal). Most applications require measurement of complex signals with many frequency components in it (immense demand for such sensors is due to the fast growing digital communications market). US-A-5 266 888 describes a wide power range radiation monitor including a pair of diodes with their cathodes interconnected and with an additional capacitor coupled in parallel to one of the diodes. The detector of the radiation monitor provides two outputs: a low power output and a high power output, which outputs are connected to the anodes of the detector diodes. Each output is supplied to a respective non-inverting input of two differential amplifiers, and one of the two outputs of the amplifiers is selected by means of a relay switch in dependence on the measured output of one of the amplifiers. US-A- 4873484 describes a probe for sensing radio frequency power incorporating two independtly driven radio frequency-sensitive diodes driven through a common RF input. The dioder are inequally padded to provide an extended sensitivity range. The diodes generate two respective outputs which are supplied to a commun power meter, which switches between the two outputs. EP -A-0412392 describes a power amplifier for a transmitter in which the output is measured using a parallel arragement of output limer, each containing a diode, one of which also contains an attenuating resistor. The desired output is selected using a two-way switch selectively connected to one of the two diodes. US-A- 4943764 describes a wide dynamic range radio-frequency power sensor having a low-power sensor portion and a high-power sensor portion. The radio-frequency input is supplied, via a de-blocking capacitor, to the low-power sensor portion and, via both the de blocking capacitor and a Further coupling capacitor, to the high-power sensor portion. The low-power sensor portion comprises a diode, the output of which is connected to a de path the dc path being connected to ground via a capacitance, the low power sensor also including a switch which to operative to open the dc path when the radio-frequency signals exceed the square law region of the low power diode sensor. SUMMARY OF THE INVENTION It would be desirable to provide a true average wide dynamic range power sensor (TA-WDR) that can be used to make accurate power measurements from - 70 dBm to + 20 dBm or more (wide dynamic range), regardless of the format of the signal (true average). In accordance with the present invention there is provided a true average wide dynamic range power sensor as defined in claim 1. Other features and advantages of the present invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and form a part of this specification and in which like numerals represent like components, illustrate arrangements of sensors and, together with the description, serve to explain the principles of the invention. However, the arrangements illustrated in Figs. 1 to 7 do not constitute embodiments of the present invention. Figure 1 shows one implementation of a true average wide dynamic range power sensor. Figure 2 shows another implementation of a wide dynamic range power sensor. Figure 3 shows a wide dynamic range power sensor with a switch in the RF path. Figure 4 shows a wide dynamic range power sensor with a passive RF circuit. Figure 5 shows a wide dynamic range power sensor with a switch in the DC path. Figure 6 shows a schematic diagram of a diode detector circuit. Figure 7 shows a schematic diagram of a diode-attenuator-diode circuit. Figure 8 shows a schematic diagram showing the switch in the DC path, in accordance with an embodiment of the present invention. Figure 9 shows the effect of the switch of Figure 8 on linearity. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made in detail to arrangements of sensors which verre to explain the principles examples of the present invention, of which are illustrated in the accompanying drawings. The present invention provides a true average wide dynamic range power sensor that is capable of making very accurate true average wide dynamic range power measurements. While there are several techniques for designing true average wide dynamic range power sensors, all these ideas consist of using a diode for measuring low power signal (-70 dBm to -20 dBm). It is important to realize that the ability to make a measurement of the lowest power signal is a limiting factor. Since a higher power signal can always be attenuated (by a known factor) easily to bring it down to a level that can be measured by a low power sensor. However, there is no simple technique to measure signals below -70 dBm (below which the diodes become too noisy). Thus in various possible implementations of a true average wide dynamic range sensor, a diode sensor is used to measure low power signals. To measure higher-power signals, an attenuator could be switched-in in the RF signal path of the same diode sensor, as shown in Figure 1, which shows the RF signal on the RF signal path 10 input to low power diode sensor 12. The diode sensor 12 receives low power RF signals and the switched attenuator 14 effectively removes high power RF signals by reducing such signals to the region of the square law. In another arragement, shown in Figure 2, the incoming signal is directed into two separate paths 20, 22, one RF path 20 going to a diode 24 which receives the RF signals for low power measurement and the other RF path 22 going to a high power sensor 26, which could be a thermistor, thermocouple, or an attenuated diode sensor (called, in general, a "high power sensor") which receives the RF signals to measure the high power signal. In this second arrangement