Abstract:
A power spectrum waveform is obtained by logarithmically amplifying a signal received by a frequency-sweep operation, detecting the amplified output of each frequency sweep, converting the detected output into a digital signal value in decibels, and converting this digital signal value into an antilogarithmic power value in watts for each display point within the width of the frequency sweep. Upon completion of the frequency sweep operation, the power values which have been converted into antilogarithmic values for each frequency sweep are averaged for each display point, the average power values are converted into logarithmic values, and the logarithmic values are displayed as a spectrum display.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a spectrum analyzer that repeats to sweep frequencies of a received signal, logarithmically amplifies the received signal, detects the amplified output, and converts the detected output into a digital signal value to display the spectrum of the received signal. More particularly, the present invention relates to an apparatus and a method for displaying a power spectrum of the received signal. 
     FIG. 1 shows a conventional spectrum analyzer. An input signal from an input terminal  11  is inputted to a frequency converter  12 , where the input signal is frequency-mixed with a local signal from a local oscillator  13  to be converted into an intermediate frequency signal. This intermediate frequency signal is passed through an intermediate frequency bad-pass filter  14 . The output of the filter  14  is logarithmically amplified by a logarithmic amplifier  15 , and its dynamic range is compressed. The logarithmically amplified output is envelope-detected by a detector  16 . The detected output is sampled by an AD converter  17  at a constant period, and each sample is converted into a digital value. This digital value is stored in a memory  18 . 
     On the other hand, a central frequency of the signal to be received, a frequency band width, a frequency-sweep speed, a reference level and the like are inputted to the spectrum analyzer from input means  19 . In accordance with those inputted data, a CPU (a central processing unit) reads out a program stored in a program memory  22 , decodes and executes the program, and sets an oscillation central frequency of the local oscillator  13 . In addition, the CPU controls a sweep signal generator  20  then the sweep signal generator  20  sweep-controls the oscillation frequency of the local oscillator  13  so that receivable frequencies of the input signal from the input terminal  11  can repeatedly be swept. 
     In the conventional trace-average displaying method, digital values P in  (dBm) obtained in the repeated frequency-sweeps for each of display points (pixels) i (i=1,2, . . . , N) on a frequency axis (usually lateral axis) of a display device  24  are read out from the memory  18 , and the average value of those digital values Pin is obtained using a following equation by an average value calculating part  25 .          P   iavg     =       1   M            ∑     n   =   1     M            P     i                 n            (   dBm   )                                  
     P in  is a power value obtained by nth sweep for ith display point, and the unit thereof is dBm for representing 1 mW to be 0 (zero) dB. In this manner, an average power P iavg  for each display point i obtained by M sweeps is read out from an average value memory  26  and is displayed by the display device  24  under control of a display control part  23 . For example as shown in FIG. 2, the average values P iavg  are displayed on the longitudinal axis on a screen of the display device  24  for the respective display points 1−N on the lateral axis (frequency axis). 
     In this trace-average displaying method, an averaging process of the signal power values P i  (dBm) for each display point is performed. Therefore, this method is effective for averaging noises added on top of the signal. However, the measured values P i  for each display point are values each having been logarithmically compressed by the logarithmic amplifier  15 , and an arithmetic average of those logarithmic values is simply calculated. For this reason, the calculated value of the arithmetic average is not a correct average of the power of the input signal for the display point (frequency) i. That is, the trace-average displaying method does not display a correct average power distribution. 
     A digital-modulated signal has similarities to a white noise, and its amplitude probability density function depends on characteristics of the modulation type and the base-band filter modulation bit. The function for performing a channel power measurement provided in the conventional spectrum analyzer is described, for example, in the article entitled “Measure Adjacent-Channel Power With A Spectrum Analyzer” written by J. Wolf and B. Buxton in a magazine “Microwave &amp; RF”, June 1997. This function measures a power for each display point and obtains a channel power by the following equation.        10                   log        [       CHBW     RBW   ×     k   n         ×     1       n   2     -     n   1         ×         ∑   n     1     n   2            10         P   i          (   dBm   )       10           ]            (   dBm   )                            
     In this case, n 1  and n 2  are display point numbers of both sides of the channel, CHBW is the channel band width, RBW is the resolution band width of the spectrum analyzer, P i  is a level of ith display point (dBm), and k is a correction coefficient for the resolution band width (RBW×k n =power band width). 
     That is, measured values P i  (dBm) for each display point are converted into true values 10 Pi/10  (mW) each having unit of watt (mW), and an average of the true values for each display point in the channel band is calculated. Further, power per channel band is obtained using the average value, and the logarithmic value of the power is displayed. In this case, the power of the entire channel band can be obtained, but the power of each spectrum or the power density of each display point cannot be obtained. In addition, a spectrum display of the power cannot also be performed. 
