Abstract:
A spectrum analyzer having an improved local oscillator for use in a digital step sweep is capable of minimizing a dynamic spurious response which is inverse proportional to a unit step time in the step sweep. The local oscillator includes a random clock delay which provides a random clock to a direct digital synthesizer to incorporate random timings in a time length of the unit step time for sweeping the local oscillator. In another aspect, the local oscillator includes a sweep step number control which maximizes the number of steps in the step sweep to ultimately decrease the dynamic spurious response.

Description:
This is a continuation-in-part of U.S. application Ser. No. 08/704,622 filed Dec. 23, 1996 which is a 371 of PCT/JP96/00115 filed Jan. 23, 1996. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to a wide band receiver such as a spectrum analyzer, and more particularly, to a local oscillator to be used in a frequency spectrum analyzer which is able to reduce dynamic spurious responses caused by a digital step sweep operation of the local oscillator. 
     BACKGROUND OF THE INVENTION 
     A wide band receiver such as a frequency spectrum analyzer is used to analyze frequency components of an incoming signal. An example of conventional spectrum analyzer utilizes a local oscillator whose frequency is digitally controlled by a step sweep signal through a direct digital synthesizer (DDS) technology. Such a conventional example of spectrum analyzer is explained with reference to FIGS. 4,  5  and  6 . 
     In the example of FIG. 4, the frequency spectrum analyzer is formed of a frequency converter  50 , a detector  62 , a display arithmetic unit  64 , and a display  68 . As also shown in FIG. 4, the frequency converter  50  includes an attenuator  51 , a first frequency mixer  52 , a local oscillator  30 , a second frequency mixer  53 , a fixed local oscillator  54  and a band pass filter (BPF)  55 . 
     The frequency converter  50  receives an input signal  100  to be analyzed and converts the input signal to an intermediate frequency signal when the local oscillator sweeps its frequency for a specified frequency range. In this example, the intermediate frequency signal is created by mixing the input signal  100  with a first local signal from the local oscillator  30  and a second local signal from the fixed local oscillator  54 . The intermediate frequency signal is filtered by the BPF  55  to a predetermined band width and is then provided to the detector  62 . 
     The BPF  55  may be formed of a plurality of band pass filters having a variety of resolution bandwidth which are set by a user. When two or more frequency spectrum in the input signal have small frequency differences, a band pass filter of sufficiently small bandwidth must be used to fully distinguish the frequency spectrum from the others. 
     The detector  62  detects DC voltages, i.e., envelope voltages, of the intermediate frequency signals from the BPF  55 . The detected voltages are provided to the display  68  through the display arithmetic unit  64 . The frequency spectrum contained in the input signal are displayed on the display  68  in a frequency domain wherein the horizontal axis is a frequency having a variable frequency span (frequency range on a full display) and the vertical axis is a power level. 
     The local oscillator  30  is a sweep oscillator which can sweep a desired frequency range in a step manner with use of the DDS (direct digital synthesizer) technology. As shown in FIG. 5, the local oscillator  30  includes a DDS time base  32 , a DDS  40 , a D/A (digital to analog) converter  34 , a LPF (low pass filter)  35 , a phase comparator  36 , a divider  37 , an integrator  38 , and a YTO (YIG-tuned oscillator)  39 . A phase lock loop (PLL) is formed of the YTO  39 , the divider  37 , the phase comparator  36 , and the integrator  38 . 
     The DDS time base  32  receives a reference clock  31  and sweep conditions  33  which includes a span (sweep frequency range) and a sweep time T swp  and delivers a clock signal  32   ck  to the DDS  40 . The clock signal  32   ck  has a unit step time T step  which is produced by dividing the reference clock  32  by a division factor Div, i.e., T step =(reference clock  31 )/Div. Thus, one clock time period of the clock signal  32   ck  is equal to the unit step time T step  in the step sweep of the local oscillator  30 . 
     The DDS  40  is a synthesizer which generates digital data representing a digital sine wave of a desired frequency. As shown in FIG. 6, the DDS  40  is formed of a frequency register  42 , an adder  44 , and a ROM table memory  46 . 
     The frequency register  42  stores advance phase data  42   dt  and provides the phase data  42   dt  to one input of the adder  44 . The advance phase data is  32  bit data, for example, and defines a magnitude of phase advance of a sine wave to be generated by the local oscillator  30 . By this data, as shown in a stepped ramp signal of FIG. 7A, a unit step frequency  92  is accumulated at every unit step time T step , which results in one sweep time T swp =M×T step . Here, M is a constant number of steps, such as 2,048 steps, in an overall sweep. 
