Abstract:
An RF power controller arrangement prevents excessive RF power-based thermal loading of an RF signal processing device, such an as acousto-optic modulator, by controllably constraining the product of the average on-duration of a baseband modulation signal and RF input power to realize no more than a prescribed value of RF energy supplied to the modulator.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates in general to signal processing systems and components therefor, and is particularly directed to a new and improved apparatus and method for preventing excessive RF power-based thermal loading of an RF signal processing device, such an as acousto-optic modulator.  
         BACKGROUND OF THE INVENTION  
         [0002]    Many RF signal processing applications require that an amplitude modulated RF signal be applied to a load, such as, but not limited to an acousto-optic modulator, whose known power handling capabilities may be exceeded under conditions where the total energy delivered in a given period of time may cause damage to the device. For the most part, damage may be attributable to excessive heat generation, which must not be permitted to exceed a maximum safe level in order to ensure proper operation of the device. At the same time, the instantaneous power level delivered to the load may be quite large, as in the case of intermittent pulse radar or low duty cycle drive requirements for certain acousto-optic modulator (AOM) applications.  
           [0003]    In a typical AOM application, a reduced complexity diagram of which is illustrated in FIG. 1, a source of RF carrier power is coupled by way of an RF input port  11  to an input gain stage  12 . The output of the input RF gain stage is coupled by way of a variable gain attenuator or amplifier (VGA)  13  to a first, RF carrier input port  14  of an amplitude modulator stage  15 . Amplitude modulator stage  15  has a second, baseband input port  16  to which a baseband modulation signal Vam is supplied. The output of the baseband modulator  15  produces a modulated RF signal which is amplified in a main RF power amplifier  17  and then applied to an output device or load, such as an acousto-optic modulator  18 . If the baseband signal Vam is not controlled by the user in such a manner as to limit the amount of RF energy delivered to the load within a given time interval (i.e., in a manner effectively limiting the average RF power level over that interval), damage to the load may occur.  
           [0004]    Conventional approaches for limiting RF power rely on direct measurement of the instantaneous modulated RF envelope being supplied to the load, with post integration of the detected RF power level. Post integration is most often performed by means of a low pass filter network, whose output depends on the impedance of the RF power detector and the baseband spectrum, both of which contribute to reduced output with very short duration pulses (e.g., less than ten nanoseconds). As these approaches are expensive, there is a need for a more ‘practical’ (i.e. cost effective) mechanism for limiting RF output power to safe levels, that will not induce thermal overload in the output device.  
           [0005]    Theory of Operation  
           [0006]    Referring to FIG. 1, let the source RF carrier power level of RF in  be given by P in . Further let G A  be the product of all constant RF gain sources in the RF signal path, with variable gain G V  from a variable gain attenuator or amplifier, and a gain G M  associated with the RF modulator. Then the total output energy E o  supplied to the load in a time τ is given by:  
             E   o   =∫P   o ( t ) dt   (E.1)  
           [0007]    where P o (t), the instantaneous output power delivered to the load, is just the product of P in  and the total gain. Rewriting (E.1) in terms of the product gains gives:  
             E   o   =G   A   ·G   V   ·Pi   n   ∫G   M ( t ) dt   (E.2)  
           [0008]    Further, if G M (t) and V AM (t) are related to within a suitable scaling constant and offset, ie. a linear relationship exists between the two, then ignoring the offset:  
           G M ( t )=κ·V AM ( t )  (E.3)  
           [0009]    and,  
           κ·∫V AM ( t ) dt =∫G M ( t ) dt   (E.4)  
           [0010]    The leftmost integral may be determined directly in any circuit application by use of a simple analog video integrator, where the output voltage from the integrator V o  is given by:  
               V   o     =         -       A   V       τ   o         ·       ∫   0   τ              V   AM          (   t   )                          t           =     C1   ·       ∫   0   τ            V   AM                        t                     (     E   .              5     )                               
 
           [0011]    with a suitable video scaling constant Av applied, where τ 0 =R·C is the integrator time constant, given by the product of the input resistance R and feedback capacitance C. Note there are no restrictions placed on the form of V AM (t), other than its suitability for application to a video integrator. From (E.4) and (E.5) we obtain the result:  
                 κ   C1     ·     V   o       =       ∫   0   τ              G   M          (   t   )                          t                 (     E   .              6     )                               
 
