Abstract:
In accordance with a method of this invention, and circuitry for performing the method, there is disclosed a process for operating a radio frequency (RF) signal power detector. The method includes the steps of (a) sampling the output of an RF detector circuit to obtain a measurement of a value of the output when no RF signal is input to the RF detector circuit; (b) storing the measurement; and (c) subtracting, when an RF signal is input to the RF detector circuit, the stored measurement from the output of the RF detector circuit to provide a subtracted output signal. In pulsed (e.g., TDMA) embodiments of this invention the step of sampling occurs between RF bursts, while in continuous wave (cw) embodiments the step of sampling occurs during a time that an RF connection is opened or otherwise blocked at an input of the RF detector circuit and a source of the RF signal. A sample and hold function can be provided for sampling and holding the subtracted output signal during a time that the input of the RF detector circuit is blocked. The step of subtracting includes the steps of combining the measured value with a transmitter power setting signal to obtain a corrected transmitter power setting signal; and subtracting the output of the RF detector circuit, when the RF signal is input to the RF detector circuit, from the corrected transmitter power setting signal.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to power detection circuits and method and, in particular, to methods and circuits for detecting the power in a pulsed signal. 
     BACKGROUND OF THE INVENTION 
     So-called peak detectors have been used to monitor an applied signal power in a circuit. A primary output of the peak detector is a voltage having a magnitude that is related to the power level. However, at low power levels the magnitude of the output voltage is small and, as a result, the output of the peak detector is strongly influenced by any drift in bias potentials, supply voltage, and the detecting device itself (e.g., a diode). The temperature dependance of the detector device alone has been found to have a significant impact on the accuracy of the output voltage. 
     The problem due to temperature drift has been addressed in the prior art by using an additional temperature dependent device to compensate for the drift of the detector device. The accuracy of the compensation thus relies on a uniformity of the temperature characteristics of the detector device and the compensating device, as well as on a close physical spacing of these two devices in order to avoid any deviation in temperature between the devices. 
     It has also been observed that the aging of the components may degrade the compensation, and some prior art circuits have thus required testing and tuning. 
     Reference can be had to U.S. Pat. Nos 4,523,155, 4,970,456, 5,222,104 and 5,287,555 for teaching various conventional power control circuit arrangements and detectors. 
     OBJECTS OF THE INVENTION 
     It is thus a first object of this invention to provide an improved method for detecting a power level that is not significantly influenced by drift due to temperature and other disturbances, and to thereby enable an accurate power monitoring or control to be achieved. 
     It is a further object of this invention to provide circuit embodiments that implement improved power detectors. 
     SUMMARY OF THE INVENTION 
     The foregoing and other problems are overcome and the objects of the invention are realized by methods and apparatus in accordance with embodiments of this invention. 
     This invention teaches a method wherein a difference between “off” and “on” states of a pulsed signal is detected so as to provide an accurate indication of the power of the pulsed signal during the “on” state. It is assumed that during the “off” period no significant signal power is present at the input of the detector. It is further assumed that the “on” period is sufficiently short such that no significant drift occurs during the “on” period measurement. 
     Advantages of the invention arise from the fact that the power detection is based on dynamic properties, for example, the peak envelope sensitivity of the detector device. As such, the detected voltage is determined by dynamic properties of the detector device, which are superior to the static properties (e.g., barrier voltage of the detector diode). The detected voltage representative of the power level is not significantly influenced by long term changes in supply voltage, bias potential, or the barrier voltage of the detecting device. 
     Furthermore, embodiments of this invention allow more freedom in the design of the detector device biasing network, which can be advantageous in order to stabilize the dynamic sensitivity of the detector device at low power levels. The invention can be implemented in such a way that the detected RF power level begins precisely at zero level, which is beneficial in systems using a low supply voltage. In addition, the zero level of the detector output voltage can begin at any desired level if a suitable reference potential (e.g., ground) is available. 
     The invention can be implemented in a number of ways depending on the type of application. The circuitry used for an analog implementation may be very simple and inexpensive. A digital implementation uses an analog to digital converter and a digital to analog converter and, while being more complex than the analog embodiment, offers the advantages of digital signal processing. 
