Abstract:
In one embodiment, a radio frequency (RF) power meter operates according to a variable loop bandwidth according to the nature of the RF signal to be measured. The RF power meter comprises: a first switch that switchably provides one of a first signal and a second signal, wherein the first signal is the RF signal to be measured; a first signal path for detecting an output signal from the first switch and for providing a first comparison signal according to a first bandwidth, a second signal path for detecting an output from the first switch and for providing a second comparison signal according to a second bandwidth, and a second switch that switchably provides one of the first comparison signal and the second comparison signal to the first switch to complete a closed-loop for the RF power meter.

Description:
TECHNICAL FIELD 
   Representative embodiments are directed to systems and methods for performing RF power measurements. 
   BACKGROUND 
   Measuring the radio frequency power associated with a wireless device is frequently necessary for production, final testing, repair, and other related activities. Typically, diode power meters and calorimetric power meters have been utilized to perform radio frequency power measurements associated with various cellular devices. Specifically, diode power meters are relatively well-suited to measure radio frequency power associated with continuous wave (CW) signals and other relatively narrow bandwidth signals (such as those signals associated with the AMPS standard). Calorimetric power meters are more suitable for broadband signals such as signals associated with code division multiple access (CDMA) signals. 
   An example of a diode power meter is shown in U.S. Pat. No. 5,656,929. The disclosed diode power meter switches a radio frequency (RF) detector between an input RF signal and a comparison RF signal to generate a difference signal. The difference signal is filtered, and converted into a direct current (DC) error signal by a synchronous detector that operates in step with the RF detector. The DC error signal is applied to an integrator to generate a loop control signal. The square of the loop control signal is linearly proportional to the input RF signal power when the servo loop is nulled, causing the comparison RF signal to equal the input RF signal. 
   A calorimetric power meter is a device that determines RF power utilizing, in general, a temperature-generated differential created by the signal being measured. An example of a calorimetric power meter is disclosed in U.S. Pat. No. 5,663,638. The disclosed calorimetric power meter accepts RF power into a terminating load that thermally couples generated heat to a temperature dependent resistance in one arm of a bridge. The imbalance created in the bridge produces an error signal within a servo loop. The servo loop responds by applying a direct current (DC) or low frequency power to a separate terminating load that couples heat into another arm of the bridge. The applied power is measured by metering how much power is required of the servo loop to re-balance the bridge. Calorimetric power meters are advantageous, because they measure total aggregate power contained in the signal. Thus, calorimetric power meters are suitable for wideband signals. However, calorimetric power meters suffer from certain limitations. Calorimetric power meters typically experience a relatively slow response time. Specifically, heat storage in the bridge limits the response time for the first power measurement, and after this measurement the bridge must be re-balanced by zeroing out the heat generated before a second measurement can be made. Accordingly, calorimetric power meters typically are unsuitable for wideband, non-repetitively pulsed signals such as occur in wide CDMA formats used for data applications. 
   SUMMARY 
   Representative embodiments are directed to systems and methods for measuring RF power. Representative embodiments provide a single power meter that enables RF power measurements to be performed for both narrowband and wideband signals. In representative embodiments, variable loop bandwidth may be selected according to the nature of the signal being measured. For example, a first portion of the RF power meter may provide a wideband loop that facilitates measurement of the power associated with modulation bandwidths from CW signals up to signals with 1.23 MHz modulation bandwidth, such as IS-95 CDMA signals. A second portion of the RF power meter may provide a narrowband loop to facilitate the measurement of the power associated with wide modulation bandwidth signals such as cdma2000 and WCDMA signals. In the United States, wideband CDMA is deployed under the cdma2000 Standards and, in Europe and internationally, wideband CDMA is deployed under the 3GPP Standards. Present modulation bandwidths are up to 3.84 MHz within these standards. By implementing first and second loop portions in this manner, representative embodiments enable RF power measurements to be performed for a variety of signal formats utilizing a single RF power meter. 
   Moreover, in representative embodiments, a detector block is operated within the transition region from linear to square law region when the wideband loop is selected and another detector block is operated only within a square law region when the narrowband loop is selected. By enabling detection within the square law region, the detector operates in a manner similar to a calorimetric detection but will respond much more quickly. The diode based square law detector will not require time for heat storage to balance out or heat re-balancing after the first measurement as is typical for calorimetric detection. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized that such equivalent constructions do not depart from the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIGS. 1 and 2  depict respective portions of an RF power meter that may operate according to variable bandwidth according to representative embodiments. 
   

   DETAILED DESCRIPTION 
   Referring now to the drawings,  FIGS. 1 and 2  depict respective portions of an RF power meter that may operate according to variable bandwidth according to representative embodiments. For example, the RF power meter may be operated in multiple modes. In one of the modes, the RF power meter operates according to a closed loop bandwidth that is relatively wide and is suitable for measuring the power of relatively narrowband signals (e.g., from CW signals to signals having modulation bandwidths up to 1.23 MHz). In another of the modes, the RF power meter operates according to a closed loop bandwidth that is relatively narrow and is suitable for measuring the power of relatively wideband signals that possess relatively wide dynamic range. Examples of these signals include cdma2000 and WCDMA signals with modulation bandwidths of 3.84 MHz. 