           [0012]    Correspondingly, if G M (t)=G max  is constant over the integration period τ, where G max  represents the maximum gain associated with the modulator, then:  
                 κ   C1     ·     V   max       =         ∫   0   τ            G   max     ·                   t         =       G   max     ·   τ               (     E   .              7     )                               
 
           [0013]    where V max  represents the maximum extrapolated output voltage which the integrator would produce. Dividing both sides of (E.2) by the integration period τ, gives the result:  
               P   avc     =           G   A     ·     G   V     ·     Pi   n     ·     1   τ              ∫   0   τ              G   M          (   t   )                          t           =       G   A     ·     G   V     ·     Pi   n     ·     〈       G   M          (   t   )       〉                 (     E   .              8     )                               
 
           [0014]    where P ave  is the average output power delivered to the load during the integration period, and &lt;G M (t)&gt; is the time average gain of the modulator over the same time interval. Using (E.6) and (E.7), we can express this result in terms of the integrator output voltages as:  
               P   ave     =       G   A     ·     G   V     ·     G   max     ·     P   in     ·     (       V   o       V   max       )               (     E   .              9     )                               
 
           [0015]    This is the key result as applied to the current claim, wherein a simple circuit concept is proposed which, utilizes the established gain values for G A , G V , G max  and V max , measures Pin and performs the necessary integration of the modulation signal in order to set P ave  to a safe operating level.  
           [0016]    By way of example, we take the case of simple pulse modulation, where we have the following conditions:  
                       G   M          (   t   )       ;         V   AM          (   t   )       =                G     M   ;            V   &#39;                   for                   V   i       ≥     V   th                              0   ;       0                                for                   V   i       ≺     V   th                       (     E   .              10     )                               
 
           [0017]    where the value of the modulating voltage is V′, for an input control voltage V i  which exceeds a logic threshold, and zero otherwise. Then (E.8) gives;  
               P   o     =       G   A     ·     G   V     ·     G   M     ·     P   in     ·     (       τ   on     τ     )               (     E   .              11     )                               
 
           [0018]    where τ on  is the total “on” time of the modulator during the integration interval τ, during which it exhibits a maximum constant gain G M . By performing the corresponding video integration of the modulating voltage as expressed by (E.6) and (E.7), we obtain the result;  
                 τ   on     τ     =       V   o       V   max               (     E   .              12     )                               
 