     The power detector and transmitter power controller in accordance with this invention eliminates the need to make separate measurements of a reference detector device, and further eliminates the requirement to provide temperature compensation devices for the power detector device. 
     In accordance with a method of this invention, and circuitry for performing the method, there is disclosed a process for operating a radio frequency (RF) signal power detector. The method includes the steps of (a) sampling the output of an RF detector circuit to obtain a measurement of a value of the output when no RF signal is input to the RF detector circuit; (b) storing the measurement; and (c) subtracting, when an RF signal is input to the RF detector circuit, the stored measurement from the output of the RF detector circuit to provide a subtracted output signal. In pulsed (e.g., TDMA) embodiments of this invention the step of sampling occurs between RF bursts, while in continuous wave (cw) embodiments the step of sampling occurs during a time that the RF signal is prevented from entering the input of the RF detector circuit, such as by opening a connection between an input of the RF detector circuit and a source of the RF signal. A sample and hold function can be provided for sampling and holding the subtracted output signal during a time that the input of the RF detector circuit is opened. 
     In one embodiment of this invention the step of subtracting includes the steps of combining the measured value with a transmitter power setting signal to obtain a corrected transmitter power setting signal; and subtracting the output of the RF detector circuit, when the RF signal is input to the RF detector circuit, from the corrected transmitter power setting signal. 
     In one embodiment of a cw circuit there are two RF detector circuits provided in parallel, and the steps of sampling and storing occur in one RF detector circuit simultaneously with the step of subtracting in the other RF detector circuit. In another embodiment a sample and hold circuit is located at the output of a single RF detector circuit, while in a further embodiment a sample and hold circuit is used at the output of a loop amplifier that forms a portion of a closed loop transmitter power control circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein: 
     FIG. 1 is a block diagram of a mobile station that is constructed and operated in accordance with this invention; 
     FIG. 2 is an elevational view of the mobile station shown in FIG. 1, and which further illustrates a cellular communication system to which the mobile station is bidirectionally coupled through wireless RF links; 
     FIG. 3 is a schematic diagram of a first embodiment of a power detector circuit in accordance with the teachings of this invention; 
     FIGS. 4A and 4B are illustrative graphs of a RF detector diode current versus bias potential and sensitivity versus temperature, respectively; 
     FIG. 5 is a circuit block diagram of a second embodiment of a power detector circuit in accordance with the teachings of this invention; 
     FIG. 6 is an illustrative waveform diagram showing an RF burst and various sampling points and times in accordance with this invention; 
     FIG. 7 is a circuit block diagram illustrating a third and fourth embodiments of a power detector circuit in accordance with the teachings of this invention; 
     FIG. 8 is a circuit block diagram illustrating a fifth embodiment of this invention; and 
     FIGS. 9A and 9B each depict an embodiment of the invention for use with continuous wave (cw) RF signals. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is made to FIGS. 1 and 2 for illustrating a wireless user terminal or mobile station  10 , such as but not limited to a cellular radiotelephone or a personal communicator, that is suitable for practicing this invention. The mobile station  10  includes an antenna  12  for transmitting signals to and for receiving signals from a base site or base station  30 . The base station  30  may be a part of a cellular network comprising a Base Station/Mobile Switching Center/Internetworking function (BMI)  32  that includes a mobile switching center (MSC)  34 . The MSC  34  provides a connection to landline trunks when the mobile station  10  is involved in a call. 
     The mobile station includes a modulator (MOD)  14 A, a transmitter  14 , a receiver  16 , a demodulator (DEMOD)  16 A, and a controller  18  that provides signals to and receives signals from the transmitter  14  and receiver  16 , respectively. These signals include signalling information in accordance with the air interface standard of the applicable cellular system, and also user speech and/or user generated data. The air interface standard is assumed for this invention to include a slotted frame structure wherein the mobile station  10  transmits one or more bursts of RF power during a given frame period. Suitable air interface standards include IS-136, GSM and DCS 1900, although the teaching of this invention is not intended to be limited only to these specific frame structures, or for use only with TDMA type systems. By example, and as will be made apparent below, the teaching of this invention also applies to mobile stations that transmit a cw RF signal. 
     A transmitter power detector (PD)  15  is also provided, and a number of embodiments of same are described in detail below. 
