Abstract:
Power measurements of a signal having a changing frequency are corrected by acquiring samples of the signal, matching the samples of the signal to a sequence of frequencies indexed to correction transforms, and applying the correction transforms to the samples of the signal. Additionally, an apparatus corrects power measurements of a signal having a changing frequency and includes an acquisition block for acquiring samples of the signal, a sequence controller for matching the samples of the signal to a sequence of frequencies indexed to correction transforms and a correction block for applying the correction transforms to the samples of the signal.

Description:
BACKGROUND OF THE INVENTION 
     Power meters require the user to select a carrier frequency before making power measurements. This frequency setting is mapped to a correction factor that is be applied to all subsequent power measurements. A typical graph  100  showing correction or calibration factor  101  versus frequency  103  is shown in  FIG. 1 . The graph  100  is for a 8481A thermocouple sensor from Agilent Technologies, Inc. of Santa Clara, Calif., USA. Modern diode power sensors, such as those used to characterize pulsed RF systems, require even more complex measurement corrections. These can vary considerably over frequency and make the selection of the proper carrier frequency even more important. 
     Frequency hopping signals have existed for many years. Frequency hopping moves, or hops, the carrier frequency of a signal between an upper and lower frequency range a number of times per second. This tends to spread out the signal&#39;s spectrum over that frequency range. Frequency hopping provides resistance to jamming and low probability of intercept which is useful in military radar and communication applications. 
     One current communication standard that makes use of frequency hopping is Bluetooth. Bluetooth has 79 channels each 1 MHz wide and hops from one channel to another in a pseudorandom manner 1600 times per second. Frequency hopping is also used in cordless phones following the WDCT standard and the HomeRF standard which provides for a broader range of interoperable consumer devices. 
     When using a power meter to measure frequency hopping signals, due to the nature of the signal which hops its carrier frequency within a range of frequencies, the measurement will have error if the frequency of the power meter is set to a single value. Average responding thermal sensors will not show too much degradation in accuracy. They will, however only report the average power of the transmission as it hops through all frequencies. 
     A bigger problem is encountered when the power at each frequency in the hopping sequence has to be checked. Even with fast pulse trains, some power meters have the capability to capture and analyze every pulse in a pulse train. Due to the complexity of the applied corrections there has been only one set of frequency corrections applied and thus, if all pulses are captured, there is an uncertainty added due to the wrong frequency correction being applied. One solution has been to set the power meter to the desired frequency and delay the acquisition until the point in the hopping sequence when that frequency is transmitted. This is repeated until the measurements at all desired frequencies are completed. However, this solution is very slow as it requires that the sequence be repeated N times in order to measure the power at all N frequencies in the sequence. 
     In addition to signal types that are designed to be hopping there is also the common component test scenario where the test signal is stepped through a sequence of frequencies. This type of testing is often used to measure the gain of amplifiers, for example, as a function of frequency. These test scenarios suffer from problems similar to those of frequency hopping signals as in both scenarios obtaining the results is slow or the accuracy is compromised. 
     SUMMARY OF THE INVENTION 
     The present invention provides fast frequency correction of power measurements for frequency hopping signals or more generally for signals having changing frequency. 
     In general terms, power measurements of a signal having a changing frequency are calibrated by acquiring samples of the signal, matching the samples of the signal to a sequence of frequencies indexed to correction transforms, and applying the correction transforms to the samples of the signal. Additionally, an apparatus calibrates power measurements of a signal having a hopping carrier frequency and includes an acquisition block for acquiring samples of the signal, a sequence controller for matching the samples of the signal to a sequence of frequencies indexed to correction transforms and a correction block for applying the correction transforms to the samples of the signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further preferred features of the invention will now be described for the sake of example only with reference to the following figures, in which: 
         FIG. 1  shows a typical graph of calibration factor versus frequency. 
         FIG. 2  shows a typical frequency hopping signal as a function of time as its frequency cycles through the hopping sequence. 
         FIG. 3  is a diagrammatic block diagram showing a power measurement system having fast correction of power measurements of an embodiment of the present invention. 
         FIG. 4  is a flowchart illustrating a calibrated power measurement method for using the power measurement system having fast correction of power measurements of  FIG. 3 . 
         FIG. 5  is a diagrammatic block diagram showing a power measurement system having fast correction of power measurements of another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  shows pulses  205  through  207  of a typical frequency hopping signal  200  as a function of time as its frequency cycles through the hopping sequence from a first carrier frequency f 1   201  to a carrier frequency fn  203  and back again to f 1   201 . 
       FIG. 3  is a diagrammatic block diagram showing a power measurement system having fast correction of power measurements  300  of an embodiment of the present invention. 