           [0019]    which upon substitution in (E.11) produces the same results given by (E. 9) for the general case. The approach to providing a low cost energy limiting function which fully exploits the results of the theory described, forms the basis for the present invention.  
         SUMMARY OF THE INVENTION  
         [0020]    In accordance with the present invention, this objective is successfully addressed by a low cost, pre-modulation protection circuit, which performs measurements on the RF and modulation signal transport paths, and then limits the total output RF energy delivered to the load, in a manner that is fundamentally insensitive to RF pulse duration on-time and duty cycle, while permitting the use of low cost monolithic RF detector integrated circuits. As in the architecture of FIG. 1, the invention has an RF input port coupled to a variable gain stage. This stage is operative under supervisory microcontroller control to impart a prescribed amount of attenuation or gain to the input RF carrier.  
           [0021]    The output of the variable gain stage is coupled to each of a baseband modulator and an RF power detector. The baseband modulator is coupled to receive a baseband amplitude modulation signal and has its output coupled to a main RF power amplifier which drives an output load, such as an acousto-optic modulator. The RF power detector outputs a value representative of the power of the RF carrier signal being supplied to the baseband modulator so as establish the instantaneous power level at a maximum output level. This monitored RF power value is coupled to the microcontroller and is used in conjunction with the baseband modulation signal to controllably constrain the RF output power delivered to the load.  
           [0022]    For this purpose, the baseband modulation signal is coupled to a dual integrator unit which integrates the baseband modulation signal over a prescribed integration interval (τ), in order to determine the average modulator gain produced by the baseband signal Vam(t). The use of a pair of precision integrators within the dual integrator unit, whose integration and reset times are controlled by the microcontroller, allows ‘ping-pong’ operation, to ensure continuous monitoring of the average modulator gain, and provides a pulse width and duty cycle insensitive mechanism for measuring the average power of the output waveform.  
           [0023]    The output level from the RF power detector is digitized by the microcontroller to establish an appropriate average modulator gain threshold voltage from the integrators under which safe output conditions are allowable. The values for these thresholds may be stored in memory, and are compared to the average value of the digitized output from the dual integrator unit. Knowing the nominal parameters of the baseband modulation signal enables the microcontroller to determine from the output of the integrator the average modulator gain produced by the baseband signal. Whenever the allowable average modulator gain threshold is exceeded for a given RF level, the microcontroller takes the appropriate action to either reduce the amplifier gain or inhibit modulation input.  
           [0024]    Namely, assuming that maximum RF carrier power is applied to the load for the entirety of the on-time of the baseband modulation waveform there is an associated maximum RF carrier power that can safely be coupled to the load. The longer the average on time of the baseband signal, the smaller the value of RF power that can be applied to the load without causing damage or degradation of its properties. Conversely, where the average on time of the amplitude modulation signal is relatively short (e.g., a very narrow RF pulse), the load is able to handle a much higher value of RF power.  
           [0025]    The problem to be avoided is not peak RF power, but total energy. An RF load device, such as an acousto-optic modulator, can handle a very narrow pulse (e.g., several nanoseconds) of very high amplitude RF power, whereas applying such a large value of RF power over a longer period of time would entail the application of a relatively large average amount and therefore destructive value of energy. In order to ensure that the load is never presented with an unacceptably large quantity of energy, the present invention establishes the average RF power in accordance with the average modulator gain produced by the baseband signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]    [0026]FIG. 1 diagrammatically illustrates a conventional RF signal processing path through an acousto-optic modulator;  
         [0027]    [0027]FIG. 2 diagrammatically illustrates an apparatus embodying the pre-AOM RF energy-limiting mechanism of the present invention; and  
         [0028]    [0028]FIG. 3 shows a baseband modulation signal Vam(t) as an ON-OFF keyed signal, having its amplitude controllably varied between a prescribed amplitude excursion range of Amax and Amin. 
     