     It is understood that the controller  18  also includes the circuitry required for implementing the audio and logic functions of the mobile station. By example, the controller  18  may be comprised of a digital signal processor device, a microprocessor device, and various analog to digital converters, digital to analog converters, and other support circuits. The control and signal processing functions of the mobile station are allocated between these devices according to their respective capabilities. 
     A user interface includes a conventional earphone or speaker  17 , a conventional microphone  19 , a display  20 , and a user input device, typically a keypad  22 , all of which are coupled to the controller  18 . The keypad  22  includes the conventional numeric (0-9) and related keys (#,*)  22   a , and other keys  22   b  used for operating the mobile station  10 . These other keys  22   b  may include, by example, a SEND key, various menu scrolling and soft keys, and a PWR key. The mobile station  10  also includes a battery  26  for powering the various circuits that are required to operate the mobile station. 
     The mobile station  10  also includes various memories, shown collectively as the memory  24 , wherein are stored a plurality of constants and variables that are used by the controller  18  during the operation of the mobile station. For example, the memory  24  stores the values of various cellular system parameters and the number assignment module (NAM). An operating program for controlling the operation of controller  18  is also stored in the memory  24  (typically in a ROM device). The memory  24  may also store data, including user messages, that is received from the BMI  32  prior to the display of the messages to the user. 
     It should be understood that the mobile station  10  can be a vehicle mounted or a handheld device. It should further be appreciated that the mobile station  10  can be capable of operating with one or more air interface standards, modulation types, and access types. By example, the mobile station may be capable of operating with any of a number of standards, such as GSM and IS-95 (CDMA). AMPS, narrow-band AMPS (NAMPS), as well as TACS, mobile stations may also benefit from the teaching of this invention, as should dual or higher mode phones (e.g., digital/analog (IS-41) or TDMA/CDMA/analog phones). It should thus be clear that the teaching of this invention is not to be construed to be limited to any one particular type of mobile station or air interface standard. 
     FIG. 3 illustrates a first, analog embodiment of the power detector (PD)  15  shown in FIG.  1 . An input node receives a sample of the signal (RF in ) transmitted by transmitter  14 . Resistors R 1 -R 3  provide a bias potential for a detector diode D 1 . D 1  is preferably a Schottky diode, although the invention is not limited to only this type of diode. Those skilled in the art will realize that diode D 1  and capacitors C 1  and C 2  together form an envelope detector. 
     During the time that RF in  is “off”, that is, the transmitter  14  is not transmitting a burst, a switch S 1  is held closed (“on”). S 1  being closed nulls the output (Detector Output) of the power detector  15  which is shorted to a reference potential (e.g., ground) through S 1 . Also during this time the potential appearing at the cathode of D 1  is stored on C 3 , which is a relatively large capacitance (e.g., 4.7-10 μF). That is, the magnitude of V BIAS  due to the forward current that flows through D 1  and R 3  to ground (e.g., 0.4 volts), plus any optionally applied offset voltage, is stored on C 3 . S 1  may be, by example, an NPN transistor or a FET. Control of the switch S 1  can be made from the controller  18  of FIG.  1 . 
     At a time just prior to RF in  turning “on”, i.e., the transmitter  14  begins to transmit a RF burst, switch S 1  is opened or turned “off”. D 1  then operates to detect the RF power in the sampled portion of the burst, and the voltage appearing at the Detector Output node represents the peak value of the detected RF in  signal (V DET ) plus any bias voltage potential, minus the value of the previously stored potential that was present at the cathode of D 1 . In this manner the undesired bias voltage potential is eliminated from the Detector Output signal, which is the desired result. 
     It is assumed that during the RF in  “off” period no significant signal power is present at the input of the detector circuit  15 . It is further assumed that the duration of the RF in  “on” period is sufficiently short such that no significant drift occurs during the “on” period measurement. 
     It can be appreciated that the embodiment of FIG. 3 provides a wide degree of freedom in selecting the bias point of D 1 , and also in the overall design of the biasing network, in that C 3  functions as a dc blocking capacitor. That is, C 3  blocks the D 1  bias potential from appearing at the detector output node. 