     Furthermore, it is described in the aforementioned magazine that the following equation is applied to the detected output voltage V video  of the spectrum analyzer to obtain the power of each display point.                V     rm                 s       =         1   T            ∫   0   T            V   video   2             t                     (   1   )                                
     In this case, T is a power measuring time, and V Video  is an output voltage of an envelope detector. 
     In this case, the detected output voltage V Video  must have a linear scale. That is, the amplifier  15  having a linear amplification characteristic instead of a logarithmic amplification characteristic is used. Therefore, in order to materialize a power measurement of an input signal having a large dynamic range, it is necessary to use an amplifier, a detector and an AD converter each having a linear characteristic and a large dynamic range. Consequently, the equipment becomes expensive. On the other hand, if a logarithmic amplifier is used to obtain a large dynamic range by a low cost equipment configuration, the integrated value of the equation (1) does not represent a correct power value. In addition, the arithmetic operation of the equation (1) is applied to the output of the AD converter  17  in FIG.  1 . Therefore, it is necessary to provide a specialized integration circuit for performing this arithmetic operation, and to perform the arithmetic operation at sufficiently high speed through digital calculations. It is shown that stable results can be obtained, and more accurate results can be obtained by increasing the integration time T, namely by increasing the sweep time. 
     However, it is not described in the magazine as to whether the sweep operation of the input frequencies is stopped during the time when the power value of one display point is being obtained, namely during the integration time T, or the input frequencies are swept at a uniform speed and the integrated value of the detected output V video  for each time length T is assumed to be the power value of one display point. In either case, in order to obtain the measured result, namely the stable integrated value of power, it is necessary to make the integration time T of each display point a relatively large value. In order to obtain the measured results of one channel band, namely the power spectrum display, it takes relatively long time. In addition, it is necessary to specially provide an integration circuit for performing the digital calculations of the equation (1) at sufficiently high speed, and in order to obtain the stable result during the integration time T, a considerable number of samples are required. It is necessary to considerably increase the sampling rate of the AD converter  17  in FIG. 1, and hence the AD converter  17  becomes expensive. 
     It is an object of the present invention to provide a spectrum analyzer that can display a correct power spectrum of an input signal having a large dynamic range and can be constructed at low cost, and to provide a spectrum measuring method using the spectrum analyzer. 
     It is another object of the present invention to provide a spectrum analyzer that can immediately display a power spectrum of an input signal regardless of modulation mode and can stably display more correct power values as time passes, and to provide a spectrum measuring method using the spectrum analyzer. 
     SUMMARY OF THE INVENTION 
     According to the present invention, sweeping of received frequencies is repeated, the received signal is logarithmically amplified, the amplified output is detected, the detected output is converted into a digital signal value, the digital signal value (dBm) is converted into a power value of anti-logarithm in watt (mW) dimension for each display point within a width of the frequency sweep, the power values having been converted into anti-logarithms that are obtained at respective frequency sweeps are averaged for each display point, the average power value for each display point is converted into a logarithmic value, and those logarithmic average power values for respective display points are displayed as a spectrum display. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a functional configuration of a conventional spectrum analyzer; 
     FIG. 2 is a diagram showing an example of a displayed spectrum waveform according to a conventional trace-average displaying method; 
     FIG. 3 is a block diagram showing a functional configuration of an embodiment according to the present invention; 
     FIG. 4 is a flow-chart showing an example of processing procedure of the spectrum analyzer shown in FIG. 3; 
     FIG. 5A is a diagram showing an example of power spectrum displayed after first sweep in a method according to the present invention; 
     FIG. 5B is a diagram showing an example of power spectrum displayed after second sweep in a method according to the present invention; 
     FIG. 5C is a diagram showing an example of power spectrum displayed after nth (n&gt;3) sweep in a method according to the present invention; and 