    
     DETAILED DESCRIPTION  
       [0029]    Before describing in detail the new and improved RF energy limiting mechanism in accordance with the present invention, it should be observed that the invention resides primarily in what is effectively a prescribed arrangement of conventional RF signal processing circuits and components therefor. Thus, the configurations of such circuits and components and the manner in which they are interfaced with other RF system and baseband components have, for the most part, been illustrated in the drawings in readily understandable block diagram format, so as to show only those specific details that are pertinent to the present invention, and not to obscure the disclosure with details which will be readily apparent to those skilled in the art having the benefit of the description herein, and are primarily intended to show the major components of the system in a convenient functional grouping, whereby the present invention may be more readily understood.  
         [0030]    Referring now to FIG. 2, an apparatus embodying the pre-modulator RF energy-limiting mechanism of the present invention is shown diagrammatically as comprising an RF input port  21  to which an RF carrier frequency is applied. As in the architecture of FIG. 1, RF input port  21  is coupled by way of an input gain stage  30  to a variable gain (VGA) stage  40 . Under control of a supervisory microcontroller  50 , the variable gain stage  40  is operative to impart a prescribed amount of attenuation or gain to the input RF carrier in accordance with a digital control signal applied thereto by microcontroller  50 .  
         [0031]    The output of the VGA stage  40  is coupled to a first input  61  of a baseband modulator/multiplier stage  60 , a second input  62  of which is coupled to receive a baseband amplitude modulation signal Vam(t). As a non-limiting example, the baseband modulation signal Vam(t) may comprise an ON-OFF keyed signal, such as that shown in FIG. 3, having its amplitude controllably varied in an on/off manner between extreme ends of a prescribed amplitude excursion range Amax and Amin. Baseband modulator stage  60  has its output  63  coupled to an RF power amplifier  70 , the output of which drives an output load, such as an acousto-optic modulator  80 .  
         [0032]    The output of the VGA stage  40  is further coupled by way of a directional coupler  45  to an RF power detector circuit  90 , such as a commercially available RF power detector integrated circuit (IC), which outputs a value representative of the power of the RF carrier signal being supplied to the baseband modulator  60 , so as establish the instantaneous power level at a maximum output on-level. This monitored RF power value is coupled to the microcontroller  50 , and is used in conjunction with the baseband modulation signal Vam(t) to controllably constrain the RF output power delivered to the load  80 , as will be described.  
         [0033]    The baseband signal Vam(t) that is applied to the baseband input  62  of modulator  60  is further coupled to a precision dual or ‘ping-pong’ integrator unit  100 . This dual integrator unit is operative to integrate the baseband modulation signal over a prescribed integration interval (τ), in order to determine the fractional on-time (See E.12 for ON-OFF keyed signal example) of the baseband signal Vam(t). As pointed out above, the use of a pair of precision integrators within the integrator unit  100 , whose integration and reset times are controlled by the microcontroller  50  allows ping-pong operation, so as to assure continuous monitoring of the fractional on-time, and provides a pulse width and duty cycle insensitive mechanism for measuring the on-time of the modulation waveform.  
         [0034]    The output level from the RF power detector  90  is digitized by microcontroller  50 , which may employ a prescribed number of most significant bits to establish an appropriate on-time threshold under which safe output conditions are allowable. The values for these thresholds may be stored in a resident EEPROM within the microcontroller, and are compared to the average value of the digitized provided from the dual integrator unit  100 .  
         [0035]    Knowing the nominal parameters of the baseband modulation signal Vam(t), microcontroller  50  determines from the integrated output from the integrator unit  100  the average on-time of the baseband signal Vam(t). Whenever the allowable on-time threshold i-s exceeded for a given RF level, the microcontroller takes the appropriate action to either reduce the amplifier gain or inhibit the baseband modulation input.  
         [0036]    Namely, as described previously, for a given on-time of the baseband modulation signal Vam(t), there is an associated maximum RF carrier power that can safely be delivered to the load, assuming that the maximum RF carrier power is applied to the load for the entirety of the on-time of the baseband modulation signal Vam(t) The greater the fractional on-time of the baseband modulation signal Vam(t), the smaller the value of RF power that can be applied to the load during this on-time interval without causing damage or degradation of its properties. Conversely, where the fractional on-time of the amplitude modulation signal Vam(t) is relatively short (e.g., a very narrow RF pulse), then the load is able to handle a much higher value of RF power.  
         [0037]    As noted above, the issue is not peak RF power, but total energy. An RF load device, such as an acousto-optic modulator, can customarily handle a very narrow pulse (e.g., an on-time of only several nanoseconds) of large peak RF power, whereas applying such as large value of RF power applied to the load over a longer period of time would entail the application of a relatively large average amount and therefore destructive value of energy. In order to ensure that the load is never presented with an unacceptably large quantity of energy, the present invention establishes the peak RF power in accordance with the fractional on-time, or more generally the time average modulator gain, produced by the baseband signal as measured by the integrator unit  100 . In accordance with the invention, microcontroller  50  may control the magnitude of the RF carrier, such as by reducing RF amplifier gain, controlling the setting of the variable gain stage  40 , or inhibiting the input to the amplitude modulator  60 , so that the peak power is no greater than the threshold associated with the detected average gain of the modulator, as determined by the dual integrator unit  100 . In this manner, the invention ensures that the load/modulator  80  will never be presented with a potentially destructive amount of RF energy, irrespective of the amplitude of the RF carrier input.  
         [0038]    As will be appreciated from the foregoing description, by performing measurements on the RF and baseband modulation signal transport paths, the pre-modulation protection circuit of the invention is able to limit the total output RF energy delivered to a load such as an acousto-optic modulator, in a manner that is fundamentally insensitive to RF pulse duration on-time and duty cycle, while permitting the use of low cost monolithic RF detector integrated circuits.  
         [0039]    While we have shown and described an embodiment in accordance with the present invention, it is to be understood that the same is not limited thereto but is susceptible to numerous changes and modifications as known to a person skilled in the art. We therefore do not wish to be limited to the details shown and described herein, but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.