     FIG. 1  depicts signal path  100  that completes a closed loop according to a relatively wide bandwidth of an RF power meter according to representative embodiments. As shown in  FIG. 1 , switch  102  switches between RF input line  101  that receives an RF signal to be measured and line  114  that provides a comparison signal from a closed loop as will be discussed in greater detail below. As controlled by the operation of switch  102 , the servo loop obtains a sample of the input RF signal and compares that sample to the comparison or feedback signal. 
   Switch  102  is controlled by a signal present at node A or node C depending upon the mode of operation of the RF meter via the operations of switch  117 . One of signal path  100  and signal path  200  (shown in FIG.  2  and to be discussed below) receives the output signal from RF connection  102  via, for example, another RF connection (shown as RF connection  103  at point E in FIG.  1 ). Signal path  100  provides a signal path to complete the closed loop for feedback to switch  102  according to the mode associated with a relatively wide bandwidth. Accordingly, by operating RF connection  103  to provide the respective signals to signal path  100 , the power of relatively narrowband RF signals may be measured efficiently. 
   Signal path  100  begins with detector block  104 . Detector block  104  includes diode stacks  121  arranged in an anti-parallel manner. An example of a diode stack implementation is discussed in greater detail in U.S. Pat. No. 6,242,901, which is incorporated herein by reference. Diode stacks  121  are terminated by capacitors  122 . The time constants of capacitors  122  are selected to allow the incoming RF signal to be tracked inside the loop bandwidth with no slewing at the detector  104  output. Furthermore, detector block  104  may be implemented as a differential diode detector utilizing a plurality of operational-amplifiers  123 . 
   Detector block  104  may be adapted to operate diode stacks  121  within a linear region, a transition region, and a square law region. Specifically, as is well-known, the current versus voltage equation for an ideal diode is given by: I=I 0 (e (nV/kT) −1), where I represents the diode current, V represents the diode voltage, T represents the diode temperature, and I 0 , n, and k are constants. Over a limited range of voltages, this equation may be approximated by: I=a(V 2 ), where a is a constant. The range of voltages where this approximation holds is referred to as the square law region. In another limited range of voltages (referred to as the linear region), the diode equation may be approximated by a linear relationship between the diode current and the diode voltage. Also, a range of voltages exist between the linear region and the square law region which is referred to as the transition region. A major advantage of the high bandwidth closed loop design is the attendant large gain of the loop. This feature enables the closed loop detector output to closely follow the RF signal for narrowband, pulsed, and signals with modulation bandwidths up to, for example, 1.5 MHz. 
   Bandpass filter  105  filters the output from detector block  104  to obtain information centered around the switching frequency of switch  102 . The output from bandpass filter  105  is amplified by variable gain amplifier  106 . The amplified signal is provided to synchronous detector  107  which is controlled by the signal present on node B. Specifically, the signal present on node B is a delayed version of the signal present on node A (as delayed by element  112 ). The amount of delay provided by delay element  112  is selected to equal the amount of delay associated with detector block  104 , bandpass filter  105 , and variable gain amplifier  106 . 
   Synchronous detector  107  generates a direct current (DC) or low frequency error signal that may be utilized to null the servo loop thereby causing the voltage of the comparison signal to approximate or equal the voltage of the RF signal being measured. That is, synchronous detector  107  generates a signal that is indicative of or related to the difference in voltage between the comparison signal and the signal being measured (i.e., the signal received via line  101 ). Synchronous detector  107  may be implemented in a number of ways. For example, synchronous detector  107  may be implemented to include a switch (not shown) and a differential amplifier (not shown). The signal from amplifier  106  could be provided to alternate inputs of the differential amplifier using the switch as controlled by the signal present on node B as disclosed in U.S. Pat. No. 5,656,929, which is incorporated herein by reference. 
   The error signal generated by synchronous detector  107  is provided to integrator  108 . Because of the wide loop bandwidth and large loop gain the voltage associated with the output of integrator  108  is linearly proportional to the voltage of the signal being measured. Accordingly, the output of integrator  108  is provided to output block  116  that utilizes the linear relationship defined by closed loop signal path  100  to provide the user with information indicative of the voltage and/or power of the signal being measured. Also, the integrated error signal is utilized as a loop control signal. Specifically, the output of the integrated error signal is provided to linear multiplier  109  (e.g., a suitable analog multiplier) that is coupled to 50 MHz reference oscillator  110  as an example. The output of multiplier  109  is provided to variable gain amplifier  113 . Switch  114 , when operated in the mode to cause the closed loop to be defined by signal path  100 , causes the signal present at node F to be provided to switch  102 . Specifically, the output of variable gain amplifier  113  is provided to switch  102  thereby completing the closed loop. 