of a true average wide dynamic range sensor, it has been determined that the presence of the low power diode sensor has very detrimental effects when a high power signal is input into this circuit and the high power arm of this circuit attempts to make an accurate measurement. Thus, one must electrically isolate the effects of low power diode sensor from the circuit when the signal level goes beyond its square law region. This can be done using a switch 30 of some type (electronic, mechanical, micro-mechanical) in the RF signal path to isolate the low power diode sensor, as shown in Figure 3. This could also be accomplished using a passive circuit 34 shown in Figure 4 that provides high isolation between the high power sensor and the low power diode sensor (for example using a high isolation splitter circuit, or a high isolation directional coupler etc.). The switches and the passive circuits mentioned above are in the RF signal path and thus must be designed with utmost care. Due to the very nature of these circuits, they will introduce frequency bandwidth limitations (at least with switches available at this time). The proper functioning of the high power sensor can be affected due to the excessive DC current flowing through the low power path diode when, inevitably, its input terminal encounters high power RF signal, which is truly meant to be measured by the high power sensor. Most of the benefits of isolating the low power diode sensor from the RF signal path can be achieved simply by disconnecting the DC path of the diode, thus preventing any DC current flow through the diode. This arrangement is shown in Figure 5, which shows the switch 38 in the DC path 40. Unlike disconnecting or isolating the RF path, it is very simple to disconnect the DC circuit using readily available switches (this could be any convenient switch that does not deteriorate the performance of the diode sensor when it is in fact being used for making low power measurements). Diodes are extensively used in all kinds of electronic circuits. The non-linear current versus voltage characteristics (I-V characteristics) of diodes are exploited in several circuits to perform useful tasks. The non-linear I-V characteristic results in varying diode impedance as the current through the diode varies. In microwave circuits, matching the impedance of a circuit to the impedance of a source is of prime importance to achieve efficient signal transfer with low levels of signal reflections. In many cases, the varying diode impedance is an inevitable fact that the design should try to compensate for. In other applications where the diode is not actively used, much of the detrimental effects of diode impedance variation can be eliminated simply by disconnecting the DC path of the diode from the circuit using either a mechanical or an electronic switch. This technique has been demonstrated in the true average wide dynamic range power sensor and has resulted in significant improvements in the accuracy and performance of this instrument. The influence of DC current on diode impedance will now be described. The relationship between voltage and current in a diode is I ( V ) = I s ( e α V − 1 ) where I is the current through the diode when voltage, V, is applied across it. I s is the saturation current, which is a constant at any given temperature. In general, if V consists of a DC component, V 0 , and an AC component, v, as follows V = V o + ν The diode impedance, commonly known as video resistance, Rv , is R ν = [ d I d V | V o ] − 1 = 1 α ( I o + I s ) where I 0 is the bias current due to DC voltage, V 0 . Thus, Rv, is inversely proportional to I 0 + I s . At zero bias (I 0 =0), R v is R ν = [ d I d V | V o ] − 1 = 1 α I s RF power detection and power measurement is one of the most important applications of diodes in microwave circuits. A schematic of a detector circuit is shown in Figure 6. In this circuit, the RF voltage is applied across the parallel combination of a matching resistor, R m , and the diode D. For small RF signals, a rectified DC voltage proportional to the square of the RF voltage appears at the output terminal. Under these conditions, the diode is said to be in its square-law region of operation. The capacitor C b shorts the RF signal at the diode output, ensuring all of the RF voltage appears across the diode D. In addition, being a dc-block, it allows the detected voltage, V d , to be tapped at the output terminal. The load resistor, R L , acts as a discharge path for the voltage built-up across the capacitor. For a given V d , the load resistor causes a DC current equal to V d / R L , which must flow through the diode D as well. From the circuit diagram, it is clear that the RF matching impedance of the detector is equal to the parallel combination of R v and R m . To maintain good match, it is required that the diode resistance remain high compared to R m . However, R v can not be too high as it decreases the diode sensitivity. A compromise between good low-power sensitivity and match requires R v to be in the 1 to 2 kilo-ohms range. The match remains good when the diode D is in its square-law region, where the detected voltage is small and the DC current through the load resistor (typically 10 kilo-ohms) is very small. Until recently, diode detectors in power sensors have always been used with the diode in its square-law region. Thus, the changing value of R v has not been an issue. At higher input power levels, where the detected voltage is higher, the DC current through the diode can be so high that the diode resistance may become too small to maintain a good match. In recent power sensors, a diode has