       FIG. 4  is a flowchart illustrating a calibrated power measurement method  400  for using the power measurement system having fast correction of power measurements  300  of  FIG. 3 . The first steps of the method  400  provide a correction transform generation method  401 . At STEP  403  a frequency sequence table  301  is populated by a user based on a user&#39;s knowledge of the hopping sequence of the frequency hopping signal  200 . In other words, the sequence of frequencies can be known ahead of time based on the predetermined hopping sequence of the hopping carrier frequency. The sequence of frequencies stored in the sequence table is thus also predetermined depending on the predetermined or fixed sequence of the frequency hopping signal  200 . Alternatively, the frequency sequence table  301  can be populated automatically or semi-automatically. The signal  200  has “n” channels, hopping from one to another, (e.g. from the channels ranging from carrier frequencies f 1   201  to fn  203 ) and returning to the first channel f 1   201  after the nth hop at the frequency fn  203 . Values for the hopping frequencies  303  (i.e. from f 1  to fn) are stored in the frequency sequence table  301 . 
     More generally the signal can have any type of changing frequency. For example, as described above in the testing used to measure the gain of amplifiers as a function of frequency. In general the values of the changing frequencies can be used to populate the frequency sequence table  301  to practice the present invention. 
     At STEP  405  a correction transform section  305  generates a correction transform for each of the frequencies  303  stored in the sequence table  301 . The correction transforms can be generated either through manipulation of calibration data or from the evaluation of functions derived to model the calibration data. The correction transform can be the simple calibration factors  101  of  FIG. 1 . These calibration factors are numerical values that represents the ratio between the sensor output voltage at a particular frequency and sensor output voltage at a calibrating frequency, given the same input power at both frequencies. The calibration factors vary at different frequencies and can be expressed as a percentage, with 100% corresponding to the calibration factor at the calibration frequency. This will generate a linear transformation of voltage to power. 
     For some sensor types the transformation is non-linear and the process is often referred to as linearity correction. This linearity correction may vary with frequency hence the need for linearity correction at each of the frequencies in the hop sequence. 
     More generally, the voltage to power transform (or V2P@f in abbreviated form) at a given frequency is a function of the applied power and the sensor temperature. So for a given frequency and temperature a look up table can be generated (the table made up of “correction transforms”) by the correction transform section  305  to convert the voltage to power. 
     At STEP  405  the arm signal  308  can be sent to a sequence controller  315  to allow the sequence controller to ignore trigger events (the sequence trigger  313  and “signal” triggers  317  described below) until the tables are all in place. 
     The correction transform section  305  can be a CPU  306  or other processor, for example. 
     At STEP  407  the correction transforms are passed to a correction transform storage table  309  by passing them through a linearity corrections buffer  307 . The linearity corrections buffer is the store of linearity corrections. In many power meters the linearity correction is performed through the use of a look-up table, so in that case the linearity corrections buffer has all the look-up tables for the sequence of frequencies. 
     At STEP  409  a full set of correction transforms  311  is stored in the transform storage table  309 . The correction transforms  311  are indexed to the frequencies  303  of the sequence table  301 . 
     Further steps provide a trigger, acquisition and correction method  411 . 
     At STEP  413  the sequence of power measurements is initialized by a sequence trigger  313  fed into the sequence controller  315 . The sequence trigger  313  synchronizes the selection of the frequency values  303  to incoming raw samples  321 . The raw samples  321  can be the voltage output by a sensor in response to receiving the power the pulses  205  through  207  forming the frequency hopping signal  200  of  FIG. 2 , for example. The sequence trigger can come from the user. A control command can be issued at the start of a measurement or from an external signal that is synchronous with the start of the hopping sequence. Alternatively it can be the first signal trigger that comes along after the correction transforms are set up. 
     At STEP  415  subsequent “signal” triggers  317  gate the acquisition of the raw samples  321  through an acquisition block  319 . The signal triggers come from either an external signal synchronous with the signal pulses or from the signal transitioning through a supplied threshold. The sequence trigger is synchronous with the hopping sequence while the signal trigger is synchronous with the signal pulses. At STEP  417  the raw samples  321  are acquired by the acquisition block  319 . Captured raw samples  331  are then output from the acquisition block  319  to a correction block  323 . 
     At STEP  419  the subsequent “signal” triggers  317  are also fed into the sequence controller  315  to gate a correction transform  329  (selected from one of the correction transforms  311  corresponding to one of the stored frequencies  303 ) into the correction block  323 . Upon each of the subsequent “signal” triggers  317 , the next frequency value  303  for the next element in the frequency sequence table  301  is selected. The next correction transform  311  corresponding to the next frequency value  303  is also fed into the correction block  323 . 