     The adder  44  is for example a 32 bit accumulator to advance the unit phase of the above noted unit frequency  92  of the sine wave. At every clock signal  32   ck  from the DDS time base  32 , one input terminal of the adder  44  receives the advance phase data  42   dt  from the register  42 , while the other input terminal receives the data from a register  44   r  connected to the output of the adder  44 . The register  44   r  holds the output data of the adder  44  produced in the previous accumulation cycle. Thus, the adder  44  accumulates the data at the two input terminals and the result is latched in the register  44   r  for the next cycle. 
     The ROM table memory  46  converts the received data to step like sine wave data. For example, the ROM table memory  46  uses data in the upper 10 bit of 32 bit data from the adder  44  as address data to read 10 bit sine wave data  46   dt  from the table memory  46 . The sine wave data  46   dt  is supplied to the D/A converter  34  shown in FIG.  5 . 
     The D/A converter  34  in FIG. 5 converts the 10 bit sine wave data  46   dt  to a step like analog signal. The LPF  35  removes frequency components of the clock signal  32   ck  in the step like analog signal to make a sine wave analog signal and provides the sine wave analog signal to one input terminal of the phase comparator  36 . 
     The phase comparator  36  detects phase differences between the two input signals and generates voltage signals representing the phase differences. Namely, the phase comparator  36  receives a reference phase signal from the DDS through the LPF  35  as well as an oscillation signal  39   osc  of the YTO  39  whose frequency is divided by 1/N at the divider  37 . The YTO  39  is a voltage controlled oscillator. The phase comparator  36  compares the phases of the two input signals and generates a voltage signal representing the phase differences between the two input signals. The voltage signal is applied to the integrator  38  which is typically a low pass filter. The integrator  38  integrates the voltage signals to produce an analog DC voltage which is supplied to an voltage control input of the YTO  39 . 
     The YTO  39  is a variable resonance oscillator in a microwave frequency band using, for example, a YIG (Yttrium Iron Garnet) crystal. The YTO  39  receives the analog DC voltage from the integrator  38  and generates the step sweep frequency signal  39   osc  which is phase locked by the PLL loop noted above. The sweep signal  39   osc  is supplied to the frequency mixer  52  in the frequency converter  50  to convert the frequency of the input signal  100  to the intermediate frequency signal through the first and second frequency mixers  52  and  53 . 
     As in the foregoing, the sweep operation of the local oscillator  30  is performed in the step manner. Therefore, as shown in FIG. 7A, the frequency of the local signal is also swept in the step like manner, and thus, the swept frequency varies discontinuously. As a result, a dynamic spurious response which is inverse proportional to the unit step time T step , i.e., Δf=1/T step  is induced as shown in FIG.  7 B. Because the spurious frequency Δf is close to a center frequency f o  of the input signal under measurement, it is difficult to remove this dynamic spurious by a filter circuit. Thus, the spurious will be displayed at the frequency positions f o ±Δf on the display of the frequency spectrum analyzer even though the input signal does not have the frequency spectrum Δf. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide a frequency spectrum analyzer having a step sweep local oscillator which is capable of minimizing a dynamic spurious response which is inverse proportional to a unit step time of the step sweep. 
     It is another object of the present invention to provide a step sweep local oscillator to be used in a frequency spectrum analyzer for minimizing a spurious response by incorporating a random clock to each of the unit step times in sweeping the local oscillator. 
     It is a further object of the present invention to provide a step sweep local oscillator to be used in a frequency spectrum analyzer for minimizing a spurious response by increasing the number of steps in sweeping the local oscillator when the sweep speed is decreased. 
     In the first aspect of the present invention, it is included in the spectrum analyzer a random clock delay which provides a random clock to a direct digital synthesizer (DDS). The random clock is a clock where a time length of a unit step time for step sweeping the local oscillator is modified to be random. 
     The random clock delay, by making the timing edges of the unit step time random, diffuses the dynamic spurious frequency Δf=1/T step  generated by the local oscillator where T step  represents the unit step time. Accordingly, the spectrum analyzer of this invention is considerably less affected by the dynamic spurious of the local oscillator which is swept by a digital step sweep through the DDS. 
     In another aspect of the present invention, a sweep step number control is provided to increase the number of steps M within a sweep time of the local oscillator. Here, the sweep time T swp =(number of steps M)×(unit step time T step ). The sweep step number control determines a division factor for dividing the reference clock signal so as to increase the step number M when the sweep time is relatively long and provides the division factor to a DDS time base wherein the reference clock is divided based on the division factor. The sweep step number control  22  also provides advance phase data to the DDS corresponding to the increased step number M. 
     By increasing the step number M with the use of the sweep step number control, and thus sweeping the local oscillator with smaller steps, the dynamic spurious will be reduced by a square of the step number M. 
     The dynamic spurious will be further reduced by combining the first and second aspects of the present invention noted above. 