     FIG. 6 is a flow-chart showing an example where a part of the method according to the present invention is changed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows an embodiment of the present invention, and portions in FIG. 3 corresponding to those in FIG. 1 have the same reference signs affixed thereto as those in FIG. 1. A anti-logarithm converting part  31  converts, in nth sweep, a measured value P in  (dBm) for ith display point into a anti-logarithm PP in  in watt (mW) dimension using the following equation.                PP     i                 n       =       10       P     i                 n       10            (   mW   )               (   2   )                                
     Actually, a digital value V in  having been converted into the digital value by the AD converter  17  is stored in the memory  18 , and this digital value V in  is converted into P in  having unit of dBm by a dBm converting part  32 . This dBm converting part  32  is one that is provided in a usual spectrum analyzer, and performs the following arithmetic operations. When the input signal becomes the maximum, a reference level is set such that the maximum signal level can be displayed at the maximum level position of the display frame on the screen shown in FIG.  2 . When a converted digital value outputted from the AD converter  17  for the reference level signal is A T , a converted digital value outputted from the AD converter  17  for the minimum level signal of the display frame is A B , the dB value between the maximum level and the minimum level of the display frame is X, a value in the unit representing 1 mW of the reference level to be 0 dB is Rf dBm, and a digital value for ith display point i in nth sweep is V in , P in  can be obtained by the following equation.                P     i                 n       =     Rf   -           (       A   T     -     V     i                 n         )        X         A   T     -     A   B                         (   dBm   )                 (   3   )                                
     An average value calculating part  34  sums a anti-logarithm PP in  (mW) converted by the anti-logarithm converting part  31  and a summed value PP iA  of ith anti-logarithms up to the previous ((n−1)th) sweep stored in a summed value memory  33  to update the summed value PP iA  stored in the summed value memory  33 . Then the average value calculating part 34 divides a summed value PP iA  of ith anti-logarithms up to nth sweep by n to obtain an average value P ia =PP iA /n. 
     A logarithm converting part  35  calculates a logarithm of the average value P ia  to obtain a display data P i =10logP ia  for a display point i, and stores the display data P i  in a logarithmic value memory  37 . 
     Next, there will be explained with reference to FIG. 4 the procedure that a detected output of an input signal is converted into a digital value by the AD converter  17 , the digital value is stored in the memory  18 , and the digital values stored in the memory  18  are read out in the sequential order to be processed. First of all, the display point number i is set to 1, and the number of frequency sweeps is set to 1 (S 1 ). A digital value V in  is taken out from the memory  18  (S 2 ). A calculation of the equation (3) is performed with respect to the digital value V in  by the dBm converting part  32  to obtain a value P in  in dBm unit (S 3 ). 
     A calculation of the equation (2) is performed with respect to this value P in  (dBm) by the anti-logarithm converting part  31  to obtain a anti-logarithm PP in  in mW unit (S 4 ). 
     Next, a summed value PP iA  of the anti-logarithms for a display point i is taken out from the summed memory  33  (S 5 ). The anti-logarithm PP in  and the summed value PP iA  are summed (S 6 ) by the average value calculating part  34 , and the summed value PP iA  in the summed value memory  33  is updated by this summed value (S 7 ). Then the summed result PP iA  in the step S 6  is divided by the number of sweeps n to obtain an average value P ia =PP iA /n (S 8 ). This average value P ia  is stored in an average memory  36  to update the previous average value P ia  (S 9 ). In addition, a logarithm of the average value P ia  is calculated by the logarithm converting part  35  (S 10 ), and the calculated logarithmic value P i =log10P ia  is stored in the logarithmic value memory  37  (S 11 ). 
     Then the display point number i is incremented by one (1) (S 12 ), and a check is made to see if i is grater than the maximum display point number N (S 13 ). If i is equal to or less than N, the process returns to the step S 2 , and a digital value V in  is taken out from the memory  18  to perform similar processes. If i is greater than N, the display point number i is set to 1 (S 14 ), and the number of sweeps n is incremented by one (1) (S 15 ). Then a check is made to see if the number of sweeps n is greater than a predetermined value M (S 16 ). If n has not exceeded the predetermined value M, the process returns to the step S 2 . If n has exceeded the predetermined value M, the summed values PP iA −PP NA  in the summed value memory  33  are updated by the average values P ia −P Na  in the average value memory  36 , respectively (S 17 ), and the process returns to the step S 2  after setting the number of sweeps n to  2  (S 18 ). 