     Referring to FIG. 4A, the bias point (BP) of D 1  is preferably established so as to operate D 1  on the non-linear portion of the diode transfer curve. A suitable value of diode bias current (I D ) is in the range of 150 μA to 200 μA. Referring also to FIG. 4B, it can be seen that the effect of biasing D 1  is to shift the curve of voltage sensitivity (V SENS ) versus temperature so as to increase the voltage sensitivity for a given temperature. In FIG. 3 the values of biasing resistors R 1  and R 2  are selected as a function of the supply voltage +Vcc, and as a function of the value of R 3 . The value of R 3  is selected, in conjunction with the value of C 2 , so as to provide an RC time constant that is significantly longer than the period of RF in . By example, and assuming that the frequency of the transmitted power in the RF burst is about 2 GHz (e.g., in a DCS 1900 embodiment), then C 2  may have a value of 27 pF, and R 3  a value of 2.2 KΩ. 
     The Detector Output signal may be connected to an input of an amplifier, and used with a power setting signal (Txc), as is shown generally in FIG. 8 with respect to the amplifier  56 . 
     FIG. 5 illustrates a second, digital implementation of the power detector  15  of FIG. 1 in conjunction with an entire power control loop. In this embodiment the power detector D 1 ′ is shown connected to an output of a power coupler  40  which is located at the output of the adjustable gain RF amplifier  42 , between the amplifier  42  and the antenna  12 . Power coupler  40  may be conventional in construction. D 1 ′ is assumed to include the components depicted in FIG. 3, minus the capacitor C 3  and switch S 1  (see FIG.  7 ). The amplifier  42  forms a portion of the transmitter  14  in FIG.  1 . The output of D 1 ′ (both V BIAS  and V DET ) is connected to an inverting input of a loop amplifier  44 . The output of D 1 ′ is also connected to an input of an A/D converter  46 , which in turn has an output connected to an input of a signal processor  48  (which may be included in the controller  18  of FIG.  1 ). A second input to the processor  48  is the power setting signal, referred to as Txc. Txc is a signal (analog or digital) that represents a desired power level setting for the RF amplifier  42 , and which may be supplied by the controller  18  of FIG.  1 . An output of the signal processor  48  is connected to an input of a digital to analog converter (DAC)  50 , which has an output connected to the non-inverting input of the loop amplifier  44 . An optional switch S 2  is connected in series with the RF signal to be amplified, and is controlled by a timing signal that is also applied to the A/D converter  46 . S 2  is optional in that the RF amplifier  42  acts to block the RF signal when the mobile station  10  is not transmitting. If an acceptably low RF leakage level can be obtained at the output of antenna  12 , within the control range of RF amplifier  42 , then S 2  can be eliminated. 
     In operation, the state of S 2  and the sampling time of the A/D converter  46  are timed such that a sample is taken by the A/D converter  46  only when the RF burst is “off”. This sampled value is stored, and represents the magnitude of V BIAS  output from D 1 . The processor  48  produces a digital control signal according to the magnitude of Txc, and the previously measured RF “off” sample from A/D converter  46 , and provides a corrected Txc signal (Txc CORR). The corrected Txc signal is applied to the non-inverting input of the loop amplifier  44 , which operates to subtract, during the RF burst, the magnitude of (V BIAS +V DET ) from Txc CORR. 
     Initially the gain of the RF amplifier  42  is at a minimum, and provides blocking of the input RF signal. As Txc CORR begins to increase in magnitude, the output of the loop amplifier  44  rises to keep the power control loop in equilibrium. As such, the output voltage of the loop amplifier  44  depends on the gain control properties of the RF amplifier  42 , and thus may not be zero. If the gain of the RF amplifier  42  is greater than or less than the gain specified by Txc, then the output of the loop amplifier  44  will assume some value which will correct the gain of amplifier  42  in a closed-loop manner. In general, the output of the loop amplifier  44  will be within some range (e.g., 0V to +5V, −2.5V to +2.5, etc.) that is predetermined to control the gain of the RF amplifier  42  so as to provide from approximately zero output power to full output power. The end result is that the output of the loop amplifier  44  controls the RF signal output from the RF amplifier  42  such that the total detector output signal caused by the detected RF signal at the input of the RF detector D 1 ′, plus the detector bias signal (a voltage in this case), plus any offset signal, is substantially equal to the value of Txc CORR. 