     At STEP  421  the correction block  323  applies the correction transform  329  corresponding to the frequency of a captured raw sample  331  in order to correct the captured raw sample  331  to provide corrected power samples  333 . For example, when the captured raw samples  331  are samples of voltage measurements output by a power sensor, the correction block  323  can apply the voltage-to-power correction transform  329  that will convert the voltage measurement to a power measurement and correct for frequency dependency of the sensor measurements at the particular frequencies of the pulses  205  to produce corrected power samples  333 . These corrected power samples  333  are the desired measurement for the particular sample. The “signal” triggers  317  assure the correspondence of the frequencies  303  of the acquired raw samples  321  and the selection of the correction transforms  311  indexed to those frequencies  303 . 
     At STEP  423  the corrected power samples  333  are output from the correction block  323  to a measurement block  335 . The measurement block converts the corrected power samples  333  into the desired output measurement format. For example, a user may require data in the form of the average power in the pulse, in which case the measurement block would sum all the samples and divide by the number of samples. Similarly the user may desire the peak in the pulse or the average power in just the middle part of the pulse, for example. The measurement block  335  performs these calculations for converting the corrected power samples  333  to the desired output data format of measurements  337 . The measurement block  335  outputs the measurements  337  to a frequency sequence measurement table  339 . 
     At STEP  425 , as the frequency sequence  303  is stepped through, so the frequency sequence measurement table  339  is built up from the measurements  337  to fill-in the measurement table  339 . Once the sequence is complete the user can retrieve the set of measurements  337  from the table  339 . 
     The embodiment  300  has a measurement rate dictated by the rate at which the correction transforms  311  can be selected and applied to the captured raw sample  331  as illustrated by the cross-hatched blocks. Because of the pre-calculation there is no slow down. This means that in the embodiment  300  the frequency correction can be performed with no degradation in the measurement rate. This invention allows the frequency to be quickly changed without slowing down the measurement speed. 
     The frequency sequence table  301  can be stored on a computer readable media  325 , the transform storage table  309  can be stored on a computer readable media  327  and the measurement table  339  can be stored on a computer readable media  343 . The media  325 ,  327 ,  343  can be solid state storage, hard drives or any other data storage known in the art. The media  325 ,  327 ,  343  can be together on a larger combined media or can be separate. 
       FIG. 5  is a diagrammatic block diagram showing a power measurement system having fast correction of power measurements of an embodiment  500  of the present invention. The embodiment  500  of  FIG. 5  is similar to the embodiment  300  of  FIG. 3 , except that it makes use of the “Method and Apparatus for Extracting Individual Pulses from an Input Signal” as described in US Patent Publication 2007/0001887A1 to Colin Johnstone and Eric Breakenridge published on Jan. 4, 2007 (hereafter referred to as “Johnstone and Breakenridge”), which is hereby incorporated by reference in its entirety into the present disclosure. 
     An acquisition block  519  uses the method of “Johnstone and Breakenridge” to capture or extract the raw samples  321  and send captured raw samples  531  to a frequency sequence acquisition table  523  stored in a buffer or computer readable media  525 . The table  523  stores the captured raw samples  531  which are then passed to the correction block  323  as the samples  331  as is described in  FIG. 3  with respect to the embodiment  300 . In this way the samples for all the measurements are stored into memory before processing them. 
     The embodiment  500  is particularly useful for very fast frequency-hopping signals. For example where the signal is a burst of short frequency hopping pulses. In this case if each pulse were processed as it arrived, as in the embodiment  300 , some of the pulses would be missed due to the processing time. In the embodiment  500 , all the pulses in the burst can be captured and stored in the table  523  and then processed at the processing rate. There is then enough time to process the stored pulses before the arrival of the next burst. 
     The embodiment  500  has a rate of capture and acquisition of pulses dictated by the sample rate and memory size rather than the processing rate. Thus the embodiment  500  can keep pace with any pulse train that can be captured with the acquisition block  519 . Again, the cross-hatched blocks in the diagram depict processing that is carried out the rate at which the correction transforms  311  can be selected and applied to the captured raw sample  331   
     In another embodiment, measurements of pulses are captured at the beginning, middle and end of the frequency hopping signal  200  and also from the lower, middle and upper frequency ends of the hopping band. Thus, instead of processing each and every pulse in the hop sequence, just a few are selected. For example one from the low frequency end of the hop sequence, one from the middle and one from the top. For example, in the frequency hopping signal  200  of  FIG. 2 , if there is a hop sequence of  200  frequencies, then the pulses  23 ,  67  and  197  might be selected as they are placed at the 1 st , 100th and 200th frequencies. 
     The frequency sequence table  301  becomes a time frequency definition table and it selects not only the frequency of the pulse to be measured but also its position within the sequence. The table would store data indicating that pulse  23  is at frequency  1 , pulse  67  at frequency  100  and pulse  197  at frequency  200 . 
     In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. The specification and drawings are, accordingly, to be regarded in an illustrative sense rather than a restrictive sense.