     The display control part  23  displays the logarithmic values P i  in the logarithmic value memory  37  for respective display points i on the display device  24 . Therefore, in the first frequency-sweep time, when the display data for respective display points i, namely the logarithmic values P i  are obtained, those logarithmic values are displayed in the sequential order. In the second frequency-sweep time, an average value of the anti-logarithm obtained this time and the anti-logarithm for the corresponding display point obtained in the previous frequency-sweep time is obtained for each display point. When a logarithmic value of each average value is obtained for each display point, the logarithmic value is displayed in the sequential order. After this, similarly to the above operations, in each frequency-sweep time, a anti-logarithm for each display point and the summed value of the anti-logarithms up to previous sweep time for the same display point are summed to obtain their average value. Then a logarithmic value of the average value is obtained to be displayed. Therefore, immediately after the frequency-sweep is started, the display operation is performed. As the number of sweeps is increased, the number of samples to be averaged is increased. Hence the stability of the display is increased and the accuracy is also increased. For example, in the first frequency-sweep, a power spectrum shown in FIG. 5A is displayed. As illustrated, the power spectrum is terribly fluctuated. In the second frequency-sweep, a power spectrum shown in FIG. 5B is displayed. The fluctuation of this power spectrum is decreased. Furthermore, in the nth frequency-sweep, as shown in FIG. 5C, the terrible fluctuation has disappeared and a stable display is obtained. At the same time, since the averaging process is performed with respect to anti-logarithms converted from dBm values, correct values are displayed. Regarding a signal like a CDMA signal having similarities to a noise, an average of each power spectrum can be obtained, and hence the process of the present invention is very convenient. 
     If the number of sweeps n is a large number, the summed value PP iA  of the anti-logarithms PP in  becomes a large number, and hence the summed value PP iA  cannot be stored in the summed value memory  33 . Therefore, as explained in the step S 17  in FIG. 4, when n becomes a predetermined value M, for example  100 , each summed value PP iA  is updated by an average value P ia  up to then. Since this update is performed when n has exceeded M, the average value P ia  obtained in the step S 8  may be stored in the average value memory  36  only when n is n=M. That is, after a check is made, as shown by dashed lines after the step S 8  in FIG. 4, to see if n is equal to M (S 19 ), the process may move to the step S 10  if n is not equal to M, or the process may move, only when n is equal to M, to the step S 9  to store the average value P ia  in the average value memory  36  and to execute thereafter the step S 10 . 
     Further, as shown in FIG. 5C, when a marker  41  is moved to an arbitrary position on the displayed waveform and the position is clicked, it is performed by a function provided in a usual spectrum analyzer that a logarithmic average value Pi for that display point i is read out from the logarithmic value memory  37  and is displayed as a numeric value on a part of the display device  24 . 
     The process for obtaining an average of anti-logarithms PP in  may be performed as shown in FIG.  6 . Processing steps in FIG. 6 that are same as those in FIG. 4 have the same step signs affixed thereto as those in FIG.  4 . In this case, after a anti-logarithm PP in  is calculated in step S 4 , a check is made to see if the sweep is the first sweep (S 21 ). When the sweep is the first sweep (n=1), the anti-logarithm PP in  is stored in the average value memory  36  as an average value P ia  (S 22 ), and at the same time P i  is obtained by calculating a logarithm of the average value P ia  (S 10 ). Thereafter, the steps S 11 ,S 12 ,S 13  and S 15  are performed. If the number of sweeps is not one (n≠1) in the step S 21 , an average value P ia  up to the previous sweep is read out from the average value memory  36  (S 23 ), and an average P ia  is calculated by the average value calculating part  34  using the following equation based on this P ia  and the PP in  obtained in the step S 4  (S 24 ).          P   ia     ←       (       P   ia     +       PP     i                 n         n   -   1         )            n   -   1     n                              
     This average value P ia  is stored in the average value memory  36  (S 9 ), and at the same time a logarithm of the P ia  is calculated to obtain P i , namely the process moves to the step S 10  and following steps. 
     As mentioned above, according to the present invention, anti-logarithm power values in watt dimension are averaged for each display point, and a logarithm of this average value is displayed as a spectrum. Therefore, a true envelope waveform of power average of a digital-modulated wave, a noise signal or the like can be displayed. 
     Moreover, since the logarithmic amplifier  15  is used to compress the signal level, even if cheap equipments each having a relatively small dynamic range are used as an envelope detector  16  and an AD converter  17 , power of an input signal having a large dynamic range can be measured. Consequently, the spectrum analyzer can be cheaply constructed as a whole. 
     In addition, according to the present invention, the averaging process is performed for each sweep, and hence the power spectrum is displayed in short time for each sweep. Therefore, the general trend of the power spectrum can be known quickly. Moreover, an average (integral) for each of all the display points is not obtained during one sweep time. It is sufficient that sampling operations are performed N (N is the number of display points) times per one sweep by the AD converter  17 . That is, a high speed AD converter is not required as the AD converter  17 , and hence the spectrum analyzer can also be constructed cheaply from this point. 
     One of, a plurality of, or all of the anti-logarithm converting part  31 , the dBm converting part  32 , the average value calculating part  34  and the logarithm converting part  35  can also be functioned by a software by decoding and executing programs by the CPU  21 .