     In accordance with this aspect of the invention the processor  48 , in conjunction with the A/D converter  46 , modifies the value of Txc to accommodate the magnitude of V BIAS . As the temperature changes and the bias potential of D 1  varies, and/or as D 1  and the associated components age, these error sources are automatically taken into account and compensated for. The operation of processor  48  and A/D converter  46  may be considered as a digitally implemented sample and hold function for the output of D 1 ′ when no RF signal is input to D 1 ′. 
     Referring to FIG. 6 it can be seen that the RF burst is preceded by a period when, for example, S 2  is optionally opened and the A/D converter  46  commanded to sample the output of D 1 ′. During this time the processor  48  stores the digitized value of V BIAS  appearing at the output of D 1 ′. S 2  is closed prior to the beginning of the RF burst. During the beginning of the RF burst the value of Txc CORR is gradually ramped-up to the commanded level by providing a series (e.g., 32) of values to the D/A converter  50  so as provide a specified ramp-up of the transmitted burst power. 
     The maximum magnitude of the RF burst is eventually established within some tolerance (e.g., 1 dB) of the desired magnitude set by the value of Txc. At the end of the RF burst another series (e.g., 32) of values are provided to the D/A  50  to provide a controlled RF burst ramp-down period. During the burst the value of Txc CORR accommodates the previously measured value of V BIAS . 
     It should be noted that it may not be necessary to sample the output of D 1 ′ between each RF burst. By example, in a GSM embodiment the RF bursts typically occur at a rate of one every 4.6 milliseconds, while D 1 ′ may be sampled only every, by example, 5 to 10 seconds. That is, due to the typically long term nature of the drift in the output of D 1 ′, the sampling of the output of D 1 ′ is not required to be accomplished prior to every transmitted RF burst. If the optional switch S 2  is provided, then S 2  need be opened only when D 1 ′ is sampled (e.g., every 5-10 seconds). The actual sample time need only be as long as is required to measure the potential of V BIAS , including any required settling time. 
     FIG. 7 illustrates an embodiment of the power detector  15 ′ using an operational amplifier  52  to cancel the dc drift of the detector circuit D 1 ′. When the RF burst is not present (RF off) S 3  switches the output of D 1 ′ to capacitor C 4 . This action, referred to as a calibration period, stores the value of V BIAS  on C 4 . Before applying the RF signal to the input of D 1 ′ S 3  is toggled to connect the output of D 1 ′ to R 4 , which is connected to the inverting input of amplifier  52 . Due to the charge stored on capacitor C 4  the voltage at the non-inverting input of the operational amplifier  52  remains at the value determined during the calibration period. Thus the output of the operational amplifier  52  is zero for zero RF input power, assuming that the operational amplifier input offset voltage can be neglected. That is, the potential appearing on C 4  cancels the potential applied to R 4 . A response to RF power is obtained as soon as the RF burst signal is present at the input of D 1 ′. In this case the detected voltage is inverted and amplified by the ratio of resistors R 4  and R 5 , while subtracting the potential (i.e., V BIAS  plus any optional offset potential) that appears on C 4 . 
     As with the embodiment of FIG. 5, it may not be necessary to sample the output of D 1 ′ during each RF burst off period, depending on the leakage of C 4 , the input impedance of amplifier  52 , the bias or leakage current of the amplifier input, and the RF burst repetition rate. 
     It can be appreciated that the embodiments of FIG. 3,  5  and  7  can be used to advantage in mobile stations operating in TDMA based systems such as IS-136, GSM, PCN, and DCS 1900, in which the transmitted RF signal is inherently pulsed. 
     The foregoing embodiments of this invention have been described in the context of pulsed RF signals, such as those encountered in TDMA mobile stations. However, the teachings of this invention may also be applied to the detection of transmitted power in continuous wave (cw) systems, such as the North American AMPS system. 
     Continuous wave RF signals do not inherently contain off periods that can be used for measuring and cancelling the bias voltage of D 1 ′. In many cases, however, the portion of the transmitted RF signal that is coupled to the envelope detector D 1 ′ can be toggled on and off without disturbing the transmitted signal. In this case the teachings of this invention can be extended to also cover cw transmission systems. 