   Additionally, variable gain amplifiers  106  and  113  may be utilized to adjust the loop gain associated with signal path  100  to enable the operation of diode detector block  104  in the square law, transition, or linear regions. By enabling operation of diode detector block within these regions, processor control of the loop gain may facilitate RF power measurement for signal formats having differing pulse, modulation, and peak value characteristics. 
     FIG. 2  depicts signal path  200  according to representative embodiments. Signal path  200  completes the closed loop for feedback to switch  102  according to a relatively narrow loop bandwidth. Detector block  201  receives the signal (either the signal being measured or the comparison signal) from switch  102  as discussed previously with respect to FIG.  1 . Detector block  201  is adapted to operate diode stacks  221  within the square law region. This is partially accomplished by using a resistive divider (consisting of R 1  and R 2  in block  201 ) to attenuate the signal at RF connection  103  into detector block  201 . Also multiple diodes (five in this example) are used in the diode stacks to further put the diodes into the square law operation region. By adapting detector block  201  in this manner, signal path  200  is suitable for measuring relatively wideband signals that possess a relatively wide dynamic range (such as wide CDMA formats adapted for data communication as well as voice communication). Because the loop bandwidth associated with detector block  201  is relatively narrow, capacitors  222  may possess a larger capacitance relative to capacitors  122  (shown in  FIG. 1 ) and amplifiers  223  may possess a lower bandwidth relative to amplifiers  123  (shown in FIG.  1 ). 
   The output from detector block  201  is provided to bandpass filter  202 . The filtered signal is provided to variable gain amplifier  203  which may be a relatively slower speed amplifier relative to amplifier  106  (shown previously in FIG.  1 ). The amplified signal is provided to lower frequency synchronous detector  204 . Lower frequency synchronous detector  204  is driven by the signal present at node D. The signal present at node D is a delayed version of the signal present at node C. The delay provided by delay element  210  between nodes C and D is selected to approximate or equal the delay associated with detector block  201 , filter  202 , and amplifier  203 . The output of synchronous detector  204  is provided to integrator  205 . 
   Because detector block  201  is operated within the square law region, the voltage of the output of integrator  205  is related to the square root of the voltage of the signal being measured (i.e., the signal inputted into line  101  as shown in FIG.  1 ). Accordingly, the output of integrator  205  is provided to output block  116  that utilizes the square law relationship to provide the user with information indicative of the voltage and/or power of the signal being measured. 
   Additionally, the output of integrator  205  is utilized as a loop control signal. The output of integrator  205  is provided to lower frequency linear multiplier  206 . The output of multiplier  206  is provided to variable gain amplifier  207 . The loop may be closed by operating switch  114  to provide the output (the signal present at node G) from amplifier  207  to switch  102  (previously discussed in regard to FIG.  1 ). 
   Representative embodiments enable the user to define the loop bandwidth by adjusting the gain associated with variable gain amplifier  203  and/or variable gain amplifier  207 . By varying the loop bandwidth, the performance of the RF meter may be optimized. Specifically, the variation in the loop bandwidth is useful for measuring signals with different pulse duty cycles. The variable loop bandwidth for the square law detector enables a trade off of loop response time versus the pulse duty cycle being measured thereby achieving a greater degree of accuracy for the measurement of time varying (pulsed) wideband CDMA signals. 
   Representative embodiments provide further advantages by omitting the necessity of re-balancing thermal bridges. Specifically, known thermal bridge power meters requires 10 milliseconds for sampling, 6 milliseconds for settling, and 26 milliseconds for re-balancing and re-zeroing the thermal bridge. Representative embodiments enable sampling to occur within 1 millisecond for narrowband signals and within 2 milliseconds for wideband signals. In further contrast to thermal bridge designs, representative embodiments do not require any time for re-balancing or re-zeroing. 
   By implementing signal paths  100  and  200  within a single RF power meter, various circuit components may be utilized for the multiple modes of operation. That is, the high frequency components of the power meter may be used for both loops, allowing for a more compact design and a lower cost of manufacturing. For example, as shown in  FIG. 2 , a frequency divider path may be utilized to provide frequency signals to control operation of the RF power meter according to variable bandwidths. As previously discussed, 50 MHz reference oscillator  110  and frequency divider  111  provide frequency signals (as present on nodes A and B) to control operations of the RF meter according to a relatively wideband loop. Frequency dividers  208  and  209  are coupled thereto to further divide the frequency reference to provide frequency signals (as present on nodes C and D) when the RF meter is operated in the mode of operation associated with the narrow loop bandwidth. 
   Furthermore, in representative embodiments, detector block  104  and detector block  201  may be integrated on a single dual detector block utilizing modern GaAs integrated circuit (IC) process techniques. By implementing detector block  104  and detector clock  201  in this manner, excellent matching of the diodes in the two detectors and an excellent RF frequency response may be obtained. Accordingly, the accuracy of the power measurement may be improved. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one will readily appreciate from the disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.