     The random clock delay in the first aspect of the invention generates the random step time which is, in average, almost the same as the unit step time. Consequently, the step sweep for the local oscillator operates randomly and thus the frequency spectrum of a dynamic spurious frequency component Δf=1/T step  is spread in a frequency domain. Therefore, the noise level caused by the dynamic spurious is significantly reduced. 
     The sweep step control in the second aspect of the invention increases the step number M to satisfy the relationship T swp =M×T step  where T swp  is a sweep time and provides the division factor corresponding to the increased step number M to the time base. The sweep step control also provides the advanced phase data to the DDS. Thus, the local oscillator is swept with smaller steps. By increasing the step number M, the dynamic spurious is reduced by the square of the step number M. 
     As in the foregoing, according to the present invention, the improvement in the local oscillator is achieved in which the dynamic spurious accompanied by the digital step sweep is considerably reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a structure of local oscillator wherein a time length of a unit step time T step  is controlled to be random according to the first embodiment of the present invention. 
     FIG. 2 is a block diagram showing a structure of a random clock delay  12  in the first embodiment of the present invention. 
     FIG. 3A is a block diagram showing a structure of local oscillator wherein the number of steps is increased so as to sweep the local oscillator with smaller steps according to the second embodiment of the present invention. 
     FIG. 3B is a schematic diagram showing a data table in the sweep step control in the second embodiment of the present invention. 
     FIG. 4 is a block diagram showing a structure of spectrum analyzer using a conventional local oscillator which is step swept based on a digital signal. 
     FIG. 5 shows a structure of a conventional local oscillator. 
     FIG. 6 shows a structure of a direct digital synthesizer (DDS)  40 . 
     FIG. 7A is a schematic diagram showing a sweep waveform wherein the frequency of the local oscillator changes in a step manner. 
     FIG. 7B is a schematic diagram showing dynamic spurious responses accompanied by the spectrum analyzer using the conventional local oscillator. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The first embodiment of the present invention will be explained with reference to FIGS. 1 and 2. In this embodiment, the dynamic spurious response caused by a step sweep in a spectrum analyzer is reduced by modifying the unit step time T step  for sweeping the local oscillator to be random time length. Thus, the spurious responses spread randomly in a relatively wide bandwidth of the spectrum analyzer, resulting in reduced peak responses which may no longer be visible on the display. 
     In the conventional technology, because the local oscillator is swept by using a plurality of fixed unit step time T step , a dynamic spurious frequency Δf will be produced at a fixed interval of frequency as a relatively large noise level. The spurious frequency is inverse proportional to a time length of the unit step time, i.e., Δf=1/T step , For example, when a sweep time is 200 millisecond and the number of steps in one sweep is 2,000, the unit step time is 0.1 millisecond. In this situation, the spurious frequency is a fixed frequency of 10 KHz which is too low to filter out. In the present invention, the energy of this spurious responses are spread across a relatively wide rage of frequency band of the spectrum analyzer. 
     As shown in FIG. 1, a local oscillator  10  of the present invention includes a random clock delay  12  instead of the DDS time base  32  in the conventional local oscillator of FIG.  5 . Remaining structure of the local oscillator  30  is the same as the conventional technology of FIG.  5 . 
     The random clock delay  12  receives a reference clock  31  and sweep condition information  33  and generates a random clock  12   rck  having random edges but, in average, the same length of the unit step time T step . The random clock  12   rck  is provided to the direct digital synthesizer (DDS)  40 . To do this, as shown in FIG. 2, the random clock delay  12  is formed of an address counter  13 , a table memory  16  and a divider  18 . 
     The address counter  13  is, for example, an 8-bit counter which increments by one in synchronism with a reference clock  31  every time it receives the random clock  12   rck  feedbacked from the output of the random clock delay  12 . The output of the address counter  13  is provided to the table memory  16  as the address signal. 
     The table memory  16  is a memory which stores pseudo-random data. The data stored in the table memory  16  is set in such a way that a random signal be generated by accessing the data therefrom while an average of the data in all of the addresses is equal to the specified unit step time T step . By receiving the 8 bit address signal from the address counter  13 , the table memory  16  generates random data  12   rd  which is provided to the divider  18 . 
     The divider  18  receives the random data  12   rd  and divides the random data by a specified division ratio to produce a random clock signal  12   rck  which has the unit step time T step = 12   rd /(reference clock  31 ). The random clock signal  12   rck  is provided to the DDS  40  as a time base. The random clock signal  12   rck  is also provided to the address counter  13  as a count enable signal. 
     By repeating the above procedure, the step sweep for the local oscillator is modified to include the random timing edges. As a result, the dynamic spurious frequency Δf=1/T step  is spread in a wide frequency range while reducing an energy level, which achieves a local oscillator of reduced dynamic spurious. 