been used to measure RF signals up to 100 mW using essentially the same detector circuit as shown above. However, in the region beyond 10 micro-watts, where the diode detector is no longer in its square-law region, only a continuous wave signal can be measured accurately. This is a major drawback for these sensors, as many applications, especially digital communications, require accurate measurement of non-CW signals. In the arrangement of Figure 7, two separate detector circuits have been integrated on the same GaAs chip. One of the detectors 44 measures the signal directly from the source. The other detector 48 is presented with a sample of the signal that is highly attenuated using an on-chip resistive divider network, including R1, R2. The detected voltages from the direct and the attenuated detectors are used to measure low power and high power signals, respectively. The schematic is shown in Figure 7. Both detectors 44, 48 measure RF power when the diodes are in square-law region. However, when the detectors are used for measuring high power signals, the diode D1 in the direct detector 44 encounters very high RF voltages across its terminals. Thus, the detected voltage in the direct detector 44 is high. This causes a high bias current to flow through this diode D1 causing its impedance to drop and thus changing the RF voltage across the high power sensor 48. This change in impedance match causes an apparent non-linearity in the detected voltage at the attenuator detector 48, thus affecting the purpose of this new design. The present invention solves this problem. The root cause of the problem is the DC current flowing through the diode D1 in the direct detector 44 when its detection is no longer in use. Thus, its high impedance can be maintained by open circuiting its DC current path. In one embodiment, the design has been implemented with FET (field effect transistor) switch that open circuits the DC path in the direct detector, as soon as the attenuated detector voltage is used to measure high power signals. This arrangement is shown in Figure 8 showing the switch 50 in the DC path 40. Now there is no DC current flowing through the low power diode D1. A mechanical switch could also have been used instead of a FET switch. However, FET switches are more repeatable, and last much longer. This technique has significantly improved the dynamic range, accuracy, impedance match and frequency response of the power sensor according to the present invention. The most important performance parameter, the linearity of the detector, with and without the use of this technique, is shown in Figure 9 so as to highlight the benefit of this technique. Figure 9 shows that the error in linearity when the switch is not used is very high. The residual error in linearity when the switch is used is due to the attenuated detector 48 itself. Thus, the great majority of the detrimental effects of low power diode sensor have been eliminated by open circuiting the DC path in that circuit. In Figure 9, error in linearity is very high when the low power diode conducts DC current. With the switch open, circuiting the DC path and preventing DC current flow through the low power diode sensor, the high power measurement accuracy remains intact. The foregoing descriptions of a specific embodiment of the present.invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and it should be understood that many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.

Claims: A wide dynamic range power sensor comprising: means for receiving RF signals having wide dynamic power ranges; a first RF path including a low power diode sensor (44) for measuring RF signals having low power ranges, said low power diode sensor comprising a diode and a matching resistor (R m ) connected in parallel with the input of the low power diode sensor (44); a second RF path including a high power sensor for measuring RF signals having high power ranges; and wherein the output from the diode is connected to a dc path (40), the dc path being connected to ground via a capacitance that is connected to the dc path at a first node adjacent the diode and via a resistance that is connected to the dc path at a second node adjacent the first node and distal to the diode, and the low power sensor also includes a switch (50) that is disposed in the dc path between the first and second nodes, the switch being operative to open the dc path when the RF signals exceed the square law region of the low power diode sensor thus preventing the flow of dc current through the diode and maintaining a high diode impedance with respect to the matching resistor R m . A power sensor as in Claim 1 wherein the wide dynamic power range is from - 70 dBm to + 20 dBm or more. A power sensor as in Claim 1 or Claim 2 including attenuation means (R 1 , R 2 ) for attenuating the high power RF signals. A power sensor as in Claim 3 wherein the attenuation means is a resistive divider network (R 1 , R 2 ) for attenuating the high power RF signals in the second RF path. A power sensor as in any preceding claim wherein the high power sensor (48) is a diode sensor preceded by an attenuator. A power sensor as in any one of Claims 1 to 4 wherein the high power sensor (48) is a thermistor sensor. A power sensor as in any one of Claims 1 to 4 wherein the high power sensor (48) is a thermocouple sensor. A power sensor as in Claim 1 wherein the dc path switch (50) is a FET switch.

Inventors: PRABHU AJAY A, PRABHU, AJAY A.
Applicants: AGILENT TECHNOLOGIES INC, AGILENT TECHNOLOGIES, INC. (A DELAWARE CORPORATION)
Application year: 2000
Main classification: G01R21
IPC codes: G01R21, G01R15, G01R29