     If the detector D 1 ′ is part of a closed loop control system, the toggling of the RF signal coupled to D 1 ′ may disturb the operation of the loop. During the calibration period (RF of f) the output of the detector or the loop amplifier should be held constant in order to avoid an undefined state of the control system. This is accomplished by providing, as is illustrated in FIG. 7, an analog sample and hold (S/H) circuit  53  comprised of an amplifier  54  connected as a voltage follower, a sampling switch S 4 , and a hold capacitor C 5 . It should be noted that the S/H function can be implemented either at the output of the power detector  15 ′, or at the output of the loop amplifier (see FIGS.  9 A and  9 B). A digital loop amplifier may also be used if programmed in such a way that the output would remain unchanged during the short calibration period of the detector. 
     In this embodiment S 4  is opened during the RF off period, i.e., when the detector  15 ′ is being calibrated. During this time the previous value of the output of amplifier  52 , which was stored (held) on C 5  when S 4  was closed, is output from the amplifier  54 . In this manner a continuous output is provided from the output of the S/H to the power control loop. 
     If the loss of tracking for even a short detector calibration period cannot be tolerated, then two detectors, connected in parallel as shown in FIG. 8, can be used alternately. That is, while one detector circuit  15 ′ is being calibrated (S 5  disconnects the input from RF in ) the other is connected to the control loop through S 6  and amplifier  56 . Capacitor C F  is a filter capacitor which may be provided to reduce or eliminate any switching transients resulting from the operation of S 6  and S 5 . It should be noted that in this embodiment S 5  and S 6  are operated in-phase with one another. 
     FIGS. 9A and 9B illustrate in greater detail the cw embodiments of this invention, wherein a switch (S BLOCK ) is positioned between the output of the power coupler and the detector D 1 ′, for the embodiment of FIG. 9A, or the detector/drift correction circuit  15  (see FIG.  3 ), for the embodiment of FIG.  9 B. In both of these embodiments S BLOCK  is opened when it is desired to sample the magnitude of V BIAS  from the detector diode D 1 . For the embodiment of FIG. 9B the above-described switch S 1  is operated out of phase with S BLOCK , that is, when S BLOCK  is open S 1  is closed, and vice versa. 
     In greater detail, during the detector bias level sampling period the closed control loop operation is temporarily suspended. This is because the RF input into the detector D 1  is blocked, and no feedback indication regarding the output power is available. Thus, the closed loop control is temporarily inoperative during the detector bias level sampling period. The S/H  53  is used during this period to hold the gain control signal of the RF amplifier  42  at a fixed potential. The fixed potential has the value that was last sampled during closed loop operation, just before the closed loop operation was suspended. During this period the output of the S/H  53  supplies the RF amplifier  42  power control signal, not the loop amplifier  44 . Immediately after opening S BLOCK , and placing the S/H  53  in the hold state, V BIAS  sampling of the detector output can occur. During the detector calibration period the output power is not controlled by the closed loop, but it can be assumed that for the relatively short calibration period the output power remains substantially constant. At the end of the calibration period S BLOCK  is first closed, and thereafter the RF amplifier gain control signal is allowed to be controlled again by the output of the loop amplifier  44 , thereby reestablishing normal closed loop operation. 
     It is also within the scope of this invention to use the embodiment of FIG. 7 at the input to the loop amplifier  44  (i.e.,  15 ′ followed by the S/H  53 ), and to then couple the output of the loop amplifier directly to the gain control input of RF amplifier  42 . 
     Although described in the context of several presently preferred embodiments, it should be realized that a number of modifications to these teachings may occur to one skilled in the art. By example, the various component values, burst repetition rates and the like that were described above are exemplary, and should not be read as a limitation on the practice of this invention. Also, the RF input to the power detector can be blocked in a number of ways, such as by providing a switch at the input to the RF detector to periodically short the RF signal to ground. 
     Also, when operating in the TDMA embodiment it should be realized that it is possible that an RF signal is continuously provided from the modulator  14 A of FIG. 1, but that the RF signal contains modulated information only during a burst time. In this case the actual RF burst can be formed in conjunction with the transmitter amplifier  42 . 
     Thus, while the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.