     The second embodiment of the present invention is explained with reference to FIGS. 3A and 3B. In this embodiment of the present invention, by controlling the direct digital synthesizer (DDS) when sweeping the local oscillator in a manner to increase the number of steps in the step sweep of the local oscillator, dynamic spurious components can be easily removed. In this situation, the step number M is increased and thus the unit step time becomes correspondingly short so that the local oscillator is swept with smaller steps. Consequently, the spurious frequency Δf=1/T step  becomes relatively high and apart from a signal to be analyzed, resulting in easy removal by, for example, the band pass filter in the spectrum analyzer. 
     In the prior art technology, the step number M in the step sweep is a fixed number, such as 2,048, which forms one cycle of the step sweep. In other words, the fixed step number M is used all the time whether the sweep speed is high or low. When the sweep speed is high (sweep time is short), since the dynamic spurious Δf is sufficiently apart from the center frequency f o  of an input signal to be tested, the spurious will not appear as a noise level on the display of the spectrum analyzer. 
     However, when the sweep speed is low (sweep time is long), since the dynamic spurious Δf is close to the center frequency f o  of the measuring signal, it is difficult to remove the spurious frequency. This is because it requires a very sharp, narrow band and highly stable filter to remove the spurious frequency without affecting the frequency spectrum of the signal under test. Therefore, in the present invention, in controlling an operation of the direct digital synthesizer (DDS) of the local oscillator, the step number M is regulated to be maximum as long as the specified sweep time satisfies the relationship T swp =M×T step  to reduce the dynamic spurious. 
     The effect of the invention is mathematically expressed as follows. Where a frequency range of a sweep is Span and the number of measurement points in the sweep is P (same as the number of steps M), a step frequency Δf step  (frequency change per step) is equal to Span/P. A step frequency time Δf m  is equal to P/T swp  where T swp  is a sweep time. The dynamic spurious level S under this relationship is expressed as: 
     
       
         S [dB]=20 log(Δf step /2×Δf m )  
       
     
     
       
         =20 log (Span×T swp /2×P 2 )  
       
     
     By this expression, it is known that the dynamic spurious will be reduced by the square of the measurement points P (the step number M). For example, when the step number M is doubled, the dynamic spurious will be improved four times, i.e., 12 dB. 
     To do this, the local oscillator  20  in FIG. 3A includes a sweep step number control  22 . Remaining components in the local oscillator  20  are the same as in the conventional local oscillator. 
     The sweep step number control  22 , based on the advance phase data  42   d  and the sweep condition  33 , determines the maximum available step number M which satisfies T swp =M×T step  but within the operable range of the local oscillator. The sweep step number control  22  provides the division factor Div which satisfies this relationship to the time base  32 . The sweep step number control  22  also determines and provides the advance phase data  22   d  (frequency for each step) to the DDS  40 . As an example, when M=2,048 and Div=100 in the conventional arrangement, by making Div=25 and M=8,192 in the present invention, the step number is increased four times. As a result, the dynamic spurious will be reduced by 24 dB according to the above equation. 
     For example, the sweep step number control  22  is formed of a micro controller such as a digital signal processor (DSP) and includes a data table such as shown in FIG.  3 B. Sweep frequency data  42   d  including Span data (frequency range of one whole sweep) and sweep condition  33  including sweep time data. Based on the sweep frequency Span and the sweep time t, the sweep step number control  22  selects the highest possible measurement points P (step number M) in the table. The sweep control  22  further determines advance phase data  22   d  (step frequency) based on Span/P and division factor Div in the table. The advance phase data  22   d  is sent to the DDS  40  to define the frequency change in each step while the division factor Div is sent to the time base  32  to define the reference clock rate to the DDS  40 . 
     In the foregoing first embodiment, the random clock delay  12  is formed of the address counter  13 , the table memory  16  and the divider  18 . Other circuit arrangement is also possible such as using a PRBS (pseudo random binary sequence) generator which generates a maximum length sequence random signal. 
     According to the present invention configured as in the foregoing has the following effects: 
     The random clock delay  12  in the first embodiment generates the random step time which has, in average, the same time length as the unit step time T step . Consequently, the step sweep for the local oscillator operates randomly and thus the dynamic spurious frequency component Δf=1/T step  is spread to a wider frequency range with smaller peak energy. Therefore, the noise level caused by the dynamic spurious is significantly reduced. 
     The sweep step number control  22  in the second embodiment increases the step number M (measurement points P) to satisfy the relationship T swp =M×T step  and provides the division factor Div corresponding to the step number M to the time base  32 . The sweep step control  22  also provides the advance phase data  22   d  to the DDS  40 . Thus, the local oscillator is swept with smaller frequency steps. Thus, by increasing the step number M, the dynamic spurious is reduced by the square of the step number M. 
     As in the foregoing, according to the present invention, the improved local oscillator is achieved in which the dynamic spurious responses accompanied by the digital step sweep is considerably reduced.